complement traditional approaches and contribute to a better mechanistic understanding of how new species arise.
In Chapter 3, Dolph Schluter and Gina Conte emphasize a theme—ecological speciation—that would please Darwin. Under ecological speciation, reproductive isolation between populations emerges from the effects of ecology-based divergent natural selection. The authors address this speciation mode generally (with respect to the genetics of postzygotic isolation and prezygotic isolation under gene flow, and the role of standing genetic variation in the process) as well as specifically (with reference to speciation in stickleback fishes). For the sticklebacks, they develop an interesting “transporter model” of ecological speciation in which ecological selection pressures in freshwater streams consistently select for alleles different from those normally present in marine populations. However, occasional hybridization between freshwater and marine forms ensures a continual supply of freshwater alleles in the sea, at low frequency and disassembled by genetic recombination. When marine fish colonize a newly opened stream, natural selection can act on this standing pool of genetic variation to reconstitute the freshwater genotype. The analogy in the title of their model is to a fictional process in the movie Star Trek, wherein an organic body placed in the transporter is disintegrated only to be reassembled at a future time in a distant location.
The vast majority of phylogenetic diversity in eukaryotes is to be found not in the lineages of multicellular plants and animals, but rather in unicellular microbes (protists). Perhaps it is not surprising, therefore, that these microeukaryotes provide a wealth of opportunities (heretofore relatively untapped) for scientific investigations into natural selection and evolutionary operations. In Chapter 4, Julius Lukeš, Brian Leander, and Patrick Keeling exemplify the utility of protists for providing evolutionary insights by summarizing numerous phenotypic as well as genomic features in representatives of two huge protistan phylads: Alveolata and Euglenozoa. They underscore the mind-boggling diversity in protists of molecular genetic as well as phenotypic features, ranging from cellular ultrastructures to mechanisms of mRNA processing and the organization of organellar genomes. The picture that emerges is one of extraordinary evolutionary experimentation in these protists, sometimes channeled into convergent outcomes by natural selection, sometimes constrained by the idiosyncrasies of phylogeny, but always tinkered endlessly by various mixes of both chance and necessity.