Speculation on Speciation

There is one topic which somehow touches almost every major debate in Paleoanthroplogy and this is the question about how speciation actually works. There were a lot of very smart people, who already wrote about this stuff, and I even read some of them. But a large proportion of what I’m going to say about this topic comes from my own thoughts on this topic. So if someone finds a mistake in my arguments, please let me know.


When it comes to speciation, there are basically two different opinions on how speciation could work. The first one is sometimes called “punctualistic” or “phylogenetic” and says that speciation usually results in a split in which the ancestral species splits into two different “offspring-species” (there is probably a more suitable term for it, I just can’t remember the English translation).

The second one is called “gradualistic” or “anagenetic” speciation and says that one species can, if there is enough time; evolve into another species without any form of splitting.

Now although there are many species concepts out there, the main criterion to recognize a species is, if their members are able to interbreed with other animals. This means that no matter what type of speciation process we propose, at some point during this process there has to be some kind of “breeding barrier” (once again, I’ve no Idea how to translate this term), be it either geografical, behavioural or otherwise.


Now let’s take a look on how this stuff actually works in case of a punctualistic speciation:

Exdample for a punctualistic speciation: Assume a Species "A" lives in a certain habitat "X". Now due to some reasons (e.g. a geological event), a barrier arises in said habitat. This barrier changes the enviroment on both of its sides and furthermore prevents the now separated populations of "A" to exchange their genes. With time, those two sperate populations will adapt to their new enviroments and will become two distinct species.

Ok, this was quite easy wasn't it? But how about gradualistic speciation? Her I'll have to admit that although I spent quite some time on this question, I just could come up with one scenario:

1. Let’s assume the species we’re looking at is restricted to one habitat

2. Let us further assume that all population of said species are able to exchange there genes with on another.

3. Now the habitat of said species changes and due to the wonder of natural selection the species adapts to those changes.

4. As time goes by the species within that habitat would differ significantly from the species we had before this process started, so that we could safely say that we have discovered a new species.

There is only one big problem with this model. If we assume a constant gene flow between the seperate populations of our example species, than this means that there never was any kind of mating barrier at any point in time. So how can we be sure that the species we witnessed at the end of that process isn’t able to interbreed with the species we observed before this whole thing started? Sure, we could assume that since both “forms”, as I might call them right now, are so different that they probably wouldn’t have interbred, if they would’ve lived at the same time.
This assumption is pretty similiar to the concept of a “Chrono-species” which defines species solely after their chronological appearances in the fossil record.
The Problem here is that we’re not able to test, whether or not, those species really weren’t able to interbreed. It is as always when we have to deal with extinct species, we simply can’t be sure about it. In the end the only safe thing we can say is that every model on the evolution of a certain species, which relies on a gradualistic model of speciation, is highly speculative.


There are two recent examples within Paleoanthropology where this Problem occurs. The first one is the possibility to draw a direct line from Australopithecus anamensis to Australopithecus afarensis (Kimbel et al. 2006, Haile-Selassie et al. 2010). The other one are the genetical evidences of interbreeding between modern Humans and Neandertals (Green et al. 2010) and modern Humans and those strange people from the Denisova Cave (Reich et al. 2010).
In both cases we have clues, if not even hard evidence in the second example, of constant gene flow between several populations over a long period of time.

The interesting question now, is what we should do with these findings. Should we just keep these different species and use some kind of dodgy chrono-species concept or should we lump all those different species into one or probably two?

I have to admit that, due to my education, I am a little bit biased towards the “lumping” part.

Right now I am not convinced that gradualistic speciation is possible, or to be more precise, that it is detectable by us. So the scientifically safer way for us right now, is to stay cleer of this concept unless we can find some way to test it.

Surely, the last word isn’t spoken on this one, and right now a can think of several flaws in my own argumentation, but I’m still convinced that it is the preferable way of thinking.


References:

Kimbel, W., et al. (2006). Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record Journal of Human Evolution, 51 (2), 134-152 DOI: 10.1016/j.jhevol.2006.02.003
Green, R., et al. (2010). A Draft Sequence of the Neandertal Genome Science, 328 (5979), 710-722 DOI: 10.1126/science.1188021
Haile-Selassie, Y., et al. (2009). New hominid fossils from Woranso-Mille (Central Afar, Ethiopia) and taxonomy of early Australopithecus American Journal of Physical Anthropology DOI: 10.1002/ajpa.21159
Reich D., et al. (2010). Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature, 468 (7327), 1053-60 PMID: 21179161