Do biologists agree on a definition of "species"?

[Buzz Bloom]

I have read through the 2013 thread

https://www.physicsforums.com/threads/definition-of-species-under-attack.688447/ .​

This was the only thread title I found when I searched for "species definition".

Although the question about a definition for "species" was discussed, I found no clarity about the question I am asking in the title of this thread.

I recall the following definition component from an undergraduate biology class in the 1950s.

If there are two identifiable groups of creatures which in the wild do not interbreed more than 25%, then they are distinct species.​

I now get the impression from a variety of sources that this is not now (if it ever was) an agreed upon concept by a large majority of professional biologists. Much of what I read now seems of favor the concept of "clade" rather than "species" in discussing the varieties of organisms.

If a reader of this post knows of a definition for "species" that they believe is currently agreed upon by a large majority of professional biologists I would very much appreciate seeing it.

https://en.wikipedia.org/wiki/Species
In biology, a species is the basic unit of classification and a taxonomic rank, as well as a unit of biodiversity, but it has proven difficult to find a satisfactory definition.

Ernst Mayr's definition (1942)
groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.[28]

It has been argued that this definition is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection.[29][30][31][32] Mayr's use of the adjective "potentially" has been a point of debate; some interpretations exclude unusual or artificial matings that occur only in captivity, or that involve animals capable of mating but that do not normally do so in the wild.​

 

[jim mcnamara]

https://www.aaas.org/example-speciation-happening-now
uses a definition that you see often. For clarity, remember that distantly related animals will sometimes interbreed and produce (normally) infertile hybrid offspring.

Species has become much less a hard and fast rule. It works as a starting point.
What this means is that what we perceive as a species does not always match what occurs in nature. So-called alpha science, the first step, is purely descriptive, and is what Linnean taxonomy is all about - cubbyholes. This is where the species concept derives from. Cladistics are the next step.

Plants and animals are under zero obligation to follow our cubbyhole system or cladistic system during mating.

 

[f95toli]

I don't know much about this...but I do remember there being an extended discussion about the topic in Dawkin's book The Ancestors Tale (this is one of Dawkins less controversial books) .
I believe the most famous counter-example to the "reproductively isolated" definition is Lake Tanganyika (?) where there are many, many "species" of Cichlids that live in different "sectors of a circle" (very simplified), each population has become highly specialized to the local parameters of "their" sector .
The interesting thing is that each population can also reproduce with "cichlids " that live in adjacent sectors but NOT with ciochlids that live e.g. on the opposite side of the circle. Hence, different populations which appear quite distinct "morph" gradually into each other as your move along the perimeter of the circle (i.e. walk around the lake) which is what makes the "reproductivly isolated" definition problematic.

Very fascinating

 

[Rive]

Very fascinating

I think this is an example for 'ring species'
Kind of funny when there is enough exception to fill a definition

definition for "species" that ... is currently agreed upon by a large majority of professional biologists

I think this is one of those things which are taken with the 'I know it if I see it' approach, and influenced by history and success of publications too. Maybe what matters is to know why is the debate.

 

[Ygggdrasil]

The concept of a species is quite old, going back to the time of Aristotle, so the concept has had to undergo major changes as our understanding of biology has changed. Reproductive isolation is certainly a very widely accepted criteria for defining species, but this would apply only to the small minority of organisms that reproduce sexually. How to define species in asexually reproducing species such as bacteria, especially when there is extensive transfer of genes and genetic material (horizontal gene transfer) between different members of bacterial communities, is much more challenging. Googling gives some interesting sources on the topic (e.g. https://schaechter.asmblog.org/scha...cies-really-exist-and-why-should-we-care.html), though I am not sure whether there is any consensus around this issue (I am neither a taxonomist, evolutionary biologist, nor microbiologist).

Another difficulty around the definition of species comes with going back in time. Because organisms can change and evolve over time, we know that new species can evolve, one species can split into two species, and species can disappear from extinction. However, we also know that these changes occur gradually, so defining the exact moment when one species changes into another or the point where two populations have diverged enough to be counted as separate species can be a challenge as well.

 

[Buzz Bloom]

Maybe what matters is to know why is the debate.

Hi Rive:

Sometimes I try to explain science concepts to a variety of people. Some are adults with some science education. Others are children, like my grandchildren, who have a curiosity about science. I find it very difficult to present a clear explanation about something when a key concept has no accepted definition.

Regards,
Buzz

 

[jim mcnamara]

@Buzz Bloom
That is why scientists define their terms for the purposes of discussion. It helps to prevent complaints/criticism about the definition dithering getting in the way of presenting other ideas and data. Mathematicians do the same. They define clearly everything. Beforehand. And in math it is possible and often necessary to limit the discussion with definitions. Ditto Biology. Minus rigor. So they are not an impediment. Tailor your setup for grade school minds and things will be fine.

You should see the asides that papers and books by Dawkins, Gould, and others generate on the topic of Evolution. Google Punctualism or Gradualism. These both exist in Geology and Evolution. Why? Because we can find great examples of both.

 

[Buzz Bloom]

Tailor your setup for grade school minds and things will be fine.

Hi Jim:

Thanks for your post. I think you are advising me to choose a particular definition that I think is understandable to my audience, even though I know it is controversial. Do you also advise that I should tell my audience that the definition is controversial, and also explain, at least to some extent, why this is so? Or do you advise that I should not do that?

Regards,
Buzz

 

[BillTre]

The reproductive isolation concept can be taken as either no mating or a limited amount of mating potentially or in nature.
It is probably the most common cited species concept, but has problems because many widely accepted species can cross with other species.

There are an increasing number of cases of reticulate evolution where separate species cross together to form new species. Platy fish (Xiphorphorus) are a well documented example of this.
In addition, intromission (crossing between different species without forming a new species, but introducing genes from one species into another) is not uncommon, and has been long demonstrated. This is similar to bacteria/archaea exchanging or acquiring genetic information by horizontal transfer of genetic information.
Both of these are challenges to the reproductive isolation species concept.

Molecular biology assessments of the amounts of shared vs. distinct genetic information (sequence similarity at some threshold level) is probably a pretty commonly used method now.

The underlying idea would seem to be groups of organisms evolving together (as demonstrated by sequence similarity) compared to others that evolve independently of that group.

 

[Rive]

I find it very difficult to present a clear explanation about something when a key concept has no accepted definition.

Then turn it around and directly use it to prove that it is actually happening in this flowing present time. If your starting point is the continuous change of species, then these exceptions just become the strongest proof in one go, instead of being hindrances.
Also, these kind of controversies presents the best opportunity to throw some anecdotes in about the workings of the scientific community. People just loves this kind of anecdotes about scientists made a fool by Mother Nature.

 

[BillTre]

Then turn it around and directly use it to prove that it is actually happening in this flowing present time. If your starting point is the continuous change of species, then these exceptions just become the strongest proof in one go, instead of being hindrances.
Also, these kind of controversies presents the best opportunity to throw some anecdotes in about the workings of the scientific community. People just loves this kind of anecdotes about scientists made a fool by Mother Nature.

I like this approach.
I especially like science explained as it developed historically.
This gives you a better understanding of the basis for central concepts such as this and how the understanding of them developed.
It often explains how the jargon developed and the meanings of words change over time.
Species is a good example, as it gene.
This does require knowledge of the history or a decent book though.