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Gene flow maintains a large genetic difference in clutch size at a small spatial scale


Understanding the capacity of natural populations to adapt to their local environment is a central topic in evolutionary biology. Phenotypic differences between populations may have a genetic basis, but showing that they reflect different adaptive optima requires the quantification of both gene flow and selection1,2,3. Good empirical data are rare4. Using data on a spatially structured island population of great tits (Parus major), we show here that a persistent difference in mean clutch size between two subpopulations only a few kilometres apart has a major genetic component. We also show that immigrants from outside the island carry genes for large clutches. But gene flow into one subpopulation is low, as a result of a low immigration rate together with strong selection against immigrant genes. This has allowed for adaptation to the island environment and the maintenance of small clutches. In the other area, however, higher gene flow prevents local adaptation and maintains larger clutches. We show that the observed small-scale genetic difference in clutch size is not due to divergent selection on the island, but to different levels of gene flow from outside the island. Our findings illustrate the large effect of immigration on the evolution of local adaptations and on genetic population structure.

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This study would have been impossible without all those who collected data on Vlieland, and H. van Eck in particular. J. Visser maintained the database. L. Keller, K. Lessells, K. van Oers and M. Visser provided comments on earlier versions of the manuscript. E.P. is supported by ALW-NWO and a Marie-Curie fellowship.

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Competing interests

The authors declare that they have no competing financial interests.

Correspondence to Erik Postma.

Supplementary information

  1. Supplementary Methods

    Contains analyses that show that clutch size can be considered as a trait of the laying female. Also, information on how manipulated clutches were dealt with is provided. Finally, we give additional information on pedigree reconstruction and animal model analyses. (DOC 28 kb)

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Further reading

Figure 1: Clutch sizes on the island of Vlieland.
Figure 2: Spatial variation in clutch size.
Figure 3: Variation in fitness in relation to origin and area of breeding.
Figure 4: Immigration into the East and the West.
Figure 5: Clutch size and fitness in relation to an individual's relatedness to immigrants.


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