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Ocean currents mediate evolution in island lizards


Islands are considered to be natural laboratories in which to examine evolution because of the implicit assumption that limited gene flow allows tests of evolutionary processes in isolated replicates1. Here we show that this well-accepted idea requires re-examination. Island inundation during hurricanes can have devastating effects on lizard populations in the Bahamas2,3. After severe storms, islands may be recolonized by over-water dispersal of lizards from neighbouring islands3. High levels of gene flow may homogenize genes responsible for divergence, and are widely viewed as a constraining force on evolution4,5. Ultimately, the magnitude of gene flow determines the extent to which populations diverge from one another, and whether or not they eventually form new species6,7. We show that patterns of gene flow among island populations of Anolis lizards are best explained by prevailing ocean currents, and that over-water dispersal has evolutionary consequences. Across islands, divergence in fitness-related morphology decreases with increasing gene flow5. Results suggest that over-water dispersal after hurricanes constrains adaptive diversification in Anolis lizards, and that it may have an important but previously undocumented role in this classical example of adaptive radiation.

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Figure 1: Map of the study islands showing the polarity and magnitude of ocean currents (thin arrows) and gene flow (thick arrows) between adjacent islands.
Figure 2: Standardized morphological divergence (mean = 0, unit variance) in four morphological characters, as a function of the standardized number of migrants (Nm) between islands.

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We thank P. Beerli and N. Manoukis for technical support using Migrate. T. Georges, J. Harlan and T. Lee provided helpful discussions concerning the action of ocean currents. D. Irwin, J. Levine, J. Losos, B. Mila and B. Sinervo provided helpful comments that improved the manuscript. We thank the Commonwealth of the Bahamas for permission to conduct research, and NSF and UCLA for financial support.

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Correspondence to Ryan Calsbeek.

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Calsbeek, R., Smith, T. Ocean currents mediate evolution in island lizards. Nature 426, 552–555 (2003).

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