Funk, D. J. Isolating a role for natural selection in speciation: Host adaptation and sexual isolation in Neochlamisus bebbianae leaf beetles. Evolution 52, 1744–1759 (1998).

The molecular genetic basis of a plumage colour polymorphism in the Gouldian finch (Erythrura gouldiae).

Word History

Turner, T. L., Hahn, M. W. & Nuzhdin, S. V. Genomic islands of speciation in Anopheles gambiae. PLoS Biology 3, e285 (2005). doi:10.1371/journal.pbio.0030285

Evolutionary genetics of the phenotypic response to environmental change in the North American red squirrel

While drift may not commonly result in speciation unilaterally, Templeton (2008) argues that it is erroneous to consider speciation a binary drift/selection process. Rather, drift and selection could work simultaneously and/or interact during divergence. A potential example involves the role of chromosomal rearrangements in reproductive isolation. Chromosomal rearrangements, such as inversions, can have an impact on divergence by creating linkage groups of loci involved in multiple forms of reproductive isolation that cannot be disrupted by recombination ( Noor et al. 2001; Rieseberg 2001). Empirical evidence suggests that such chromosomal inversions may contribute to the maintenance of species boundaries despite interspecific gene flow (e.g., Brown et al. 2004). Chromosomal inversions may sometimes be fixed by genetic drift, but the loci within the inversions may be subject to selection. Reproductive isolation could therefore be a product of both the adaptive loci within the inversion and the inversion itself, resulting in speciation that cannot be unambiguously defined as either ecological or nonecological. SEXUAL SELECTION AND SEXUAL CONFLICT Inland and coastal forms of M. guttatus experience selection for different growth and flowering times resulting in flowering phenology with little overlap.

Moreover, the suggestion that the ecological speciation perspective affords investigators a new opportunity to study the mechanisms of reproductive isolation fails to recognize that Dobzhansky (1937) and Mayr (1942) had already established a complete inventory of reproductive isolating barriers more than a half century ago. Given this historical appreciation of ecological factors in speciation, the new perspective serves to direct attention to how the presence and strength of divergent selection affects reproductive isolation, but provides little new insight into how ecological versus non-ecological mechanisms are involved. By these contrasting perspectives, allopatric taxa that differ in habitat may or may not be different species under the BSC. Mayr's “single most important event in evolution” has either undoubtedly happened, or is yet to occur. As originally proposed, the BSC defines species as “… groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups” ( Mayr 1942). The word “potentially” accounts for species whose geographic distribution prevents complete assessment of isolation, such as the case of allopatric populations. Mayr himself clearly struggled with the problem of allopatry, removing the word “potentially” from later versions of the definition ( Mayr 1984). We suggest that Mayr's original formulation of the BSC is the correct one, and that the word “potential” allows for the assessment of reproductive isolation even in allopatric taxa. SEPARATING ECOLOGICAL FROM HISTORICAL FACTORS To resolve this paradox, we propose that differences in geographic distribution caused by adaptation to different habitats be treated like any other form of reproductive isolation, as a quantitative estimate of how much gene flow is reduced by intrinsic differences between taxa. To distinguish the independent contributions of genetic and historical processes, we separate geographic factors into ecogeographic isolation and effective geographic isolation. We define “ecogeographic isolation” as the degree to which differences in geographic distribution are based on genetic differences between taxa. “Effective geographic isolation” is the actual spatial separation experienced by populations, and includes both ecogeographic isolation and differences in distribution based solely on historical factors. Therefore, effective geographic isolation will sometimes be greater than ecogeographic isolation between populations such as the case of populations separated solely by geologic features. However, only ecogeographic isolation is relevant to speciation under the BSC, because it represents the portion of effective geographic isolation due to genetic differences between populations. This distinction sets up the possibility of discordance between status assigned under the BSC and other species concepts, as populations may diverge morphologically and/or phylogentically in allopatry without complete reproductive isolation. Ecological niche modeling and reciprocal transplant data could be combined to take advantage of the strengths of both approaches. If reciprocal transplants were performed across a wide range of climatic conditions, it would be possible to build an ecological niche model using fitness of the transplants in place of presence/absence data. Projecting this “transplant niche model” onto the geographic landscape could then be an excellent tool for measuring the overlap in ecogeographic isolation. Because these models would be based upon the fitness of organisms, they would be a more reliable predictor of adaptation to habitat. Future studies that compare results from the two methods would help determine if niche modeling is an appropriate proxy for transplant studies. Assessing the “Importance” of Reproductive Barriers

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