Ecological population genomics in the marine environment.

Marine species live in a wide diversity of environments and yet, because of their pelagic life stages, are thought to be well-connected: they have high migration rates that inhibit significant population structure. Recent innovations in sequencing technologies now provide information on nucleotide polymorphisms at thousands to tens of thousands of loci based on whole genomes, reduced representative portions of genomes (0.1-1%) or a majority of expressed mRNAs. Data from these genomic approaches are used to define and quantify single-nucleotide polymorphisms (SNPs). These SNP data tend to agree with data from older technologies (allozymes or microsatellites), which support well-connected populations with few genetic differences among populations. However, these studies also find few percentages of SNPs (1-5%) that readily distinguish genetic differences among populations on relatively small geographic scales. The magnitudes of the genetic differences (FST values) suggest that hundreds of loci with significant differences are due to positive selective pressures. Thus, these data suggest that natural selection is effectively altering allele frequencies at 100s of loci in marine populations. In this manuscript, we provide examples of these studies, the strengths and weaknesses of different genomic approaches as well as important technical aspects associated with genomic approaches.