Microgeographic adaptation in a broadcast-spawning marine invertebrate
Gurski, Lauren Marie
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The spatial scale over which adaptive evolution occurs may be much smaller than originally predicted, affecting our understanding of the maintenance of genetic diversity and adaptive capacity. Microgeographic adaptation, where patterns of genetic adaptation develop within Wright’s dispersal neighborhood, is strongly opposed by gene flow and has been considered to be rare. Here, we test whether selection can overcome migration and drive microgeographic adaptation at a very small spatial scale (6-7 orders of magnitude less than the dispersal neighborhood) in a dispersive, broadcast-spawning, intertidal patellogastropod (Cellana exarata, ‘opihi) using molecular techniques. Adults were collected from exposed rock surfaces and sheltered crevices separated by cm-m in three semi-isolated populations on different islands. Consistent with the hypothesis that selective pressures vary between the microhabitats, mean shell size was smaller on exposed surfaces than crevices. 16,387 restriction site associated DNA sequence loci were ascertained with PCR-free ezRAD in 20-39 ‘opihi per microhabitat on each island. Consistent with a pattern of microgeographic adaptation, 126 candidate loci (1 transcribed gene) exhibited a parallel pattern of differentiation between microhabitats on all three islands (F=0.03 – 0.11). While an assessment of reproductive state indicates that spawning was more likely to be delayed in crevices, the reproductive state and DNA data indicate that there is substantial opportunity for, and evidence of, gene flow between the microhabitats. There are two explanations for these results which are not mutually exclusive. (1) Individuals seek advantageous microhabitats given their particular genetic composition. (2) Individuals with disadvantageous composite genotypes in a microhabitat are more likely to perish. Given the extreme scenario tested here, microgeographic adaptation is likely to be much more common than originally anticipated.