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Hybridization and Speciation in Fossorial Mole Rats

Eviatar Nevo and Hana Bar-El
Evolution
Vol. 30, No. 4 (Dec., 1976), pp. 831-840
DOI: 10.2307/2407822
Stable URL: http://www.jstor.org/stable/2407822
Page Count: 10
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Hybridization and Speciation in Fossorial Mole Rats
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Abstract

Natural hybridization between four pairs of karyotypes of mole rats, Spalax ehrenbergi, was analyzed to test reproductive isolation and the stages of speciation of the karyotypes involved. Hybrid zones decrease progressively in width (from 2.800 km through 0.725 km to 0.320 km), regressing negatively and linearly on the chromosomal differences between the respective parental karyotypes (2n = 58-60; 54-58; and 52-58). The expected Hardy-Weinberg proportions are realized in the first hybrid zone but not in the other two zones. Both the narrowness of the zones and the lower than expected proportion of hybrids in the 54-58 and in the 52-58 hybrid zones, are taken as evidence of selection against hybrids. Dispersal of hybrids into parental territories is restricted presumably by either cytogenetic, ethologic, and/or ecological incompatibilities. Though hybrids are at least partly fertile, their overall fitness appears lower than that of the parental types. The nature and extent of hybridization in Spalax ehrenbergi reinforces the hypothesis that the chromosome forms are indeed young, closely related, sibling species at early stages of evolutionary divergence. The completion of reproductive isolation probably occurs currently in or near hybrid zones where hybrids are eliminated by natural selection, species identification is perfected, and a progressive terminalization of species formation is under way.

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