Evolution is a tricky thing – and so is inferring general theory from simplified model results. This is proven once again in a new study by Leithen K. M’Gonigle with Ulf Dieckmann as senior author, which challenges the wide held belief that sexual selection alone cannot stabilize coexistence in the absence of ecological differentiation when species ranges are overlapping.
The conclusion that sexual selection alone is not sufficient to stabilize coexistence was derived by models that worked with homogeneous space. Leithen et al. show that a slight modification of assumptions, namely an ecological neutral model featuring 1) sexual selection 2) spatial heterogeneity in habitat quality and 3) mate search costs in females allows long-term coexistence of ecologically equivalent species because it leads to local separation of species on clusters of “good” habitat (Figs. 1,4 in the paper).
Figure: Effect of variation in habitat quality on species distributions. From M’Gonigle et al.
Just some random thoughts: it seems to me that the mechanism is not globally stabilizing in the sense that it would allow the invasion of new species once space is filled by one or several resident species (at least no such results are shown). Rather, I would think that species are caught in a “local” minima by the fact that sexual selection creates a kind of local Allee effect that gives each species an advantage against invaders on the patches it occupies, and a disadvantage to invade other patches. Maybe this is also be the reason why the authors chose to speak about “long-term coexistence” rather “stable coexistence” in the title? Doesn’t challenge the conclusions drawn here, but interesting from a theoretical perspective.
Another, more ecological note: this coexistence mechanism predicts a strong local segregation of species, a pattern that is so distinctive that it should be relatively easy to test empirically. There are a few comments on that in the paper, but it would certainly be interesting to see an empirical study testing this prediction.
In any case, a very interesting study, and another example that should caution against generalizing theory from simple, particularly non-spatial models – that is not to say that such models don’t have their merits, but I think far too many “general” conclusions have vanished (I’m thinking, for example, about the conditions that allow evolution of cooperation) when things were put into (heterogeneous) space, or made stochastic, etc.