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  E-mail Message: I thought you might be interested in this item at http://www.worldcat.org/oclc/111264775 Title: A test of alternative hypotheses for the evolution of reproductive isolation between spadefoot toads: support for the reinforcement hypothesis. Publisher: ISBN/ISSN: 0014-3820 OCLC:111264775

  
  

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A test of alternative hypotheses for the evolution of reproductive isolation between spadefoot toads: support for the reinforcement hypothesis.

Author:	KS Pfennig
Edition/Format:	Article : English
Publication:	Evolution; international journal of organic evolution, 2003 Dec; 57(12): 2842-51
Database:	From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
Other Databases:	British Library Serials
Summary:	How do species that interbreed become reproductively isolated? If hybrids are less fit than parental types, natural selection should promote reproductive isolation by favoring the evolution of premating mechanisms that prevent hybridization (a process termed reinforcement). Although reinforcement should generate a decline in hybridization over time, countervailing forces of gene flow and recombination are thought to preclude natural selection from enhancing and finalizing reproductive isolation. Here, I present recent estimates of hybridization frequency between two species of spadefoot toad, Spea multiplicata and S. bombifrons. I compare these recent measures of hybrid frequency with previously published estimates and show that hybridization between these species has declined precipitously over the past 27 years. Although previous studies suggest that reinforcement possibly accounts for this decline in hybrids over time, three alternative hypotheses also can explain the observed decrease in hybridization. First, if one of the two interacting species becomes rare, opportunities for and incidence of hybridization may decrease. Second, if one of the two interacting species is initially rare, hybridization may be initially common if the rare species has difficulty locating conspecific mates. Third, if hybrids are produced only in particular environments, hybrid frequency may decline if habitat changes result in loss of those environments that promote hybrid formation. I found no support for these three alternative explanations of the decline in hybrids. Instead, reinforcement appears to best account for the evolution of enhanced reproductive isolation between these species. Moreover, the finding that hybridization declined precipitously in only 27 years suggests that many systems that have undergone reinforcement may be overlooked because reproductive isolation between the interacting populations or species may already be complete.  Read more...
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