Studies on allozyme variation in eastern plethodontid salamanders of the woodland genus Plethodon have revealed a large number of cryptic species.Genetic variation within and among species reveals patterns of speciation. In the late Miocene and early Pliocene there appear to have been only five clades of eastern North American Plethodon that still survive today. A burst of speciation took place in the Pliocene, so there are now at least 35 species in four species groups in eastern North America. It is hypothesized that the arid climates of the Pliocene isolated many populations in wetter forested moun tainous areas where in isolation allopatric speciation occurred. With the return of wetter climates, many of these species dispersed and a large number of parapatric contact zones may now be studied where closely related species are interacting. Hybridization between closely related species is frequent in these contact zones.Annu. Rev. Ecol. Syst. 1995.26:579-600. Downloaded from www.annualreviews.org by Duke University on 10/07/12. For personal use only.Quick links to online content Further ANNUAL REVIEWS
Functionally equivalent genes may evolve heterogeneously across closely related taxa as a consequence of lineage-specific selective pressures. Such disparate evolutionary modes are especially prevalent in genes that encode postcopulatory reproductive proteins, presumably as a result of sexual selection. We might therefore expect genes that mediate reproduction prior to insemination to evolve in a similar manner. Plethodontid receptivity factor (PRF), a proteinaceous salamander pheromone produced by the male, increases female receptivity during courtship interactions. To test for lineage-specific differences in PRF's evolution, we intensively sampled PRF genes across the eastern Plethodon phylogeny (27 spp.; 34 populations) to compare gene diversification, rates of evolution, modes of selection, and types of amino acid substitution. Our analyses indicate that PRF evolutionary dynamics vary considerably from lineage to lineage. Underlying this heterogeneity, however, are two well-defined transitions in evolutionary mode. The first mode is representative of a typical protein profile, wherein neutral divergence and purifying selection are the dominant features. The second mode is characterized by incessant, cyclical evolution driven by positive selection. In this mode, the positively selected sites are bound by a limited assortment of acceptable amino acids that appear to evolve independently of other sites, resulting in a tremendous number of unique PRF alleles. Several of these selected sites are implicated in receptor binding. These sites are apparently involved in a molecular tango in which the male signal and female receptors coevolve within a confined molecular space. PRF's lineage-specific evolutionary dynamics, in combination with evidence of a molecular tango, highlight the molecular action of sexual selection on a chemical signal that is used during courtship.
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