The Genomics of Speciation and Adaptation
Speciation is the process responsible for generating biodiversity. During the speciation process, populations become increasingly isolated from one another until eventually becoming distinct species. As populations diverge profound changes occur throughout their genomes. Those regions of the genome responsible for adaptive divergence and/or reproductive isolation often show the greatest changes and are the key to a gaining a greater understanding of how biodiversity arises.
Genomic regions that diverge early in the speciation process have been coined “genomic islands of speciation”. Currently there is a surge of interest in understanding how small or large these islands are, where they are distributed throughout the genome, what type of genetic changes they contain, and what roles they play in adaptation and speciation. Models of how selection and genetic hitchhiking drive the accumulation of these islands and their expansion into continents are just beginning to emerge. However, the empirical foundation for these models remains shaky. There have been a limited number of studies of genome wide patterns of divergence, such as those in mosquito, apple maggot fly, Drosophila, house mouseand sticklebacks. These studies have been highly influential and challenged our understanding of how genomes diverge, but only provide single snapshots of genomic divergence during the speciation process and patterns of divergence in these studies are highly variable. At this time, there is no clear consensus on the expected number, size and distribution of these genomic islands at different stages in the speciation process.
Characterizing genomic divergence along an evolutionary continuum of hybridizing taxa promises the most powerful insights into the complex nature of the evolving species boundary. Divergent, hybridizing taxa provide the most direct insights into the proximal causes of speciation, because, due to ongoing gene flow, often the only genomic differences maintained are those near the changes ultimately causing speciation. In this regard, hybrid zones provide powerful natural laboratories to study genomic islands of speciation. These islands of divergence between hybridizing taxa not only contain the targets of selection, but also blocks of neighboring regions that hitchhike along, both of which may create barriers of isolation. The accumulation of these islands and their expansion is expected to coincide with the accumulation of isolation barriers between the hybridizing taxa, but the actual relationship between these remains unexplored. Just as classic studies of this relationship using only handfuls of genetic markers yielded discoveries that still serve as the foundation for the genetics of speciation, studies of genomestudies of genome wide patterns of divergence across the speciation continuum promise a new level of understanding of how species are formed