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1 Natural Selection inAction During Speciation--Sara Via
Pages 5-26

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From page 5...
... Under divergent selection in sympatry, the genomes of incipient species become temporary genetic mosaics in which ecologically important genomic regions resist gene exchange, even as gene flow continues over most of the genome. Analysis of such mosaic genomes suggests that surprisingly large genomic regions around divergently selected quantitative trait loci can be protected from interrace recombina tion by "divergence hitchhiking." Here, I describe the formation of the genetic mosaic during early ecological speciation, consider Departments of Biology and entomology, University of Maryland, College Park, MD 20742.
From page 6...
... Mayr also stressed that the evolution of reproductive isolation is a fragile process that can only proceed if geographical separation renders gene flow impossible, firmly establishing allopatric speciation as the norm. Theodosius Dobzhansky (1937)
From page 7...
... . By analyzing partially reproductively isolated ecotypes or races, the genetic changes contributing to reproductive isolation can be studied before they become confounded by additional genetic differences between species that accumulate after speciation is complete.
From page 8...
... . others disregard ecologically based reproductive isolation between ecotypes because it lacks permanence and could be reversed if the pattern of divergent selection changes (Coyne and orr, 2004)
From page 9...
... . When gene exchange is physically impossible, the conditions under which reproductive isolation can evolve are nonrestrictive: allopatric speciation can be driven by strong or weak divergent selection, sexual selection, uniform selection, or even stabilizing selection.
From page 10...
... it is a maxim of population genetics that migration and genetic drift affect the entire genome, whereas the effects of natural selection are limited to genomic regions harboring loci that affect the selected phenotypic traits. in ecologically specialized populations, divergent selection on traits associated with the use of resources or habitats is strong enough to maintain divergence in the parts of the genome that affect those traits, while gene flow continues in other genomic regions (Figs.
From page 11...
... grossly overestimate the realized gene exchange experienced by divergently selected genomic regions. Because of the localized genomic effects of divergent selection on realized gene exchange, divergence early in ecological speciation with gene flow is expected to be greater in genomic regions that harbor key quantitative trait loci (QTl)
From page 12...
... , but it is now of primary interest as a signature of divergent selection that permits detection of genomic regions involved in adaptive divergence (Beaumont, 2005; storz, 2005)
From page 13...
... . however, conditions of the initial split are far less relevant to the study of speciation than is the fact that divergent selection currently maintains genetically based phenotypic differentiation and significant ecologically based reproductive isolation between sympatric populations.
From page 14...
... . surprisingly, the spatial distribution of the mapped Fst outliers suggests that the signature left by divergent phenotypic selection on neutral markers can extend far from major QTl: the average outlier was 10.6 cM from the nearest QTl.
From page 15...
... in other words, the "effective map distance" between a locally adapted QTl and a nearby marker is much less than that estimated from a linkage map made using controlled matings. We can empirically approximate the effective marker-QTl distance in pea aphids by estimating the extent to which habitat choice and divergent selection limit the opportunity for interrace mating and recombination (e.g., Fig.
From page 16...
... Although smaller than the nominal map distance of 10.6 cM, it is far from the tight effective linkage seen at present. The size of each region of divergence hitchhiking therefore depends not only on the strength of divergent selection directly on that genomic region, but also on the extent to which effective migration is reduced by the earlier divergence of other QTl alleles throughout the genome.
From page 17...
... found that QTl for different traits under divergent selection in the pea aphid host races tended to colocalize on the linkage map. Colocalization of QTl increases selection experienced by that genomic region, thereby increasing the size of the region of divergence hitchhiking, and facilitating both QTl detection and the accumulation of additional QTl.
From page 18...
... in this view, outliers are thought to be either under direct divergent selection, or tightly linked to a selected gene.
From page 19...
... This establishes the commonly observed pattern of genomic heterogeneity in divergence between incipient species, which we call the genetic mosaic. As divergent selection proceeds, ecologically based reproductive isolation increases because of resource-based selection against migrants and hybrids and the evolution of habitat choice.
From page 20...
... . otherwise, genes diverge over a long time period under any combination of divergent selection, independent responses to uniform or balancing selection, or drift, eventually all coming into concordance to produce the branching pattern of the new species.
From page 21...
... The truly neutral markers in other genomic regions will still be useful, however, for analyses of demographic events such as bottlenecks or range expansion. Preferentially using markers affected by divergent selection for phylogeographic or phylogenetic analysis of populations conflicts with the clear preference for neutral markers in species-level phylogenetics.
From page 22...
... The incipient species are now essentially "ecologically allopatric." Thus begins stage 2, in which the parts of the genome outside regions of divergence hitchhiking begin to differentiate by genetic drift or independent responses to selection within the new lineages. This secondary divergence will eventually bring all of the variation in polymorphic gene trees into widespread phylogenetic concordance with the branching pattern determined earlier by divergent selection on QTl affecting the key ecologically important traits.
From page 23...
... in contrast, in ecological speciation with gene flow, various ecologically based barriers to gene flow evolve first, as a result of adaptation under divergent selection. once migration between the incipient species is essentially eliminated, they become ecologically allopatric, and postzygotic incompatibilities can accumulate.
From page 24...
... A high Fst marker seen late in speciation or in a hybrid zone is more likely to be the result of genetic drift or a recent selective sweep within 1 species than it is to be a signature of the divergent selection that caused speciation. Therefore, outliers found between new species or in hybrid zones, or between incipient species in secondary contact after a period of allopatric divergence, should not necessarily be expected to mark genomic regions affected by divergent phenotypic selection during the initial phases of ecological speciation.
From page 25...
... There is essentially only 1 path for purely sympatric speciation: rapid divergence at genomic regions harboring QTl for traits under divergent selection, leading to significant ecologically based reduction of successful interbreeding between incipient species and ecological allopatry by the end of stage 1. Then, during stage 2, genetic incompatibilities can accumulate to reinforce the ecologically based isolation and make it permanent.
From page 26...
... however, population-level analysis of the ecological and genetic mechanisms that produce reproductive isolation between partially isolated ecotypes or races (the magnifying glass) can provide a very different perspective on the problem of speciation.


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