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Review
. 2015 Aug 26;9(1):91-102.
doi: 10.1111/eva.12296. eCollection 2016 Jan.

Geography, assortative mating, and the effects of sexual selection on speciation with gene flow

Maria R Servedio  1
Affiliations
Review

Geography, assortative mating, and the effects of sexual selection on speciation with gene flow

Maria R Servedio. Evol Appl. .

Abstract

Theoretical and empirical research on the evolution of reproductive isolation have both indicated that the effects of sexual selection on speciation with gene flow are quite complex. As part of this special issue on the contributions of women to basic and applied evolutionary biology, I discuss my work on this question in the context of a broader assessment of the patterns of sexual selection that lead to, versus inhibit, the speciation process, as derived from theoretical research. In particular, I focus on how two factors, the geographic context of speciation and the mechanism leading to assortative mating, interact to alter the effect that sexual selection through mate choice has on speciation. I concentrate on two geographic contexts: sympatry and secondary contact between two geographically separated populations that are exchanging migrants and two mechanisms of assortative mating: phenotype matching and separate preferences and traits. I show that both of these factors must be considered for the effects of sexual selection on speciation to be inferred.

Keywords: allopatry; assortative mating; migration; secondary contact; sympatry; two‐island model.

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Figures

Figure 1
Figure 1
Schematic of the modes of selection necessary to drive speciation with gene flow in the scenarios of (A) allopatric populations that are exchanging migrants and (B) sympatric speciation. The pictures associated with the labels for ‘geography’ show discrete traits for the purpose of illustration, but these scenarios are also relevant for quantitative traits, as shown under ‘selection pattern’. The populations in (A) are drawn as if they have already diverged to some extent. The arrows on the figure are color coded to match the descriptions of selection under ‘selection mode’.
Figure 2
Figure 2
The amount of trait divergence between two populations exchanging migrants under assortative mating by phenotype matching, as a function of the strength of preferences, α, for two different migration rates (m = 0.01 and m = 0.03). The lines shown are equilibrium frequencies of the trait T2 in population 2; as the identity of the ‘local’ trait is interchangeable in a model with only phenotype matching, if T2 is at the frequency shown by the top line in population 2, then T1 will be at the same frequency in population 1. The difference between the two lines can thus be considered a measure of the divergence between the traits across the two populations. The solid black line is a stable equilibrium line reached from the assumption of secondary contact between divergent populations and the dashed gray line is an unstable equilibrium. Stable lines of equilibrium where both populations are fixed for T2 or for T1 and an unstable equilibrium at 0.5 are not shown on the figure. The value of α that leads to the maximum amount of divergence between the populations, αopt, is labeled for each migration rate (differentiation cannot be maintained if migration rates are too high). Redrawn with permission from Servedio (2011).
Figure 3
Figure 3
Local trait (t 2) and preference (p 2) frequencies in population 2 at equilibrium, as a function of the strength of preferences, α. Higher values represent more differentiation between the two populations in trait and preference frequencies, while a frequency of 0.5 represents no differentiation. (A) equilibrium trait frequencies. (B) equilibrium preference frequencies [note the range of the y‐axis differs from panel (A)]. The migration rate m = 0.01; higher migration results in the same pattern but with each curve at lower values of t^2 and p^2. The strengths of selection leading to the local adaptation of trait T2 are shown in the inset on panel (A), where the locally adapted trait has a relative viability of 1 + s in each population. Redrawn with permission from Servedio and Bürger (2014).
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