Systematics and the Origin of Species On Ernst Mayr's 100th Anniversary (2005) / Chapter Skim
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Part I--THE ORIGINS OF SPECIES BARRIERS: 2 The Genetic Basis of Reproductive Isolation: Insights from Drosophila--H. ALLEN ORR
Part I--THE ORIGINS OF SPECIES BARRIERS: 2 The Genetic Basis of Reproductive Isolation: Insights from Drosophila--H. ALLEN ORR
Pages 5-23
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From page 5... ...
This question is addressed by Allen Orr in "The Genetic Basis of Reproductive Isolation: Insights from Drosophila" (Chapter 2) , and he explains what we know from the growing handful of cases in which the actual genes that contribute to low hybrid fitness have been isolated.
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From page 6... ...
. Not even populations with many rapidly evolving genes can be expected to become reproductively isolated from other populations if gene flow rates are high.
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From page 7... ...
(1936) Studies on hybrid sterility.
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From page 9... ...
More important, speciation genes are typically very rap idly evolving, and this divergence is often driven by positive Dar winian selection. Finally, I review recent work in Drosophila pseu doobscura on the possible role of meiotic drive in the evolution of the genes that cause postzygotic isolation.
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From page 10... ...
and later in Animal Species and Evolution, was that genetic drift plays a critical role in speciation. According to Mayr, large populations suffer a sort of evolutionary inertia: the conservative forces of gene flow and epistasis prevent, or at least render unlikely, the evolution of novel morphologies or new coadapted gene complexes in large, geographically widespread species.
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From page 11... ...
The consequence of this methodological dilemma is that evolutionists have been unable to address a large set of fundamental questions about the genes that cause reproductive isolation, so-called speciation genes. (This perhaps unfortunate term, which is now entrenched in the literature, refers to any locus that causes reproductive isolation, whether in F1 or later-generation hybrids, and whether the gene was among the first to cause isolation or not.)
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From page 12... ...
And do speciation genes diverge by natural selection or genetic drift? This last question is perhaps the most important: if we can identify speciation genes at the level of DNA sequences, we should be able to bring to bear a powerful set of molecular population genetic tools (e.g., McDonaldKreitman and HudsonKreitmanAguade tests)
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From page 13... ...
There is little reason to believe, however, that postzygotic isolation usually involves malignancies. The recent renaissance in the genetics of speciation has therefore featured a determined effort to find additional speciation genes regardless of the form of hybrid inviability or sterility involved.
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From page 14... ...
melanogaster. The essence of this approach is that it allows detection and mapping of recessive, and thus, normally masked, speciation genes: if a chromosomal region includes a recessive hybrid inviability gene(s)
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From page 15... ...
. In very recent work, my laboratory has turned to the attempt to identify and characterize speciation genes that cause hybrid sterility, not inviability.
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From page 16... ...
melanogaster genome. My laboratory has recently further narrowed the location of this putative hybrid sterility gene and has begun to test the few remaining candidate loci in the region for their possible role in hybrid sterility (J.
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From page 17... ...
First, speciation genes are rapidly evolving. Second, these genes often evolve by positive Darwinian selection (Barbash et al., 2003; Presgraves et al., 2003)
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From page 18... ...
Although we do not yet know the proximate mechanism of segregation distortion (e.g., classical meiotic drive in the male germ line, immotility of Y-bearing sperm in the female reproductive tract, postfertilization failure of pronuclei fusion, etc.) , we have completed a preliminary genetic analysis of this hybrid segregation distortion.
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From page 19... ...
. More suggestive still, the severity of hybrid sterility and the extent of offspring sex ratio distortion are strongly correlated across individual backcross hybrid males (Fig.
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From page 20... ...
CONCLUSIONS Molecular evolutionary analyses of speciation genes show that these loci are rapidly evolving, and that this evolution is often driven by positive Darwinian selection. Although the sample of genes characterized thus far by various laboratories remains small, and concentrated in the genus Drosophila, I suspect that these patterns may prove general, although likely not universal.
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From page 21... ...
(2000) The Drosophila melanogaster Hybrid male rescue gene causes inviability in male and female species hybrids.
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(1992) An empirical test of the meiotic drive models of hybrid sterility: Sex ratio data from hybrids between Drosophila simulans and Drosophila sechellia.
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From page 23... ...
(1998) A rapidly evolving homeobox at the site of a hybrid sterility gene.
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