What role does natural selection play in speciation?

Abstract

If distinct biological species are to coexist in sympatry, they must be reproductively isolated and must exploit different limiting resources. A two-niche Levene model is analysed, in which habitat preference and survival depend on underlying additive traits. The population genetics of preference and viability are equivalent. However, there is a linear trade-off between the chances of settling in either niche, whereas viabilities may be constrained arbitrarily. With a convex trade-off, a sexual population evolves a single generalist genotype, whereas with a concave trade-off, disruptive selection favours maximal variance. A pure habitat preference evolves to global linkage equilibrium if mating occurs in a single pool, but remarkably, evolves to pairwise linkage equilibrium within niches if mating is within those niches-independent of the genetics. With a concave trade-off, the population shifts sharply between a unimodal distribution with high gene flow and a bimodal distribution with strong isolation, as the underlying genetic variance increases. However, these alternative states are only simultaneously stable for a narrow parameter range. A sharp threshold is only seen if survival in the 'wrong' niche is low; otherwise, strong isolation is impossible. Gene flow from divergent demes makes speciation much easier in parapatry than in sympatry.

Neo-Allopatry and Rapid Reproductive Isolation

“…However, in the study species, flower morphology and phenology do not differ among regions (D. Montesinos, personal observation), and thus prepollination reproductive barriers are unlikely to be important. Potential mechanisms of postpollination reproductive isolation include pleiotropy, genetic hitchhiking, chromosomal inversions, Bateson-Dobzhansky-Muller interactions, genetic bottlenecks that occurred during the process of geographic isolation, and changes in ploidy (Barton 2010;Rieseberg and Blackman 2010;Schemske 2010;Wolf et al 2010). The three Centaurea species we studied are diploid on both continents (Heiser and Whitaker 1948;Powell et al 1974;Sun and Ritland 1998); thus, polyploid speciation is unlikely.…”

Speciation genetics: current status and evolving approaches

“…From such work on evolutionary young lineages that are at incipient states of divergence, it has increasingly been recognized that barriers to gene flow can evolve as a result of ecologically based divergent or disruptive selection. This perception is bolstered both from theoretical work evaluating the role of natural selection in a growing number of empirical systems (Gavrilets 2004;Dieckmann et al 2004a;Gavrilets & Losos 2009;Barton 2010). Clearly, a central limitation of this forward-looking approach is that one cannot foresee whether the speciation process will be driven to completion.…”

Gloger's rule in North American Barn Owls

“…At a large geographic scale, such as across a continent, ecological and climatic factors vary greatly among regions, promoting the evolution of specific phenotypes adapted to these specific local conditions (Huey et al 2000). Local adaptation requires populations to differ genetically (Blanquart et al 2013), potentially leading to speciation even in the presence of gene flow (Barton 2010). Although studying geographic variation in phenotypic traits has a long history (Wright 1943, Mayr 1956), this approach still remains essential to identifying the potential adaptive function of alternative phenotypes in changing environments (Newton 2003).…”