In the Light of Evolution Volume III: Two Centuries of Darwin (2009) / Chapter Skim
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3 Genetics and Ecological Speciation--Dolph Schluter and Gina L. Conte
3 Genetics and Ecological Speciation--Dolph Schluter and Gina L. Conte
Pages 47-64
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From page 47... ...
We propose that ecological speciation has occurred multiple times in parallel in this group via a "transporter" process in which selection in freshwater environments repeatedly acts on stand ing genetic variation that is maintained in marine populations by export of freshwater-adapted alleles from elsewhere in the range. Selection from standing genetic variation is likely to play a large role in ecological speciation, which may partly account for its rapidity.
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From page 48... ...
. The first of these is ecological speciation, defined as the evolution of reproductive isolation between populations, or subsets of a single population, as a result of ecologically based divergent natural selection (schluter, 2000, 2001; rundle and nosil, 2005; Funk, 2009)
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From page 49... ...
here, we review several aspects of the problem. We address characteristics of genes underlying premating isolation; the evolution and genetics of postzygotic isolation; and the role of standing genetic variation as a source of alleles for the evolution of reproductive isolation.
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From page 50... ...
Felsenstein (1981) pointed out that in the absence of very strong reproductive isolation between hybridizing populations, recombination between genes governing assortative mating and genes under divergent natural selection will cause existing levels of assortative mating between populations to decay and inhibit the evolution of even stronger assortative mating.
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From page 51... ...
Without more evidence on the mechanism of selection we are unable to determine which, if any, of the genes recently discovered to underlie intrinsic postzygotic isolation in Drosophila, yeast, and mice (Coyne and orr, 2004; Brideau et al., 2006; lee et al., 2008; Mihola et al., 2009; Tang and Presgraves, 2009) fixed as a result of ecologically based divergent natural selection.
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From page 52... ...
. Thus, as populations adapt to different environments, the fitness of hybrids between them evolves below that predicted from the hybrid's phenotype for the trait under divergent selection, the amount depending on genetic details such as y optimum 1 optimum 2 x Ancestor Environment 1 Environment 2 FiGUre 3.2 A model for the buildup of postzygotic isolation between 2 popula tions descended from a common ancestor adapting to distinct ecological environments, after Barton (2001)
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From page 53... ...
ECOLOGICAL SPECIATION FROM STANDING GENETIC VARIATION speciation occurs from standing genetic variation when reproductive isolation between 2 or more populations evolves from alleles already present within the common ancestral population, rather than from new mutations. Theory to describe the buildup of Dobzhansky-Muller incompatibilities often explicitly assumes that speciation occurs from new mutations [e.g., Barton (2001)
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From page 54... ...
. standing genetic variation can result in the same alleles being used repeatedly in separate speciation events, enhancing the probability of parallel evolution of reproductive isolation.
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From page 55... ...
The marine species or ecotype is the ancestral form to all freshwater populations. Most marine populations are anadromous, migrating to streams to breed and returning to sea afterward.
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From page 56... ...
3.4A) is that each freshwater population was formed by individuals immigrating directly from other freshwater populations nearby, via the sea, and that subsequent gene flow between adjacent marine and stream populations erased the signature of freshwater-to-freshwater colonization at neutral marker loci.
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From page 57... ...
Genetics and Ecological Speciation / Germany A Iceland Scotland Norway Iceland Massachusetts Massachusetts Germany Nova Scotia Japan Japan Alaska British Columbia California California British Columbia British Columbia Washington Alaska SEA OF JAPAN Marine Iceland Norway Freshwater B Massachusetts Germany Scotland British Columbia British Columbia Alaska Alaska British Columbia Washington California California ~10 mya California Japan Massachusetts Germany Iceland Nova Scotia ~2 mya Alaska California British Columbia Japan British Columbia SEA OF JAPAN G wheatlandii C "high" British Columbia Kitlg British Columbia allele British Columbia "low" Washington Kitlg allele California
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From page 58... ...
Possibly, strong selection maintaining phenotypic differences between adjacent marine and stream populations negatively impacts the fitness of hybrids, which would also contribute to postzygotic isolation. These considerations justify looking to genes causing phenotypic differences between marine and freshwater populations as a first step toward understanding the genetics of reproductive isolation in this group.
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From page 59... ...
Given these findings for Eda and Kitlg, it would not be far-fetched to suppose that the same process of repeated selection from standing genetic variation has occurred at many loci. selection from standing variation provides a ready explanation for rapid parallel evolution in so many phenotypic traits that characterizes this group, including those affecting reproductive isolation.
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From page 60... ...
The hypothesis does not address where or when an advantageous mutation arose, but proposes that once a mutation reaches appreciable frequency in a freshwater population it could participate in the transporter process. A key assumption of this transporter hypothesis is that existing freshwater populations are the source of standing genetic variation in the marine ancestral population.
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From page 61... ...
Genetics and Ecological Speciation / 1 x 2 3 4
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From page 62... ...
instead, we attribute the similarity of Eda alleles between nearby populations to the transport of low alleles from one population to the other by a process that included an intermediate stage during which alleles exported from freshwater were present as standing variation in the marine population. such a transport process may continue today in moving allele copies between freshwater populations that still hybridize with marine populations.
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From page 63... ...
freshwater colonization, then they should show a similar history to that of neutral markers. however, under the transporter process, genes underlying reproductive isolation in freshwater populations were positively selected from standing variation maintained in the sea, in which case the genes should be most closely related to gene copies found in other freshwater populations nearby.
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From page 64... ...
other genetic processes during ecological speciation become apparent when there is gene flow between populations, either continuously during their divergence or after secondary contact. With significant gene flow, premating isolation is unlikely to arise or persist unless the genes underlying it somehow overcome the antagonism between selection and recombination, such as by pleiotropy or by reduced recombination with genes under divergent natural selection.
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