Evolving Thoughts

Evolution, culture, philosophy and chocolate! John Wilkins' continuing struggle to come to terms with impermanence... "Humanus sum, nihil humanum a me alienum puto" - Terence

Friday, February 24, 2006

Speciation of invaders by natural selection

Continuing my theme of late that species are formed by natural selection even if they are not maintained in sympatry with their nearest relatives that way, comes this item in PNAS by Daniel Funk at Vanderbilt, Patrik Nosil at Simon Fraser, and William Etges from the University of Arkansas.

Funk et al. analysed indices of reproductive isolation (RI) and genetic distance, standing in for the time since speciation, of species that had invaded novel areas and speciated. They used an ecological indicator (ecological divergence, or ED) to see how ecological adaptation and reproductive isolation correlated. They correlated rather well. In over 500 species ranging from flowering plants to birds, fishes, butterflies and other invertebrates, ecological factors correlated strongly with speciation. Somebody who can read statistics needs to comment on this, but it looks to me as if Funk and co. just gave the idea of sympatric speciation a real boost.

The proximate factors here include sexual selection, but even allowing for that, ecological factors on size, diet, habitat selection and so on all indicate that selection plays a crucial role in speciation. Or does it? Nobody denies, not even the most ardent antiadaptationist, that aspects of organisms are strongly subject to selection, whether during speciation or after it. The critical issue is whether selection is a cause of speciation itself.

The allopatric consensus view allows for local adaptation, of course, when isolated from the parent metapopulation. What it denies is that selection for RI occurs - how could it when speciation is occurring without contact with the reproductively isolated populations? There is selection of RI, of course, since RI on that account is a byproduct of changes in the population that are selectively favoured for ecological reasons. But not selection for RI itself [the selection of and selection for distinction is due to Elliot Sober]. So, argue allopatrists such as Jerry Coyne and Allan Orr, selection is not a cause of speciation in allopatry. And this seems right.

The sympatrists claim, on the other hand that some speciation at least occurs when the ranges of the two populations coincide at least in part. In this case, if there is selection for a local adaptive niche that differs from the parental species' niche, there is also selection against hybrids that are equally maladapted for each niche, neither fish nor fowl, as it were. Hence, on a sympatric view, there is selection for RI.

What Funk et al. have demonstrated is that ecology plays a role in shaping the function and structure of new species. This was never really at issue. Does it have any further import?

I think it does. The import lies not in the fact that species are isolated reproductively from their relatives, but that they are cohered with their own by selection. In short, what causes a species is that it is a genetically compatible group with a shared reproductive "reach". And this contributes to the fitness of variants, largely for the very obvious reason that if you can't interbreed with your neighbours, you have a zero fitness. What causes isolation is either geography (allopatry) or divergent selection in sympatry. So being a different species is due to RI; being the same species is due to selection for a range of properties, including reproductive reach and ecological adaptation.

I distinguish between extrinsic and intrinsic selection for this conspecific cohesion. Some aspects of being a species are caused by extrinsic environmental selection - of course we should expect that, for those that cannot flourish cannot breed. But also, in sexual organisms, one has to be compatible with potential mates, and so selection for intrinsic properties that facilitate this must also play a role.

If we think of speciation as "what makes a species" then we get ecological and other selective processes. If we think of speciation as "what makes it not the same species", then the explanatory focus shifts, and here the answer is, in cases when divergent selection is not going on, populations simply drift away from the reproductive reach of the ancestral population.

[Thanks to Pete Dunkelberg for spotting some errors]