15 Gems of Evolution (Part 9)

9) Differential dispersal in wild birds

Gene flow caused, for instance, by migration, can disrupt adaptation to local conditions and oppose evolutionary differentiation within and between populations. Indeed, classical population genetics theory suggests that the more that local populations migrate and interbreed, the more genetically similar they will be. This concept seems to accord with common sense, and it assumes that gene flow is a random process, like diffusion. But non-random dispersal can actually favour local adaptation and evolutionary differentiation, as Ben Sheldon of the Edward Grey Institute of Field Ornithology in Oxford, UK, and his colleagues reported in 2005. Their work was part of a multi-decade study of the great tits (Parus major) that inhabit a wood in Oxfordshire,
UK. The researchers found that the amount and type of genetic variation in nestling weight in this songbird differs from one part of the wood to another. This pattern of variation leads to varying responses to selection in different parts of the wood, leading to local adaptation. The effect is reinforced by non-random dispersal; individual birds select and breed in different habitats in a way that increases their fitness. The authors conclude that “when gene flow is not homogeneous, evolutionary differentiation can be rapid and can occur over
surprisingly small spatial scales”.

In another study of great tits on the island of Vlieland in the Netherlands, published in the same issue of Nature, Erik Postma and Arie van Noordwijk from the Netherlands Institute of Ecology in Heteren found that gene flow, mediated by non-random dispersal, maintains a large genetic difference in clutch size at a small spatial scale, again illustrating, as these scientists put it, “the large effect of immigration on the evolution of local adaptations and on genetic population structure”.

References
Garant, D., Kruuk, L. E. B., Wilkin, T. A., McCleery, R. H. & Sheldon, B. C. Nature 433, 60–65 (2005).
Postma, E. & van Noordwijk, A. J. Nature 433, 65-68 (2005).
Additional resource
Coltman, D. W. Nature 433, 23–24 (2005).
Author websites
Ben Sheldon: http://www.zoo.ox.ac.uk/egi/people/faculty/ben_sheldon.htm
Erik Postma: http://www.nioo.knaw.nl/ppages/epostma
Arie van Noordwijk: http://www.nioo.knaw.nl/PPAGES/avannoordwijk
David Coltman: http://www.biology.ualberta.ca/faculty/david_coltman