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STRONG SELECTION AGAINST HYBRIDS AT A HYBRID ZONE IN THE ENSATINA RING SPECIES COMPLEX AND ITS EVOLUTINARY IMPLICATIONS
Author(s) -
Alexandrino João,
Baird Stuart J. E.,
Lawson Lucinda,
Macey J. Robert,
Moritz Craig,
Wake David B.
Publication year - 2005
Publication title -
evolution
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.84
H-Index - 199
eISSN - 1558-5646
pISSN - 0014-3820
DOI - 10.1111/j.0014-3820.2005.tb01783.x
Subject(s) - cline (biology) , biology , linkage disequilibrium , hybrid zone , evolutionary biology , transect , hybrid , introgression , genetics , biological dispersal , selection (genetic algorithm) , allele , ecology , haplotype , genetic variation , population , gene flow , botany , artificial intelligence , computer science , gene , demography , sociology
Abstract The analysis of interactions between lineages at varying levels of genetic divergence can provide into the process of speiation through the accumulation of incompatiable mutations. Ring species, and especially the Ensatina eschscholtzii system exemplify this approach. The plethodontid salamanders E. eschscholtzii xanthoptica and E. eschscholtzii platensis hybridize in the central Sierran foothills of California. We compared the genetic structure across two transects (southern and northern Calaveras Co.), one of which was resampled over 20 years, and examined diagnostic molecular markers (eight allozyme loci and mitochondrial DNA) and a diagnostic quantitative trait (color pattern). Key results across all studies were: (1) cline centers for all markers were coincident and the zones were narrow, with width estimates of 730 m to 2000 m; (2) cline centers at the northern Calveras transect were coincident between 1981 and 2001, demonstrating repeatability over five generations; (3) there were very few if any putative F 1 S , but a relatively high number of backcrossed individuals in the central portion of transects; and (4) we found substantial linkage disequilibrium in all three studies and strong heterozygote deficit both in northern Calaveras, in 2001, and southern Calaveras. Both linkage disequilibrium and heterozygote deficit showed maximum values near the center of the zones. Using estimates of cline width and dispersal, we infer strong selection against hybrids. This is sufficient to promote accumulation of differences at loci that are neutral or under divergent selection, but would still allow for introgression of adaptive alleles. The evidence for strong but incomplete isolation across this centrally located contact is consistent with theory suggesting a gradual increase in postzygotic incompatibility between allopatric populations subject to divergent selection and reinforces the value of Ensatina as a system for the study of divergence and speciation at multiple stages.