Article

Evidence for Ecological Speciation and Its Alternative

Biodiversity Research Centre and Zoology Department, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.
Science (Impact Factor: 31.48). 03/2009; 323(5915):737-41. DOI: 10.1126/science.1160006
Source: PubMed

ABSTRACT Natural selection commonly drives the origin of species, as Darwin initially claimed. Mechanisms of speciation by selection fall into two broad categories: ecological and mutation-order. Under ecological speciation, divergence is driven by divergent natural selection between environments, whereas under mutation-order speciation, divergence occurs when different mutations arise and are fixed in separate populations adapting to similar selection pressures. Tests of parallel evolution of reproductive isolation, trait-based assortative mating, and reproductive isolation by active selection have demonstrated that ecological speciation is a common means by which new species arise. Evidence for mutation-order speciation by natural selection is more limited and has been best documented by instances of reproductive isolation resulting from intragenomic conflict. However, we still have not identified all aspects of selection, and identifying the underlying genes for reproductive isolation remains challenging.

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    • "Recent studies using multilocus data provide increasing evidence that divergent selection is more common than previously thought (Rundle and Nosil 2005; Schluter and Conte 2009; Via 2012; Shafer and Wolf 2013). Divergent selection reduces overall gene flow between populations, leading to a pattern of isolation by environment (IBE), which can eventually affect all loci and result in ecological speciation (Nordborg and Innan 2002; Nosil et al. 2009; Schluter 2009; Strasburg et al. 2012; Orsini et al. 2013; Shafer and Wolf 2013). However, when speciation is incomplete, divergent selection would only act on particular areas of the genome, resulting in mosaic patterns of genetic differentiation, where different parts of the genome will show different degrees of genetic differentiation between incipient species (Via and West 2008; Via 2012; Feder et al. 2014). "
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    ABSTRACT: We explored the role of isolation by environment in a white pine species complex: Pinus flexilis, Pinus strobiformis and Pinus ayacahuite distributed from Canada to Central America. We predict that species differentiation would match genetic structure of candidate genes associated with significant differences in climatic niche in the species complex. To test this prediction, we sequenced five candidate genes for drought tolerance and three housekeeping genes, in individuals from across the entire range of each species. We performed neutrality tests, estimated genetic differentiation and performed partial mantel correlations, to test for isolation by environment in the species complex. Our results show that different loci vary in degrees of genetic differentiation within species and contrast in patterns of differentiation among species. This is considered to be a mosaic pattern of genetic differentiation. There was also significant isolation by environment in candidate genes. P. flexilis was genetically differentiated for candidate genes and P. ayacahuite for housekeeping genes. There was also an overall pattern of shared ancestral polymorphism followed by independent evolution. Nonetheless, all loci together recovered groups that correspond to the recognized taxonomy. In conclusion, the pattern of isolation by environment in candidate genes support the idea of ecologically driven differentiation of this species complex, especially in the case of P. flexilis. The observed difference in housekeeping genes between P. strobiformis and P. ayacahuite can be due to limited gene flow. The mosaic pattern of differentiation suggests that speciation is recent and ecological differences could be a factor in the diversification of pines in North America.
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    • "Predation constitutes a major selective source for diversification, divergence and speciation (Schluter, 2009), resulting in a variety of defensive armour and avoidance strategies (Edmunds, 1974; Kerfoot & Sih, 1987). The adaptive benefits of those antipredator features have been revealed in many studies (e.g. "
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    • "Ecological divergence generates phenotypic differences as a result of adaptation to different ecological conditions. Therefore, studies of this topic provides a wealth of information on how, and under which conditions, natural selection leads to evolutionary change (Nosil & Crespi 2006; Grant & Grant 2009; Schluter 2009). It can even lead to ecological speciation, not only in the classical scenario of absence of gene flow among populations (allopatry, Coyne & Orr 2004; Price 2007; Grant & Grant 2009), but also when there is gene flow among populations (sympatry and parapatry, Barluenga et al. 2006; Bolnick & Fitzpatrick 2007; Huber et al. 2007; Seehausen et al. 2008). "
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