Question about evolution of animal species

[mikelepore]

Please explain something I don't understand about the evolution of animals that have sexual reproduction.

I thought that being a separate species means an animal will have sexual intercourse resulting in fertile offspring only if the partner is of the same species.

I shall cite the giraffe as an example because of an explanation about its evolution that was once taught to me.

If I understand correctly, the giraffe with a long neck probably came into existence because at one time a mutation occurred inside in a shorter-neck parent, such that the offspring had a longer neck.

Am I correct in saying that the new long neck animal will continue as a distinct species only if does not mate with short neck animals? But why? Would two animals, which are otherwise sexually attracted to each other, stop and think, "I'm unwilling to mate with you because I can see that your neck is too long (or short)"? Or did the mutation that made the offspring's neck longer also make the reproductive cells or organs physically incompatable with those of the short neck animals?

In either case, how did that long neck animal find a mate, so that the new species could survive? Did there have to be at least two mutations in the same geographical area, in both cases having the effect of causing short-neck parents to have long-neck offspring, so that the long neck animal could find a mate?

Thank you. Please keep your explanation simple because my background is physical science, not life science.

 

[chill_factor]

Here's how it works qualitatively:

You have giraffes with normally distributed heights. Those below a certain cutoff height always get eaten by lions because they're too short to see the lions through the grass until its too late. Those above a certain cutoff height never get eaten by lions because they can always see them in time. Now this is a huge simplification but bear with me here.

As time goes on, the short giraffes get wiped out and the tall giraffes reproduce more and more. This raises the average height of the entire population because there's no "short end" to pull down the average. In addition, as tall giraffes mate with average giraffes, their offspring might have height *between* theirs, and become the "new average".

https://www.physicsforums.com/insights/what-is-evolution-a-beginners-guide/

 

[mikelepore]

Thanks for replying. I probably chose a bad example because height is a continuous variable. Perhaps I should have selected an example of any either/or characteristic, such as a reptile with a three-chambered heart evolving into a bird with a 4-chambered heart, or the first time feathers replaced scales. In that case my question puzzles me. If it is a new species then it can only remain a distinct line by not mating with its neighbors, but then I don't understand how it finds a mate to permit it to reproduce.

 

[onomatomanic]

Perhaps I should have selected an example of any either/or characteristic, such as a reptile with a three-chambered heart evolving into a bird with a 4-chambered heart, or the first time feathers replaced scales.

There aren't any such examples. Your notion is similar to the one referred to as "irreducible complexity" by proponents of intelligent design ("the eye couldn't have evolved gradually because each of its components is crucial to its function", et cetera), and the way mainstream science rejects that one is by showing that the eye could indeed have evolved gradually, NOT by arguing that non-gradual evolution is an option as well.

In short, speciation is always the end result of accumulating a sufficiently large number of minute changes, to the point at which the members of the emerging species no longer can or at least no longer do mate with the members of the mother or sister species. It is never the result of the same sort of drastic mutation occurring in a male and a female at the same time and the two just happening to find each other more attractive than any non-mutated specimen. The latter is, as you suggest, absurdly improbable (and I use that claim with caution, because ID proponents tend to throw it around a lot when it doesn't really apply). The key concept which you may want to read up on is exaptation.

As a caveat, I should say that I'm not entirely certain that what you described as "either/or" evolution might not sometimes occur in species which reproduce asexually. It doesn't seem implausible for the occasional drastic mutation to turn out to be useful in some way, and for the clonal offspring of the parent with that mutation to displace the offspring of all other parents or to radiate into a new ecological niche. I'm also not certain how one would go about confirming or falsifying that idea, considering the rarity of useful drastic mutations. Maybe someone more knowledgeable can run with this...

 

[Pythagorean]

Yes, it mostly continuous. Say a species gets separated into two groups geographically and evolve continuously but differently, given their different environments. At a certain point, they drift too far from each other and can't mate anymore by the time they return to each others company. In some cases (horse and donkey) they retain sexual compatibility despite being two different species.

 

[mishrashubham]

If it is a new species then it can only remain a distinct line by not mating with its neighbors, but then I don't understand how it finds a mate to permit it to reproduce.

A single mutation in animal resulting in a drastic phenotypic change doesn't suddenly make it a new species. Speciation is a process which happens to populations and not single organisms. We measure allele frequency for genes in different populations. When in two different populations, the dominant alleles become very different for many genes, so much so that they cannot produce viable offspring after mating with each other, they become two different species.

What happens in your scenario is the creation of a new allele (although it is unlikely that neck length is controlled by a single gene) within a population. Depending on whether the phenotype is beneficial or not, it may be carried forward to the next generation.

But why? Would two animals, which are otherwise sexually attracted to each other, stop and think, "I'm unwilling to mate with you because I can see that your neck is too long (or short)"?

Yes that does happen. The females of many animals (and sometimes males too) often look for certain traits in the opposite gender based on which they choose mates. Why do you think the male peacock has such a huge and heavy tail, which would make it really easy for predators to hunt it down? In case of Giraffes, the males use their long necks to fight each other. So having a short neck won't actually get you any females. In fact this is one of the two hypotheses for the evolution of the long neck. Check Sexual Selection

 

[Ken Natton]

As others have pointed out, the key thing you seem to be missing is to understand that large scale observable changes that occur are always an accumulation of many tiny changes that you would barely notice. Individual genetic mutations that are the origin of the morphological change are never anywhere near significant enough to make that individual organism’s gametes incompatible with the gametes of another member of the same species with the opposite sex. Another example I have encountered that might help is bat wings. If you have ever seen a bat wing you will have noticed how recognisably – if spookily – it resembles any mammalian upper limb, but just with hugely elongated fingers. According to something I read some time ago, this elongation of the fingers was controlled by a group of genes that make ‘bone morphetic proteins’. The key ‘aha’ for me was to understand that the mutations to these genes did not so much say ‘grow long fingers’ as it said, ‘grow fingers at a faster rate’. It will still have taken a long and complex sequence of separate mutations for the bat’s arm to develop into the wing frame you can observe in modern bats.

 

[mikelepore]

As for all of the changes being gradual, thanks for correcting my misconceptions. I had visualized the mutations involved in evolution as occasionally being [I don't what word to use here] "categorical", forming definite branches on the taxonomic tree -- a warbler gets hit by a gamma ray and then its offspring is a thrush, a halibut gets hit by an alpha particle and then its offspring is a mackeral. I don't remember ever hearing this explained in school -- I'm a teacher myself so I'm vigilant about the issue of what we sometimes forget to say. Also thanks for the helpful links.

 

[Darwin123]

Please explain something I don't understand about the evolution of animals that have sexual reproduction.

I thought that being a separate species means an animal will have sexual intercourse resulting in fertile offspring only if the partner is of the same species.

I shall cite the giraffe as an example because of an explanation about its evolution that was once taught to me.

If I understand correctly, the giraffe with a long neck probably came into existence because at one time a mutation occurred inside in a shorter-neck parent, such that the offspring had a longer neck.

Am I correct in saying that the new long neck animal will continue as a distinct species only if does not mate with short neck animals? But why? Would two animals, which are otherwise sexually attracted to each other, stop and think, "I'm unwilling to mate with you because I can see that your neck is too long (or short)"? Or did the mutation that made the offspring's neck longer also make the reproductive cells or organs physically incompatable with those of the short neck animals?

You are incorrect. The new long neck animal could become a new species even if it mated with short necked individuals. The new long necked animal may not even be a separate species, technically. The new species would develop over time, after many mutations.

In either case, how did that long neck animal find a mate, so that the new species could survive?

A hybridization barrier is not a toggle. There are gradations to hybridization barriers. Many times, two populations within the same species can hybrids with a slightly decreased viability. Some small genetic difference makes the hybrid individuals slightly more vulnerable, but the "pure" individuals with a gene slightly more fit.
My favorite example occurs within the human population. Human beings have at least two Rh blood types: Rh positive and Rh negative. You agree that we are the same species, regardless of our Rh blood type. You agree that this is merely a small variation in the same species. Yet, there is a very slight hybridization barrier between the two populations.
There is an allele in most primates, including that contains genes that determine the blood type. If a person has two copies of either type of gene, then the baby will be the same blood type as both parents. However, a hybrid baby may be a different blood type then the mother. Then, there is a chance that the mothers immune system will attack the baby during childbirth. This is called blue-blood syndrome.
I have heard the argument that "Rh baby syndrome" is a minor problem because modern hospitals know how to handle it. Just use prenatal blood transfusions! However, it probably was close to lethal before the 17th century.
Here you see a hybridization barrier within a species (human beings) which can not be eliminated by dilution. Furthermore, it has not been eliminated by natural selection.
If an Rh woman mates with an Rh positive man, the second and third babies may be sick with Rh baby syndrome. I doubt this will lead to a new species of human now that medicine can treat it. However, it is an interesting hybridization barrier.
http://en.wikipedia.org/wiki/Rh_disease
“Rh disease (also known as Rhesus isoimmunisation, Rh (D) disease, Rhesus incompatibility, Rhesus disease, RhD Hemolytic Disease of the Newborn, Rhesus D Hemolytic Disease of the Newborn or RhD HDN) is one of the causes of hemolytic disease of the newborn (HDN). The disease ranges from mild to severe, and typically occurs only in some second or subsequent pregnancies of Rh negative women where the fetus's father is Rh positive, leading to a Rh+ pregnancy. During birth, the mother may be exposed to the infant's blood, and this causes the development of antibodies, which may affect the health of subsequent Rh+ pregnancies. In mild cases, the fetus may have mild anaemia with reticulocytosis. In moderate or severe cases the fetus may have a more marked anaemia and erythroblastosis (erythroblastosis fetalis). When the disease is very severe it may cause haemolytic disease of the newborn (HDN), hydrops fetalis, or stillbirth.”

http://www.marchofdimes.com/baby/birthdefects_rh.html
“The Rh factor is an inherited protein found on the surface of red blood cells. Most people have this protein and are called Rh-positive. However, some people don't have protein; they are called Rh-negative. Rh-negative pregnant women are at risk of having a baby with a potentially dangerous form of anemia called Rh disease. Fortunately, treatment usually can prevent Rh disease.”

Did there have to be at least two mutations in the same geographical area, in both cases having the effect of causing short-neck parents to have long-neck offspring, so that the long neck animal could find a mate?

No.
“Diluting” a mutated gene does not destroy it. Meiosis does not destroy mutated genes, it only mixes them up. Natural selection follows copies of a mutated gene, not the individual that has the gene. Unless the mutated gene is instantly lethal, natural selection can not destroy a mutated gene in one generation.
Hybridization and reproduction barriers come in many forms. Mating behavior is only one way to keep populations separate. Other changes restrict gene flow between populations. There is a large number of such changes. “Dilution” by cross breeding can’t destroy a gene. Only natural selection can totally eliminate a gene and only after more than one generation.
These hybridization barriers are seldom toggles. One mutation does not automatically make the population different. Even using Mendelian laws, it is easy to see that genes don’t get destroyed by cross breeding populations. Most important genes have more complicated dynamics then are indicated by Mendelian laws. Even in those cases, cross breeding doesn’t eliminate the existence of a gene.
In the case of the giraffe, the first mutation that first made the neck longer didn’t need a hybridization barrier to propagate. Genes don’t get diluted continuously. If a giraffe with a slightly longer neck mates with a giraffe that doesn’t have the gene, then the gene for a longer neck will pass on to some of the offspring. The gene doesn’t have to be protected from contact with its opposite allele.
If the gene was a classic Mendelian type gene, with one major phenotype tied to one site on one chromosome chromosome, crossing can never get rid of it. An animal either has one copy, two copies or no copies. If the trait involved several alleles, then natural selection only occurs in those hybrids that have the correct gene on all alleles. The effect of these genes would then be additive. However, hybridization would still preserve the gene whether or not it is advantageous. The animal with the gene would have to be killed by natural selection to eliminate the gene.
This is especially true if the associated phenotype is recessive. The first mutation probably has a gene of only one allele, so the trait isn’t even expressed in the first mutation. The recessive gene won’t be expressed until it has already passed on to quite a few descendents. Then, the animal would need the same recessive gene in two alleles to express itself. If the trait is advantageous, then natural selection will favor animals with two genes. So the natural selection will actually act on the population, not the individual. There is a statistical favoring of the traite.
In your giraffe example, how do you know that the first giraffe with the mutation for longer neck even had a longer neck? The gene could have been recessive. Thus, it wouldn’t express itself in the first generation. If the species isn’t inbred, then it may be a few generation before the gene for a longer neck is expressed. So the selection wouldn’t act on it until long after the mutation had occurred. The population would gradually increase the fraction of animals with a gene for a longer neck. Then another mutation occurs, which could be a duplication of the allele on a chromosome. That duplicate gene could make an even longer neck. The contribution to neck length of the two alleles could add. Pretty soon one would have multiple copies of the long neck gene. The number of such multiple copies would vary in the population, but natural selection would favor more copies of the original allele.
The hybridization barrier, whatever it is, won’t be useful until a few animals already have this gene. Once the advantageous gene has spread through the population, natural selection would favor those animals which cross with those animals that have the advantageous gene. A second mutation, favoring animals which mate with those animals which show the trait, could occur randomly much later. Natural selection would then favor such animals.
Note that in your giraffe case, there is a type of geographical isolation. The short necked giraffe would find food most easily in a jungle or place with short plants. The okapi is an extant animal that lives in a jungle which is like a giraffe in many ways. However, the mutant with the slightly longer neck would find food at the edge of a forest, or any place with long trees. They would graze in the regions where they were the most comfortable. These regions would overlap but have areas outside the overlap. This would not separate the two species completely. However, it would reduce the amount of cross breeding slightly.

http://onlinelibrary.wiley.com/doi/10.1111/j.1439-0469.1999.tb00979.x/abstract
“The frequency of the occurrence of hybrids between Chironomus thummi thummi and Chironomus thummi piger is estimated to be 0.047% in the wild. The rare hybridization events are the consequence of the sexual isolation mechanism of different swarming behavior of thummi and piger. Under laboratory conditions hybrids are easily obtained.”http://ukpmc.ac.uk/abstract/MED/12782734/reload=0;jsessionid=zOfty1718zCKvs7waG8r.12
“The success or failure of interspecific crosses is vital to evolution and to agriculture, but much remains to be learned about the nature of hybridization barriers. Several mechanisms have been proposed to explain postzygotic barriers, including negative interactions between diverged sequences, global genome rearrangements, and widespread epigenetic reprogramming. Another explanation is imbalance of paternally and maternally imprinted genes in the endosperm.”

One example with marine animals. More on this mating.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3338553/
“Sympatric assemblages of congeners with incomplete reproductive barriers offer the opportunity to study the roles that ecological and non-ecological factors play in reproductive isolation. While interspecific asynchrony in gamete release and gametic incompatibility are known prezygotic barriers to hybridization, the role of mating system variation has been emphasized in plants. Reproductive isolation between the sibling brown algal species Fucus spiralis, Fucus guiryi (selfing hermaphrodite) and Fucus vesiculosus (dioecious) was studied because they form hybrids in parapatry in the rocky intertidal zone, maintain species integrity over a broad geographic range, and have contrasting mating systems. We compared reproductive synchrony (spawning overlap) between the three species at several temporal scales (yearly/seasonal, semilunar/tidal, and hourly during single tides).”


Here is an article concerning a hybridization barrier based on mating preferences. Note that this is an example of a saltation. Natural selection acts on the mutation over only a few generations. This fast type of evolution is rare. They call it a saltation. Notice that even in this case, the natural selection takes more than one generation to make an effect. So even with the saltation, using a Mendelian type allele, the mutant gene spreads over more than one generation.
http://www.la-press.com/redirect_file.php?fileId=604&filename=EBO-2-Norrstrom-et-al-(2)&fi...
“Coevolution of exploiter specialization and victim mimicry can becyclic and saltational
Darwin’s Principle of Divergence explains sympatric speciation as gradual and directional. Contradicting evidence suggests that species’ traits evolve saltationally. Here, we model coevolution in exploiter-victim systems. Victims (resource population) have heritable, mutable cue phenotypes with different levels of defense. Exploiters have heritable, mutable perceptual phenotypes. Our simulations reveal coevolution of victim mimicry and exploiter specialization in a saltational and reversible cycle. Evolution is gradual and directional only in the specialization phase of the cycle thereby implying that specialization itself is saltational in such systems. Once linked to assortative mating, exploiter specialization provides conditions for speciation.”

 

[Darwin123]

Here's how it works qualitatively:

You have giraffes with normally distributed heights. Those below a certain cutoff height always get eaten by lions because they're too short to see the lions through the grass until its too late. Those above a certain cutoff height never get eaten by lions because they can always see them in time. Now this is a huge simplification but bear with me here.

As time goes on, the short giraffes get wiped out and the tall giraffes reproduce more and more. This raises the average height of the entire population because there's no "short end" to pull down the average. In addition, as tall giraffes mate with average giraffes, their offspring might have height *between* theirs, and become the "new average".

This is a huge simplification. The OP is asking how the species evolves to avoid cross breeding.The OP may argue that the giraffes that don't live in grassy areas, such as jungle regions, are better off with with short necks. Hence, the final result should be a giraffe with intermediate size necks that live
There are other selective pressures. Male giraffes fight each other using their necks. The female takes the winner, who usually has the longer neck. Thus, there is a pressure to maintain the long neck even in areas with shorter plants. So the fighting instinct prevents long necked giraffes from mating with short necked giraffe-like animals.
There is a short necked giraffe. The okapi is a short necked giraffe-like animal that lives in jungles. There are fossils of different giraffe-like species that lived in different environments. So geography on some scale may be involved with how these different species of giraffe separated.
You can imagine a population with a mixture of short and long necks. Giraffes, like other animals, move to the areas where they can obtain the most food. So the different varieties of giraffe have a tendency to settle in different areas where they can get the most food. This reduces cross breeding.
So maybe the first long necked giraffe simply moved out to a neighborhood that was better for him.

 

[Pythagorean]

As for all of the changes being gradual, thanks for correcting my misconceptions. I had visualized the mutations involved in evolution as occasionally being [I don't what word to use here] "categorical", forming definite branches on the taxonomic tree -- a warbler gets hit by a gamma ray and then its offspring is a thrush, a halibut gets hit by an alpha particle and then its offspring is a mackeral. I don't remember ever hearing this explained in school -- I'm a teacher myself so I'm vigilant about the issue of what we sometimes forget to say. Also thanks for the helpful links.

Naw, the branches are more-or-less "what remains". What I've gathered is that pretty much every time you see a split, it was because a single species was separated into two groups in two different environments and thus had two different averaged selection pressures on them (and given enough time to evolve "away" from each other).

There may be other ways to cause the split, but I can't concieve of them (I've never taken an evolution class though, I've just studied evolutionary neuroscience and worked in labs looking at the evolution of the respiratory control system).

 

[Darwin123]

As for all of the changes being gradual, thanks for correcting my misconceptions. I had visualized the mutations involved in evolution as occasionally being [I don't what word to use here] "categorical", forming definite branches on the taxonomic tree --

The word that you are looking for is "saltation". A "saltation" is a mutation with a huge phenotypic change that is expressed in the first generation. The theory that you are describing is that species are generated by saltations, rather than by the gradual accumulation of large numbers of genes altered by mutations with small phenotypical effects.
A saltation has also been called a monster. The phenotypic change is so huge that the next generation seems almost unnatural.
My opinion is that speciation by saltation is extremely rare since saltations usually die in the first or second generation. However, I suspect that it must happen sometimes. Most of evolution is very, very gradual. However, in biology there is an exception to every rule. So there must be an exception to "evolution is gradual".
Here is an article on evolution that describes several variations of the theory of evolution, including "saltation theory".
http://davidpratt.info/evod3.htm#e1
“A number of influential biologists have seen large-scale mutations as the most probable way in which new types of organisms have emerged. An extreme form of evolution by saltation was proposed in 1940 by geneticist Richard Goldschmidt, with his theory of the ‘hopeful monster’. He held that every so often a spontaneous ‘systemic mutation’ or macromutation – a massive reorganization of the genome of an individual organism – would occur, resulting in a ‘monster’. Most of these would be unviable and perish, but occasionally a ‘hopeful monster’ would appear which would be preadapted to a new environmental niche and become a successful new species. He proposed, for instance, that at one time a reptile laid an egg and a bird was hatched from the egg. He believed that such events accounted for all the major gaps in the fossil record. Goldschmidt was excommunicated by the darwinist establishment and regarded as a lunatic for the rest of his life, though his theory did find favour with palaeontologist Otto Schindewolf, another opponent of gradualism.”

Almost all saltations are lethal in the first generation. Therefore, they probably don't contribute very much to speciation. Unfortunately, saltations are the easiest mutations to study. They show themselves when the mutation first occurs, which is in the very first generation.
So there is a lot of studies of such saltations. One of the best studied types of saltation is the homeotic saltation. This is where a big change occurs in a homeobox gene. A lot of studies have been done on homeotic saltations. Homeotic mutations have helped scientists understand a lot about heredity. However, homeotic saltations are not very likely to contribute to evolution and speciation.
There are a lot of subtler mutations which are probably more common. However, they are very hard to detect in the first or second generation. They are hard to detect even after that, since the effect is small compared to environmental of variation. These mutations have a much greater chance of surviving a few generations, and so contribute much more to evolution. An example would be a balanced chromosomal translocation. Here is an article on chromosomal translocation. Note that there have been studies of this type of mutation. However, most of the experiments have to be done a few generations after the mutation first occurs because the detection is difficult.
http://en.wikipedia.org/wiki/Chromosomal_translocation
“In genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. A gene fusion may be created when the translocation joins two otherwise separated genes, the occurrence of which is common in cancer. It is detected on cytogenetics or a karyotype of affected cells. There are two main types, reciprocal (also known as non-Robertsonian) and Robertsonian. Also, translocations can be balanced (in an even exchange of material with no genetic information extra or missing, and ideally full functionality) or unbalanced (where the exchange of chromosome material is unequal resulting in extra or missing genes).”

 

[mikelepore]

Reply to Pythagorean:

Do you know of some examples how groups can become separated into two different environments? Is it realistic to say water level changing to produce a new island, or a river changing its course? What else can do it?

Reply to Darwin123:

Could it be that saltation and gradualism in the ideas of biologists have some historical or philosophical parallels to the ideas of catastrophism and uniformitarianism in the idea of geologists, respectively? The two dichotomies seems parallel to me. I hope I'm not just making a silly metaphor.

Grateful for everyone taking the time to instruct me.

 

[Pythagorean]

Reply to Pythagorean:

Do you know of some examples how groups can become separated into two different environments? Is it realistic to say water level changing to produce a new island, or a river changing its course? What else can do it?

Of course, it was never directly observed and evidence of specific examples of what exactly occurred are probably rare. But here is the general theory that gives two main mechanisms. The first is geological processes as you have given examples of, the second is population dispersal: like migration or accidental relocation.

For some specific examples and more theory:

http://en.wikipedia.org/wiki/Allopatric_speciation

An example from another page:

"An example of vicariance is the separation of marine creatures on either side of Central America when the Isthmus of Panama closed about 3 million years ago, creating a land bridge between North and South America."

http://www.pbs.org/wgbh/evolution/library/05/2/l_052_03.html

 

[Darwin123]

Reply to Pythagorean:

Do you know of some examples how groups can become separated into two different environments? Is it realistic to say water level changing to produce a new island, or a river changing its course? What else can do it?

There are lots of examples in the literature of geographic separation in evolution. However, I have a particular fascination with the "Rh baby syndrome". Therefore, I will focus this reply on Rh baby syndrome. I promise to discuss other examples in future posts.
I discussed “Rh baby syndrome in a separate post. I presented it as a special case of a “hybridization barrier” within a single species. The single species in this case is Homo sapien. However, I did not demonstrate that there was a geographical separation mechanism for the Rh blood types..
It turns out that the ratio between the Rh blood types in a population has a strong geographical variation. The largest percentage of Rh negatives in the world occurs among the Basque people on the border of France and Spain. This probably explains the high infant mortality that they have had.
There were legends that the Basque people were under some type of curse that caused infant mortality. Now it seems that what they have been suffering from is a high percentage of Rh positive genes. If an Rh negative mother has an Rh positive fetus, then every Rh positive fetus that follows will have severe anemia. No other region in the world has such a large problem with “Rh baby syndrome.”
Furthermore, the percentage of Rh positive genes is lowest in Asia. The population with the lowest percentage of Rh negatives is in Mongolia. They have almost no problem with “Rh baby syndrome”.
There has obviously been some geographic separation process between the two gene types. No one is sure what it is. However, there is one theory that is currently being examined.
The theory is that the Rh positive antigen provides a resistance to the infectious disease caused by the protozoa species, Taxoplasmosis gondii. There is even a theory that hybrids, people with both Rh positive and Rh negative genes, have the highest resistance to Taxoplasmosis gondii. There is a correlation between the areas with a high percentage of infection with Taxoplasmosis gondii and the Rh positive blood type. The disease is usually asymptomatic. However, there is a slightly higher percentage of mental illness and suicide among people with the disease.
Both Rh baby syndrome and symptomatic Taxoplasmosis are rare, one could say that neither has a strong fitness value. However, evolution has had hundreds of generations to work. So it is not too surprising, given evolutionary theory, that there has been a slight bias in the geographical distribution of the Rh blood types.

Here are some links to articles related to the correlation between the Rh-positive blood type, Taxoplasmosis infection, and geography.

http://rhnegativeregistry.com/the-basque-history-of-the-world-rh-negative-origin.html
“But then, in 1937, came the discovery of the rhesus factor, more commonly known as Rh positive or Rh negative. Basques were found to have the highest incidence of Rh negative blood of any people in the world, significantly higher than the rest of Europe, even significantly higher than neighboring regions of France and Spain. Cro-Magnon theorists point out that other places known to have been occupied by Cro-Magnon man, such as the Atlas Mountains of Morocco and the Canary Islands, also have been found to have a high incidence of Rh negative.
Twenty-seven percent of Basques have O Rh negative blood. Rh negative blood in a pregnant woman can fatally poison a fetus that has positive blood. Since World War II, intervention techniques to save the fetus have been developed, but it is probable that throughout history, the rate of miscarriage and stillborn births among the Basques was extremely high, which may be one of the reasons they remained a small population on a limited amount of land while other populations, especially in Iberia, grew rapidly.”

http://bloodjournal.hematologylibrary.org/content/3/4/414.full.pdf
“If the Rh factor played the same role in all races as it does in the Caucasian, then one would expect the incidence of erythroblastosis to correspond to the frequency of the Rh-negative blood type. This expectation has apparently been fulfilled in the Mongolian race, since erythroblastosisis extremely rare among these peoples.”

http://web.natur.cuni.cz/flegr/pdf/rh.pdf
“Toxoplasma and reaction time: role of toxoplasmosis in the origin, preservation and geographical distribution of Rh blood group polymorphism”
…
Here we demonstrated for the first time that among Toxoplasma-free subjects the RhD-negative men had faster reaction times than Rh-positive subjects and showed that heterozygous men with both the RhD plus and RhD minus alleles were protected against prolongation of reaction times caused by infection with the common protozoan parasite Toxoplasma gondii.”

 

[Darwin123]

Reply to Pythagorean:
Could it be that saltation and gradualism in the ideas of biologists have some historical or philosophical parallels to the ideas of catastrophism and uniformitarianism in the idea of geologists, respectively? The two dichotomies seems parallel to me. I hope I'm not just making a silly metaphor.

Saltation and gradualism really concern the source of the inherited variation in organisms. They don't directly concern the fossil record. I don't think many scientists think that saltation explains the discontinuities in the fossil record. Or that the rarity of saltations explains the long periods of stasis in the fossil record.
Saltation has made an odd comeback in the theory called "evolution by self organized complexity". The idea seems to be that there are "islands" of fitness on the parameter landscape that can only be crossed by big sudden changes. In this theory, small changes in phenotype aren't very viable. It is only by rather improbable leaps with huge phenotypic changes that any improvement can be seen. Self organized complexity contradicts the idea of gradualism.
I am not a fan of evolution by self organized complexity. It sounds like physics, but really isn't. I am a physicist so I have an educated bias on this. Evolution of self organized complexity hasn't really been applied in a quantitative manner to data. There have been interesting processes modeled by self organized complexity. However, biological evolution is not one of them.
I see saltation and gradualism as mostly about ontogeny, not phylogeny. One is making a hypothesis in these theories about how the mutation impacts the development of an organism. If the morphology of the organism allows large changes without killing the organism, then it is very likely that evolution occurs from saltation to saltation. However, if small changes in development are less lethal than large changes, then the accumulation of small mutations is more likely to influence evolution. Thus, the real discussion for these two theories should concentrate on how organisms develop (i.e., ontogeny).
Catastrophism and uniformitarianism actually concern how the nonbiological environment affects the evolution of an organism. No hypothesis is made on the type of mutation that is probably. Catastrophism is the idea that sudden changes in environment drive evolution. The environment changes in discontinuous steps. Uniformitarianism is the idea that the environment itself changes, if at all, in small steps. Uniformitarianisms assumes that the parameters of environment are effectively continuous in time.
Catastrophism and uniformitarianism are mechanisms that were developed to explain the fossil record. Both mechanisms appear in the theory called punctuated equilibrium.
You obviously think that catastrophism and saltation are the same. I doubt that catastrophism and saltation are even consistent. I don't think catastrophism and saltation have much to do with each other.
Take the extreme case of both being true. In physical sciences, one checks out the self consistency of a theory by taking the theory to extreme limits, because the extreme limits are simpler to analyze. Suppose that evolution only occurs as a series of saltations. Also suppose that evolution can only advance shortly after a huge catastrophe.
Evolution could only take place when a big catastrophe occurs simultaneously with a saltation. If they did not take place at the same time, then the species couldn't evolve. It would simply go extinct. Yet, the hypothesis is that large catastrophes and saltations are both extremely unlikely. So the probability of both occurring at the same time is small.
Catastrophism and saltation are not equivalent as shown by the fossil record. I think that the fossil record shows that catastrophes really influence evolution. However, I think the fossil record contradicts "saltationism". If saltations were really that common, then we would see fossil evidence for them even in the periods of equilibrium. However, we see evidence of large catastrophes. Very often, we see a sudden diversification of organisms after a catastrophe. Many species of organisms diversify at the same time.
The idea of saltations and catastrophes occurring simultaneously is rather difficult to believe. I find it hard to believe that viable saltations occurred to all these species at the same time. I can see one or two species having the correct saltation at the same time as a catastrophe.
Look at it this way. The evolutionary development of a new species of organism is perhaps very slow. However, the extinction of a species can happen overnight. These are really the two hypotheses of punctuated equilibrium. So a catastrophe that occurs in a single night can change the direction of evolution for an organism that evolves very slowly. This is consistent with the fossil record.

 

[Darwin123]

Reply to Pythagorean:

Do you know of some examples how groups can become separated into two different environments? Is it realistic to say water level changing to produce a new island, or a river changing its course? What else can do it?

The boundary between two populations does not have to be sharp before the two populations diverge to become species. The geographical division can grow slowly in small steps. Furthermore, the geographical boundary does not have to be sharp for the two populations to diverge.
What is important for speciation is that there has to be a change in environment between two geographical areas. The difference in environment itself vsn generate a boundary between two populations so that they become two varieties.
Allopatric peciation is when two populations are separated by a very sharp geographical barrier resulting in the eventual divergence into separate species. Hypothetically, there is no overlap between the two geographical regions. The division of a geographical area into two regions separated by a sharp boundary is called vicariance.
Sympatric speciation is where two populations diverge when there is considerable overlap between the two geographical areas.

The type of evolution that you are probably imaging is called allopatric speciation.
Definition of vicariance
http://en.wikipedia.org/wiki/Vicariance
“Vicariance [from Latin vicarius, derived from vicis; to change, alternate, substitute: root has calendrical associations] is a process by which the geographical range of an individual taxon, or a whole biota, is split into discontinuous parts by the formation of a physical barrier to gene flow or dispersal.
…
Once a species has been split by vicariance into multiple populations with little to no genetic exchange, the populations begin to drift independently. Thus vicariance is a necessary precursor to allopatric speciation.”

Definition Allopatric speciation
http://en.wikipedia.org/wiki/Allopatric_speciation
“Allopatric speciation (from the ancient Greek allos, "other" + Greek patra, "fatherland") or geographic speciation is speciation that occurs when biological populations of the same species become vicariant — isolated from each other to an extent that prevents or interferes with genetic interchange.”

Here is an example of allopatric speciation.
http://www.eeb.yale.edu/near/22.pdf
“RAPID ALLOPATRIC SPECIATION IN LOGPERCH DARTERS (PERCIDAE: PERCINA)”

“Theory predicts that clades diversifying via sympatric speciation will exhibit high diversification rates. However, the expected rate of diversification in clades characterized by allopatric speciation is less clear. Previous studies have documented significantly higher speciation rates in freshwater fish clades diversifying via sympatric versus allopatric modes, leading to suggestions that the geographic pattern of speciation can be inferred solely from knowledge of the diversification rate. We tested this prediction using an example from darters, a clade of approximately 200 species of freshwater fishes endemic to eastern North America.”


Two populations often speciate even while there is considerable oberlap between the two geographical areas of occupation. I described an example when I described “Rh baby syndrome. However, there are a lot of other examples.
Definition of sympatric speciation
http://en.wikipedia.org/wiki/Sympatric_speciation
“Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that they occur together at least in some places.”

This link tabulates some of the cases of sympatric specieation.
https://webspace.utexas.edu/dib73/Bolnicklab/pdfs/Online%20Supplement%20Table%201%20Empirical%20cases.pdf
“A collection of putative cases of sympatric speciation. Listing a particular
case study does not imply that we advocate its status as a case of sympatric speciation, merely that it has been claimed as such. We therefore distinguish between cases that have been generally (though perhaps not universally) accepted, those that seem like possible candidates given further information, and ones that appear to be unlikely or have been firmly rejected. The list of rejected cases is not exhaustive.”

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0034260
“Evidence is growing that not only allopatric but also sympatric speciation can be important in the evolution of species. Sympatric speciation has most convincingly been demonstrated in laboratory experiments with bacteria, but field-based evidence is limited to a few cases. The recently discovered plethora of subterranean diving beetle species in isolated aquifers in the arid interior of Australia offers a unique opportunity to evaluate alternative modes of speciation. This naturally replicated evolutionary experiment started 10-5 million years ago, when climate change forced the surface species to occupy geographically isolated subterranean aquifers.”

Theory of sympatric speciation.
http://ukpmc.ac.uk/abstract/MED/17927777
“Recent theory suggests that frequency-dependent disruptive selection in combination with assortative mating can lead to the establishment of reproductive isolation in sympatry. Here we explore how temporal variation in reproduction might simultaneously generate both disruptive selection and assortative mating, and result in sympatric speciation.”

 

[Ken Natton]

Do you know of some examples how groups can become separated into two different environments?

Notwithstanding Darwin123’s more substantial reply to this point I would like to offer this little perspective. One of the key things that link Darwin and Wallace is that both reported very similar epiphanies on this very point. For Darwin, it was finches with different beaks on the islands of the Galapagos. For Wallace it was beetles with subtly differing features in different valleys in the jungles of the Malay and Indonesian archipelagos. But both had a blinding flash of inspiration when they suddenly realized that the differences between similar species related to the geography of the landscapes in which they were found. That was when they realized what was going on – that they were adapting to their differing environments. So such examples as you are asking for lay right at the very origin of the idea.

 

[Darwin123]

Notwithstanding Darwin123’s more substantial reply to this point I would like to offer this little perspective. One of the key things that link Darwin and Wallace is that both reported very similar epiphanies on this very point. For Darwin, it was finches with different beaks on the islands of the Galapagos. For Wallace it was beetles with subtly differing features in different valleys in the jungles of the Malay and Indonesian archipelagos. But both had a blinding flash of inspiration when they suddenly realized that the differences between similar species related to the geography of the landscapes in which they were found. That was when they realized what was going on – that they were adapting to their differing environments. So such examples as you are asking for lay right at the very origin of the idea.

He seems to be asking about the most extreme limit of the idea, rather than the broad spectrum. I hypothesize that he has been exposed to the idea of allopatric speciation, but not sympatric speciation. So I want to emphasize that the allopatric and sympatric speciation are poles on a spectrum of speciation mechanisms.
I am not sure whether those examples shown by Darwin and Wallace should be considered sympatric speciation or allopatric speciation. You are implying that they are allopatric, providing that no intermediate species lived between valley or island.
However, they could be sympatric. There may be intermediate beetles living between the valleys described by Wallace. There could be some continuous interchange of birds between the Galapagos Islands described by Darwin. So even in these cases, the boundary to gene flow may not be sharp.
I want to make the point that the geographical barrier doesn't have to be a sharp boundary. In fact, it is unlikely that any sharp boundary grows overnight. I want to point out that in some ways it doesn't matter how sharp the boundary is. Even a porous barrier, allowing some interbreeding, can result in speciation. This is why I am emphasizing sympatric speciation.
The geographic barrier can be very gradual rather than abrupt, resulting in sympatric evolution. One possible result of sympatric evolution is the “ring species”.
Here is a link describing different “ring species”. Note that there are geographical barriers in all four cases of the ring species. However, they are not abrupt geographical barriers.
http://en.wikipedia.org/wiki/Ring_species
"In biology, a ring species is a connected series of neighboring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" species. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing a "ring".
…
Currently, there are four known forms of life that appear to be matching the definition of a ring species. The classic example is that of the Larus gull, which in 1925 was found to form a chain of varieties around the Arctic Circle by Jonathan Dwight, but lately doubts have arisen if this is an actual ring species or not. In 1938, Claud Buchanan Ticehurst hypothesized the Greenish Warbler spread from Nepal around the Tibetan Plateau, while adapting to each new environment, meeting again in Siberia where the ends could no longer interbreed.[3] These and other discoveries led Ernst Mayr to first formulate a theory on ring species in his 1942 study Systematics and the Origin of Species. Also in the 1940s, Robert C. Stebbins recognised the Ensatina around the Californian Central Valley as a ring species.[4] Finally in 2012, the first example of a ring species in plants was found in a Spurge form around the Caribbean Sea."

In these four examples, the boundary to gene flow is not sharp. It is gradual. This confuses the issue of whether they are truly species or varieties. However, evolution has clearly occurred in all four groups.

 

[Ken Natton]

I want to make the point that the geographical barrier doesn't have to be a sharp boundary.

I seem to remember a nature program I saw once that showed a particular species of hare that lives in one of the eastern former Soviet Republics. There was one group that lived high up a mountain among the snow and had thus evolved a pure white coat, because that offers the best camouflage in those circumstances; and another group that lived in an adjacent valley that retained the more normal dark brown coat. Again the two populations were perfectly capable of interbreeding and there was no physical barrier preventing contact between them. But the realities were that the white ones lived at the top of the mountain and the brown ones lived in the valley.

 

[Darwin123]

I seem to remember a nature program I saw once that showed a particular species of hare that lives in one of the eastern former Soviet Republics. There was one group that lived high up a mountain among the snow and had thus evolved a pure white coat, because that offers the best camouflage in those circumstances; and another group that lived in an adjacent valley that retained the more normal dark brown coat. Again the two populations were perfectly capable of interbreeding and there was no physical barrier preventing contact between them. But the realities were that the white ones lived at the top of the mountain and the brown ones lived in the valley.

There are lots of examples like that, where the two populations remain perfectly capable of interbreeding. The famous "moth" dichotomy is like that. I have seldom heard Creationists deny that natural selection contributed to the change in moth color.
The argument that I have always heard from Creationists is "Yes, they obviously have diverged due to natural selection. However, they are the same species. For instance, the two populations are perfectly capable of interbreeding. Obviously, natural selection can't create new species."
The examples that I gave were cases where the two populations diverged due to some geographical variation in environment. However, the two populations have lost the perfect ability to interbreed. Even if the barrier went down, the two populations would have some difficulty in interbreeding.
That is why I like "Rh baby syndrome" so much as a natural history phenomenon. Yes, both populations (Rh+ and Rh-)are the same species. Both populations are varieties of the species Homo sapien, which is us.
The blood types range over sociological "races". One couldn't even tell from looking or smelling whether a person is Rh-positive or Rh-negative. Yet, the two varieties of people (Rh+ and Rh-) have some problem interbreeding.
It isn't a large problem interbreeding. It isn't totally lethal. However, it would exist even if the "geographical barriers" that first segregated them disappeared. Even if every Taxoplasmosis gondii on Earth were wiped out in a massive public health program, Rh- mothers would still give birth to Rh+ offspring with anemia.
The Basque people have the problem caused by interbreeding worse. However, the Basque people all look Caucasian. The two populations look identical. However, they have the highest rate of Rh baby syndrome in the world. So there is a fundamental difference in biology among them, but they probably don't even recognize the difference in their culture.
The Basques probably have the lowest rate of Taxoplasmosis in the world. The protozoa create the geographical boundary, although it isn't a sharp geographical boundary. However, the hybridization barrier would persist even if this boundary disappeared. Even if all the Taxoplasmosis gondii protozoa were killed off, maybe by killing off all the cats in the world, Rh baby syndrome would persist. The hybridization barrier can outlive the geographical barrier!
The fastest way to eliminate it would be a brutal segregation of Rh+ and Rh- people. Such a brutal program would ensure a speciation event. Because any mutation that make Rh+ people and Rh- people repel each other would have a fitness advantage.
The "Rh baby" syndrome makes a much better illustration than those moths. The moths are definitely the same species. I don't know about Rh+ and Rh- people.

 

[Borek]

"Yes, they obviously have diverged due to natural selection. However, they are the same species. For instance, the two populations are perfectly capable of interbreeding. Obviously, natural selection can't create new species."

I thought there are known examples of animals that were were introduced into new ecosystems in historical times (either accidentally or for a purpose) that have changed enough to lose the ability to interbreed with the original population. I remember reading about mice or rats that came with sailors to some island and the local population is no longer capable of interbreeding with the original population. But for the life of me I don't remember where have I read it, as it was at least 20 years ago, if not 30.

 

[Ken Natton]

I thought there are known examples of animals that were were introduced into new ecosystems in historical times (either accidentally or for a purpose) that have changed enough to lose the ability to interbreed with the original population. I remember reading about mice or rats that came with sailors to some island and the local population is no longer capable of interbreeding with the original population. But for the life of me I don't remember where have I read it, as it was at least 20 years ago, if not 30.

The problem with that Borek is time scale. I know man has been migrating around the world for some significant length of time, but the general business of taking species from one environment and introducing them into other ones is generally, I think, a more recent phenomenon. It is unlikely, I think, that populations separated for only 500 years or less will already have diverged so far as to be unable to interbreed. I could be wrong. Perhaps Darwin123 will elucidate.

 

[Borek]

I know what the problem is, that's why I am asking.

Note that in the case of small rodents 500 years can mean around 2000 generations, and we start with a small population (even just a single impregnated female) that heavily interbreeds, which can have an additional effect. That's why it doesn't look completely improbable to me.

 

[Darwin123]

I thought there are known examples of animals that were were introduced into new ecosystems in historical times (either accidentally or for a purpose) that have changed enough to lose the ability to interbreed with the original population. I remember reading about mice or rats that came with sailors to some island and the local population is no longer capable of interbreeding with the original population. But for the life of me I don't remember where have I read it, as it was at least 20 years ago, if not 30.

Among my links presented in previous posts, there are a number of examples of speciation events in “historic” times. I don't know if those mice are mentioned in my links. However, there are a few examples.

A few of them have involved insects. Insects reproduce rapidly. Most insects multiply faster than mammals. Therefore, they can speciate more rapidly in time than mammals.

You quoted a belief that I was trying to refute. Speciation has been observed quite a few times where the ability to interbreed has been partly lost on a historic time scale. I was not trying to prove that speciation was impossible. I was pointing out that it has been observed quite a few times already.

One of the classic examples that I know best involve the “apple maggot”, Rhagoletis pomonella. Here are a few links to articles on the speciation of the apple maggot.

http://en.wikipedia.org/wiki/Apple_maggot
“Rhagoletis pomonella is significant evolutionarily in that the race of this species that feeds on apples spontaneously emerged from the hawthorn feeding race in the 1800 - 1850 CE time frame after apples were introduced into North America. The apple feeding race does not now normally feed on hawthorns and the hawthorn feeding race does not now normally feed on apples. This constitutes a possible example of an early step towards the emergence of a new species, a case of sympatric speciation.[2]
The emergence of the apple race of Rhagoletis pomonella also appears to have driven formation of new races among its parasites.”http://www.pnas.org/content/early/2010/04/30/1000939107.full.pdf
“The few genes under or physically linked to loci experiencing strong disruptive selection
can diverge, whereas gene flow will homogenize the remainder of the genome, resulting in isolated “genomic islands of speciation.” We conducted an experimental test of this hypothesis in Rhagoletis pomonella, a model for sympatric ecological speciation. Contrary to expectations, we found widespread divergence throughout the Rhagoletis genome, with the majority of loci displaying host differences, latitudinal clines, associations with adult eclosion time, and within-generation responses to selection in a manipulative overwintering experiment.”

http://biology.mcgill.ca/faculty/leung/articles/Feder_etal_2010EEA.pdf
“The recent shift of Rhagoletis pomonella Walsh (Diptera: Tephritidae) from its ancestral host hawthorn to apple is a model for incipient sympatric speciation in action. Previous studies have shown that changes in the over-wintering pupal diapause are critical for differentially adapting R. pomonella flies to a difference in the fruiting times of apples vs. hawthorns, generating ecologically based reproductive isolation.”

http://www.eebweb.arizona.edu/Courses/Ecol525/Readings/Berlocher%26Feder2002.pdf
“SYMPATRIC SPECIATION IN PHYTOPHAGOUS
INSECTS: Moving Beyond Controversy?
Walsh (176) was inspired by the shift of the apple maggot fly, Rhagoletis pomonella (Walsh), from hawthorn (Crataegus spp.) to domesticated apple (Malus pumila), forming what is now known as a “host race” (48).”

A wasp species, Diachasma alloeum, that parasitizes the apple maggot has speciated.
http://www.crec.ifas.ufl.edu/academics/faculty/stelinski/PDF/Stelinskietal.JIB2010.pdf
“Diachasma alloeum (Muesebeck) (Hymenoptera: Braconidae) is a solitary larval endoparasitoid attacking Rhagoletis (Diptera: Tephritidae) species. Rhagoletis pomonella (Walsh) mark the surface of fruit after oviposition with an oviposition marking pheromone (OMP) which deters conspecific female flies. Herein we demonstrate that female D. alloeum wasps reared from either apple or hawthorn race R. pomenella larvae also deposit an OMP that reduces oviposition by conspecific female wasps.”

 

[Borek]

OK, thanks.

Some googling helped me find this: http://www.genomenewsnetwork.org/articles/04_00/island_mice.shtml

(original paper here: http://www.nature.com/nature/journal/v403/n6766/full/403158a0.html, but I can't access it, and GNN has a reasonably detailed summary - hopefully accurate enough).

 

[Darwin123]

Thanks for replying. I probably chose a bad example because height is a continuous variable. Perhaps I should have selected an example of any either/or characteristic, such as a reptile with a three-chambered heart evolving into a bird with a 4-chambered heart, or the first time feathers replaced scales. In that case my question puzzles me. If it is a new species then it can only remain a distinct line by not mating with its neighbors, but then I don't understand how it finds a mate to permit it to reproduce.

Why should any of these examples be either-or? All the things that you mentioned probably developed slowly, over thousands to millions of generations.
Mammals have a four chambered heart while most reptiles have a three chambered heart. However, this was not an either-or situation. The septum that divides the ventricle in us was optional in our distance ancestors.
The development of a four chambered heart from a three chambered heart probably occurred slowly. The septum developed slowly, eventually dividing the ventricle into two sections.
The septum was not vital to the survival of our reptilian ancestors. They were cold-blooded creatures. Their arterial and venous blood could mix a little bit without killing them.
The fossil record isn't complete concerning the ventricular septum because ventricular septums don't fossilize very well. However, there is little difference between a ventricular septum in a reptile and the maturing rate in Rhagoletis pomonella.
Here is a link to an article about the reptile heart. It shows that most reptiles live with a mixture of arterial and venous blood. They don't even need a septum. A ventricular septum could hypothetically be useful to some reptiles, but these reptiles live without it.


http://www.sacklunch.net/naturalhistory/R/Reptiles.php
“The heart in reptiles consists of three chambers - two auricles and a ventricle, the latter being divided into two portions only by a partition which is generally incomplete, and allows the arterial and venous blood to mix, so that the blood is never so perfectly aerated as in the higher animals. The blood is consequently much colder than that of birds and all the more highly organized animals, where the oxygen obtains a freer access to its particles. In the crocodiles the heart has a complete septum, but there is an intermixture of the venous and arterial blood outside the heart.”

 

[Darwin123]

Thanks for replying. I probably chose a bad example because height is a continuous variable. Perhaps I should have selected an example of any either/or characteristic, such as a reptile with a three-chambered heart evolving into a bird with a 4-chambered heart, or the first time feathers replaced scales. In that case my question puzzles me. If it is a new species then it can only remain a distinct line by not mating with its neighbors, but then I don't understand how it finds a mate to permit it to reproduce.

I conjecture that you would have selected the change from one shell to two shells as an example of an either/or characteristic. I can show that in the case of mollusks this was not an either/or characteristic.
Bivalves, mollusks with two shells, apparently evolved from single shelled mollusks. Both bivalves (two shelled class) and gastropods (single shelled class) appear to have evolved from rostroconchia (single shelled class). However, the change was not discontinuous. There are a range of rostrochonchia shells with an indentation in their shells which eventually leads to the two shelled bivalves.
Note: feathers and scales are also not an example of either/or characteristic because dinosaurs had a continuous range of feathers. However, that deserves another post.
Here are some relevant links relating to the origins of the bivalvia.
http://en.wikipedia.org/wiki/Rostroconchia
“The Rostroconchia is a class of extinct molluscs dating from the early Cambrian to the late Permian. They were initially thought to be bivalves, but were later given their own class. They have a single shell in their larval stage, and the adult typically has a single, pseudo-bivalved shell enclosing the mantle and muscular foot. The anterior part of the shell probably pointed downward and had a gap from which the foot could probably emerge. Rostroconchs probably lived a sedentary semi-infaunal lifestyle. There were probably more than 1,000 species of members of this class.
…
Early, primitive rostroconchs such as Ribeiroia had a hinge in which all shell layers covered the dorsal region resulting in a very rigid shell. In Conocardium, a more advanced rostroconch, the outer shell layers do not cross the entire margin, suggesting independent steps towards the bivalve flexible hinge.”http://rstb.royalsocietypublishing.org/content/284/1001/319
“The pseudobivalved Rostroconchia, first recognized as a separate class of molluscs in 1972, may be the only extinct molluscan class. Until recently, primitive rostroconchs (ribeirioids) were generally thought to be the carapace of crustacean arthropods and advanced rostroconchs (conocardioids) were considered to be unusual pelecypods. In fact, rostroconchs were a diverse class of molluscs (2 orders, 8 families, 31 genera, 400+ species known) that grew a bivalved adult shell from a univalved larval and juvenile shell.”

http://www.shellmuseum.org/BivalvesLeal.pdf
“The bivalve shell probably originated from an evolutionary split of a single ancestral,
cap-like shell along a longitudinal line.”

 

[Borek]

Perhaps a good example of either/or is a change in the number of chromosomes. This is not a thing that can happen gradually.

 

[Ryan_m_b]

Perhaps a good example of either/or is a change in the number of chromosomes. This is not a thing that can happen gradually.

Unless there was some sort of gradual chromosomal translocation, that's not very likely though.

 

[Darwin123]

Unless there was some sort of gradual chromosomal translocation, that's not very likely though.

Originally Posted by Borek View Post
"Perhaps a good example of either/or is a change in the number of chromosomes. This is not a thing that can happen gradually." In terms of number of chromosomes, the change is sudden. For example, a highly asymmetric translocation can fuse two chromosomes together. This would decrease the number of chromosome pairs by one.
In terms of phenotypes, the change is gradual. Because the phenotype change is slight, I would not even call this a saltation. A chromosome fusion can result in one chromosome with two centromeres. This would result in almost no change in phenotype, and only a very small change in interfertility. As long as both centromeres are functional, the fused chromosomes would pair with the two unpaired chromosomes. Meiosis and recombination would largely be unaffected by such a chromosome fusion.
A fusion of chromosomes would not even be noticeable without microscopes and chromosome staining. The change in structure and behavior would largely be insignificant.
If both centromeres were functional, hybrids of fused and unfused chromosomes would have a slight chance of having a birth defect. The chances of survival for the hybrid would be slightly reduced. Later mutations could remove even this disadvantage.
Chromosome fusion may have occurred in the lineage leading from the human species to the most recent common ancestor of chimpanzees and humans.

Here are some links discussing chromosome fusion. These articles indicate that the change caused by chromosome fusion is not a saltation. The effect on anatomy is small if not negligible.

This link is interesting because the article was written by a Creationist. Notice that he explicitly admits that the result of a chromosome fusion can be viable. He provides proof that such fusions are viable. However, he tries to turn this fact around by saying that there was no new “species” created. He ignores the fact that this is consistent with Darwinian evolution. Biologists say that speciation is caused by the accumulation of a large number of very small changes, which is what this is.
http://amazingdiscoveries.org/C-deception-glossary_fusion_inversion_variation#ChromosomeFusion"
“Other populations have been discovered with chromosome numbers varying between 22 and 40. The number of chromosome arms is the same and banding studies reveal the genes to have the same structural features and pattern of genes. Obviously, in terms of their relationship, these different species are all one group.”

Here is a breeding experiment with fruit flies that shows that chromosome fusions can be viable.
http://www.esp.org/foundations/genetics/classical/holdings/Genetics/Genetics-1935-20-4-327.pdf
“In the original experiment, 14 translocations involving the X and fourth chromosomes, and viable in the male, were obtained. In 8 of these the gene string of the X appeared to be intact up to and including the normal allele of bobbed.”

There is the evidence that this happened in the evolution of the human race.
http://www.evolutionpages.com/chromosome_2.htm
“All great apes apart from man have 24 pairs of chromosomes. There is therefore a hypothesis that the common ancestor of all great apes had 24 pairs of chromosomes and that the fusion of two of the ancestor's chromosomes created chromosome 2 in humans. The evidence for this hypothesis is very strong.”

 

[Darwin123]

Thanks for replying. I probably chose a bad example because height is a continuous variable. Perhaps I should have selected an example of any either/or characteristic, such as a reptile with a three-chambered heart evolving into a bird with a 4-chambered heart, or the first time feathers replaced scales. In that case my question puzzles me. If it is a new species then it can only remain a distinct line by not mating with its neighbors, but then I don't understand how it finds a mate to permit it to reproduce.

You wanted to know about how a geographical barrier can effect the genetic distribution of a discontinuous variable. Chromosome number is an intrinsically “discontinuous” variable that is sometimes associated with speciation.
Biologists still believe that in the vast majority of cases evolution is "gradual". By gradual, I mean that the anatomy of the animal is changing in very small increments. Even in the case of changing chromosome number, the animal undergoes very small changes on an anatomical level. One would not notice the changes in each generation except with specialized equipment.
Chromosome number can result in a genetically unviable hybrid. The effect of mismatched chromosomes can be large. For example, an unbalanced translocation can fuse two chromosomes by causing an imbalanced dosage of enzymes.
Changes in chromosome number can also result in a very small difference in phenotypic expression. The degree of change in the phenotype is sensitive to the mechanism by which the chromosomes change number. Hence, chromosome number within a species can vary over a broad geographic range.
Note that the anatomy and behavior of such a mutation may be unnoticeable. A change in chromosome number doesn’t have to result in a saltation. Balance translocation can change the
In animals that with obligate sexual reproduction, mutations that change in chromosome number are usually unviable. However, there are exceptions.
Many examples of the change in chromosome number occur by sympatric speciation. Thus, there is a geographical “umbilical cord” between two populations with differing chromosome number. within the “umbilical cord”, there is continuous hybridization.
One thing a catastrophe can do is “severe” the umbilical cord. If a species is undergoing sympatric variation, the zone of hybridization is limited. If there is a mass extinction, the middle zone is wiped out and the two varieties can evolve their separate ways.
I think this is the main role of catastrophe. I don't think that it causes the speciation. Catastrophes mostly severe the "geographical umbilical cord" by killing off the varieties that originally evolved by sympatric speciation.
I don't think that a saltation has anything to do with it. "Punctuated evolution" is about disasters that cut the geographic umbilical cord.

The horse genus is well known for having a huge range of chromosome numbers. Horses on the opposite sides of this grade can not interbreed. The donkey-horse hybrids are most famous for being infertile. However, there are species within this genus that are interfertile. The only reason that they don’t interbreed is because of geographical distance. Note that in these cases the change in chromosome number is not associated with a discontinuity in anatomy.

The next two links refer to different species of horses that can interbreed even though they have different numbers of chromosomes.
http://en.wikipedia.org/wiki/Przewalski's_horse
“The karyotype of the domestic horse differs from that of Przewalski’s horse by an extra chromosome pair either because of the fission of domestic horse chromosome 5 in Przewalski’s horse or fusion of Przewalski’s horse chromosomes 23 and 24 in the domestic horse. In comparison, the chromosomal differences between domestic horses and zebras include numerous translocations, fusions, and inversions. Przewalski’s horse is known to have the highest diploid chromosome number among all equine species. Przewalski’s horse can interbreed with the domestic horse and produce fertile offspring (65 chromosomes).”

http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1479&context=gradschool_diss
“COMPARATIVE GENE MAPPING FOR EQUUS PRZEWALSKII AND E. HEMIONUS ONAGER WITH INVESTIGATION OF A HOMOLOGOUS CHROMOSOME POLYMORPHISM IN EQUIDAE
The Przewalski.s wild horse (E. przewalskii, EPR) has
a diploid chromosome number of 2n=66, while the domestic horse (E. caballus, ECA) has 2n=64.”

Most zebras are like donkeys. They don’t produce viable hybrids with the domestic horse. However, Grevy’s zebra is an exception. It can produce viable hybrids with the domestic horse. Here is a link.
http://www.princeton.edu/~dir/pdf_dir/2009_Cordingley_AnimConser.pdf
“Both species are sympatric in the Laikipia ecosystem of northern Kenya, where we have observed purportedly hybrid individuals. Using mitochondrial and Y chromosome DNA, we confirmed the hybrid status of the morphologically identified hybrids and demonstrate conclusively that all first-generation hybrids are the offspring of plains zebra females and Grevy’s zebra males.
…
Plains and Grevy’s zebra have distinct chromosomal structure, morphology, ecology and social systems. Genetically, the Grevy’s zebra has 46 chromosomes; while plains zebra has 44 (Ryder, Epel & Benirschke, 1978). There is a close chromosomal correspondence between the two species, with karyotypic differences in only eight chromosomes (Musilova et al., 2007). In captivity, viable male and female hybrid offspring produced from a plains zebra female and Grevy’s zebra male had 45 chromosomes (Benirschke, 1977).”


This link describes the varying genomes of a species of gopher with varying chromosome number. Or is it two species of gopher? The law hasn’t decided yet.
http://www.nsrl.ttu.edu/personnel/RJBaker/Publications/223-Hybrid%20gophers%20and%20ESA-Jones%20et%20al-1995.pdf
“Hybrid Pocket Gophers and Some Thoughts on the Relationship of Hybrids to the Rules of Nomenclature and the Endangered Species Act
The one animal that was karyotyped possessed a diploid number of 72 (Table 1)…The karyotype was indistinguishable from the karyotype of G. b. major, but distinct from those of G. knoxjoni and G. texensis, both of which have possesses 2n=72.”

 

[Darwin123]

Thanks for replying. I probably chose a bad example because height is a continuous variable. Perhaps I should have selected an example of any either/or characteristic, such as a reptile with a three-chambered heart evolving into a bird with a 4-chambered heart, or the first time feathers replaced scales.

It is important as a science teacher that you don’t teach your students a misrepresentation of evolutionary theory. I am primarily addressing your skepticism that evolution is gradual. I have been posting a number of messages, with links, to show that evolution generally proceeds in a series of very small steps even in animals where sexual reproduction is obligatory. Evolution seldom, if ever, proceeds by a saltation in anatomy.
You claim that scales and feathers can are discontinuous. However, your definition of scale is rather general. Extant reptiles have a flat scales. However, some extant reptiles also have follicles (i.e., tubes) in addition to flat scales. Follicles may have preceded scales.
Please note that extant birds still have flat scales in addition to feathers. This is especially pronounced in birds that haven’t been domesticated until recently. Look at the legs of a turkey, for example. However, feathers did not evolve from these flat scales. What feathers evolved from were follicles such as some lizards and crocodilians have.
Hypothetically, feathers evolved from follicles in a series of very small steps. Maybe flat scales also evolved from follicles. The diversity of dinosaur follicles shows that feathers did not have to develop from follicles in one generation. A continuous variety of follicle forms existed in the Mesozoic. In fact, there are a variety of follicle forms

1)
http://www.mcorriss.com/Prum_&_Brush_2002.pdf[/URL]
“Whatever their function, feathers evolved by selection for a follicle that would grow an emergent tubular appendage. Feathers are inherently tubular structures. The homology of feathers and scales is weakly supported. Feathers are composed of a suite of evolutionary novelties that evolved by the duplication, hierarchical organization, interaction, dissociation, and differentiation of morphological modules. The unique capacity for modular subdivision of the tubular feather follicle and germ has fostered the evolution of numerous innovations that characterize feathers. The evolution of feather keratin and the molecular basis of feather development are also discussed.”

2)
[url]http://icb.oxfordjournals.org/content/40/4/631.full[/url]
“It is likely that feathers evolved from a conical shaped tubercle rather than a plate-like structure. Although the morphology of the presumably most primitive feather is unknown, minimal conditions for its production include the cellular capacity to synthesize feather proteins (=ϕ-keratin) which provides the molecular phenotype, and a follicular mechanism for production and assembly of molecular and gross structure. Once the minimal structural element, presumably recognizable as a barb, existed, a variety of phenotypes followed rapidly. A tubercular growth center of appropriate size could produce a simple barb-like element, with cortex and medulla. This might be recognized externally as a bristle, but need never existed as a separate morphological unit.”

3)
[URL]http://www.nature.com/nature/journal/v464/n7293/full/nature08965.html?free=2[/URL]
“In combination with the wide distribution of proximally ribbon-like pennaceous feathers and elongate broad filamentous feathers among extinct theropods, this find suggests that early feathers were developmentally more diverse than modern ones and that some developmental features, and the resultant morphotypes, have been lost in feather evolution.”

One of the most common lies is that evolution requires saltations. Let me show you an article which presents such a lie regarding the evolution of feathers.
4)
http://darwinism-watch.com/index.php?git=makale&makale_id=1228
“Evolutionists claim that feathers evolved from the scales on birds’ alleged ancestors—reptiles. The fact is, however, that scales are folds in the skin, whereas feathers emerge from follicles in the skin, in a similar manner to hairs. Feathers consist of barbs, barbules and hook. Furthermore, the places where barbules and scales emerge from are very different.
…
Evolutionists suggest random mutations for that alleged mechanism. However, it is a known fact that mutations do not add new genetic information to living things, and can therefore have no evolutionary effect.
…
That advantage deals a fatal blow to the evolutionist scenario that the transition to flight took place after thermal insulation. According to that scenario, feathers assumed to have evolved initially for insulation should have a contoured structure, in which case only feathers which provide greater thermal insulation, in other words more contoured feathers, will be selected. For that reason, the hypothetical progress from a contoured structure to an aerodynamic one will be eliminated.
...
According to the theory of evolution, the genetic information for feathers, which did not at that time yet exist in nature, must have been new, and furthermore this information must have been added to the reptile DNA by a natural causes-based mechanism. Evolutionists suggest random mutations for that alleged mechanism. However, it is a known fact that mutations do not add new genetic information to living things, and can therefore have no evolutionary effect.
…
The irreconcilable differences between scales and feathers clearly reveal the invalidity of the evolutionists’ claims. In addition, the oldest known bird, Archaeopteryx, has an asymmetrical feather structure identical to that of modern flying birds. In other words, the oldest known bird emerged with the most perfect feather structure, whereas the theory of evolution would expect a “primitive” structure. The researchers L. Martin and S. A. Czerkas state that, “The oldest known feathers … are already modern in form and microscopic detail.”

“Evolutionists” now believe that feathers evolved from follicles (link #1). This scenario does not require that the feathers initially evolved for insulation should have a contoured structure, since the property of being contoured is a continuous variable.
The feathers are said to have first evolved in dinosaurs, not birds (links 1, 2 and 3). Since dinosaurs had follicles, this is not a problem. Archaeopteryx is no longer the oldest known bird, as there are fossil birds that seem to have come before. Birds and dinosaurs were not very different, so calling Archaeopteryx a “bird is a matter of sophistry. Some dinosaurs even had fused clavicles, something not mentioned in link #4.
The author’s entire argument is based on a tautology. A bird is a dinosaur with contoured feathers. If Archaeopteryx had merely barbed follicles that were not contoured, then it would not be called a bird.
Feathers can be contoured and still serve as insulation. A bird that flies to a tropical island wouldn’t need insulation anyway. The assumption here is that a dinosaur would have to fly perfectly well to evolve a feather. Obviously, it isn’t true.
Also note that the "known fact" that "mutations do not contain information". This is two lies. It is not a fact. No where does any experimenter or theorist say that mutations do not contain information. However, the bigger lie is "that it is a known fact". There is no reference, let alone links, to this known fact. It is a rumor, not a fact. I bet that you can't find a reputable study where this is explicitly proven. I bet you can't even find a study where biological information is quantitatively defined. How can this be a "known fact" when "information" is not quantitatively defined?
If you have other objections to the theory of evolution, that is fine. I just want to refute the urban legend that evolution takes place in discontinuous steps. Evolution is a gradual process.