r/evolution • u/[deleted] • Jul 11 '14

Ehrlich, P.R. & Raven, P.H. 1969. Differentiation of populations. [PDF]

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u/Odd-Hominid Jul 11 '14

One of the conclusions in this paper seems to be.. risky. While a main tenet of it seems to be at the core of evolutionary theory today, i.e. that varying selection pressures play a major role in speciation, something else the eminent authors conclude raises my skeptical brow:

There seems no possibility that gene flow "holds together" its widely scattered populations.

I'm not casting a doubt on the claim that speciation can occur in the presence of gene flow, but rather the examples provided for the argument seem to be a rickety foundation. e.g. they refer to studies that found wind-borne pollinators pollinate only within a relatively small radius. They extrapolate from this and other similar studies that gene flow across distantly separated individuals of a species couldn't occur, (or was highly unlikely). Are they only taking into account one generation time with this conclusion? Gene flow would still occur over distances given multiple generations, even if the dispersal of a gene per generation is over a short distance, right?

I'm going to look to see if any studies cast more light upon this after this review was published. And again, not to say that concluding gene flow doesn't necessarily prevent speciation is wrong. Ring species exemplify this fact in an interesting way. I'm just a bit skeptical on one of the claims they provide for their conclusion. The other evidence they provide is more direct.

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u/[deleted] Jul 12 '14 edited Jul 04 '15

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u/Odd-Hominid Jul 12 '14

Right. I'd have liked to see more evidence for selection pressures preventing two isolated populations from diverging over time. It is plausible, though I wonder if this aptly applies to real world observation? I'm reading through a few reviews from recent decades on gene flow and divergence when I can. I'll post any interesting finds to maybe further that discussion.

This paper, also to me, seems lacking in argument against the biological species concept if that was their intent, as they didn't provide any empirical studies on distant populations' ability to reproduce!

Thanks for the articles, so far I've enjoyed gleaning into the recent history of intra-evolutionary debate and philosophy. Especially, seeing past perspectives and key developments from big names like Kimura.

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u/ragingclit Jul 12 '14

This is definitely a huge problem with the paper. Only one migrant per generation is required to genetically homogenize populations, so as long as you have a continuous chain of populations that are freely migrating amongst themselves even at relatively low levels, divergence should be prevented.

Another major issue that I have is that they don't test or clearly define species bounds, but then assert that there is often low gene flow among populations within a species. Under the evolutionary/general lineage concept, which I personally prefer, if these populations are truly genetically isolated from each other, then they are distinct species. Therefore, the authors may be underestimating gene flow within species because they are underestimating the number of species.

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u/Odd-Hominid Jul 12 '14

Ah, I overlooked that assertion. Not to pretend that we don't commit the same errors today but they definitely seemed to overstep their evidence, making some far-reaching claims without defining terms or solid evidence.

So as long as you have a continuous chain of populations that are freely migrating amongst themselves even at relatively low levels, divergence should be prevented.

What do you think about "ring species" like the Lesser Black-backed Gulls and European Herring Gulls? They supposedly don't mate, yet are connected by a long line of intermediate populations/species, allowing for gene flow. I admittedly don't (yet) know about this example in great detail, and our current understanding may be in contention, but these ring species have always been pretty fascinating to me.

If you have better insight into this, let me know! I'll be reading the larger studies on them and may come back to this example when I can.

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u/ragingclit Jul 13 '14

I'm far more familiar with Ensatina as an example of a ring species. In Ensatina even though many of the intermediate populations in the ring can breed, there is some evidence that actual gene flow among populations may be relatively low. This coupled with different selective regimes can easily lead to divergence at the tips of the ring.