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the rate of evolution of gamete recognition proteins. The animation method used by Mayr is generally supported by molecular phylogenies. However, the existence of multiple rates in the acquisition of reproductive isolation complicates placement of different genera in an evolutionary series.

Ernst Mayr built an argument for the way speciation occurred based on the geographic patterns of variation among closely related species (Mayr, 1942, 1963). He showed that there was a hierarchy of species descriptions that could be ordered in a series of increasing complexity. Some descriptions pertained to recently diverged species, with morphologically identical populations inhabiting a continuous range. Other descriptions were of polytypic species, those with slightly differentiated populations inhabiting different parts of the range. Further along the speciation axis were superspecies, taxa with morphologically distinct, allopatric populations. Still later in the series, Mayr identified groups of related species in which some taxa were sympatric. The trajectory from homogeneous populations to overlapping sympatric species encompassed Mayr’s view of the process and pacing of geographic speciation. In addition to describing these separate elements, a major contribution by Mayr was to order these elements in a series. The elements thus served as separate frames in an evolutionary animation that sped up the slow process of speciation so that it could be viewed and studied by biologists.

The geographic distributions of species, subspecies, varieties, and slightly divergent populations constituted the database in Mayr’s analyses. He made the implicit assumption that the genetic and evolutionary divergence of these groups increased from population- to species-level distinction and used morphological differentiation as a proxy for evolutionary time. Mayr established sister-species relationships on the basis of morphological similarity and included a tacit phylogenetic framework for his animations based on the best information available at the time.

One difficulty faced by Mayr was that few concrete phylogenetic analyses were available during the development of these ideas. Since that era, molecular phylogenies have made it possible to obtain a statistically robust view of phylogenetic relationships, divergence order, and sister-species status (Hillis et al., 1996). Molecular phylogenies can also give an indication of the timing of divergence events through the application of molecular clock calibrations. Even without precise time calibrations, the record of the order of divergence of taxa permits an examination of the causes of each splitting event. Lastly, phylogenies can provide objective data on divergence levels to test predictions of Mayr’s evolutionary reconstructions.



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