Fine-scale genetic structure due to adaptive divergence among microhabitats

It has been suggested that adaptive evolution on ecological timescales shapes communities. However, adaptation among environments relies on isolation or large selection coefficients that exceed migration effects. This reliance is tempered if adaptation is polygenic-does not depend on one allele completely replacing another but instead requires small allele frequency changes at many loci. Thus, whether individuals can evolve adaptation to fine-scale habitat variation (for example, microhabitats) is not resolved. Here we analyze the genetic divergence of the teleost fish, Fundulus heteroclitus, among microhabitats that are <200 m apart in three separate saltmarshes using 4741 single-nucleotide polymorphisms (SNPs). Among these SNPs, 1.3-2.3% have large and highly significant differences among microhabitats (mean F-ST=0.15; false discovery rate <= 1%). The divergence among microhabitats for these outlier SNPs is larger than that among populations, exceeds neutral expectation and indicates surprising population structure among microhabitats. Thus, we suggest that polygenic selection is surprisingly effective in altering allele frequencies among many different SNPs that share similar biological functions in response to environmental and ecological differences over very small geographic distances. We acknowledge the evolutionary difficulty of large genetic divergence among well-connected habitats. Therefore, these studies are only the first step to discern whether natural selection is responsible and capable of effecting genetic divergence on such a fine scale.