Reproductive isolation is the engine of evolutionary diversification. When two populations become unable to exchange genes, they’re effectively separate species, free to evolve on independent trajectories.
Biologists have documented many examples of reproductive isolation arising from all sorts of different interactions between organisms and their environments, including incompatibilities between gametes, adaptation to different pollinators (in plants), or the evolution of different sexual characteristics. The cover article for this month’s issue of Evolution describes another way reproductive isolation can arise – adaptation to different environments.
Photo by Dawn Endico.
The new paper, by Lowry et al., describes how different ecological conditions create reproductive isolation where there would otherwise be none [$-a]. The wildflower Mimulus guttatus grows all along the U.S. Pacific Coast. Some Mimulus populations grow inland, in coastal mountains, where the summers are hot and dry; others grow right on the coast, where fog provides moisture but plants have to tolerate salt spray from the sea. Plants from inland and coastal populations look quite different (inland = tall with big flowers; coastal = short with small flowers), and have previously been separated out into different subspecies. But are they actually isolated?
Lowry et al. found that inland and coastal plants perform poorly when transplanted to the others’ habitats, and that they flower at significantly different times. A population genetic analysis shows that the coastal and inland populations don’t exchange genes very often. But it’s possible to hybridize the two types in the greenhouse. In short, it looks like Mimulus is a case of what Nosil et al. called “immigrant inviability” [$-a]. Immigration between inland and coastal sites may be possible, and immigrants would (theoretically) be able to reproduce if they mated with plants from the local population – but before they get a chance, they’re nailed by summer drought (at inland sites) or salt spray (at coastal sites). So even before they’ve evolved fundamental incompatibilities, these two types of Mimulus are well on their way to being separate species.
D.B. Lowry, R.C. Rockwood, J.H. Willis (2008). Ecological reproductive isolation of coast and inland races of Mimulus guttatus. Evolution, 62 (9), 2196-214 DOI: 10.1111/j.1558-5646.2008.00457.x
P. Nosil, T.H. Vines, D.J. Funk (2005). Reproductive isolation caused by natural selection against immigrants from divergent habitats. Evolution, 59 (4), 705-19 DOI: 10.1554/04-428