Environmental Changes and Sexually Antagonistic Selection

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Environmental Changes and Sexually Antagonistic Selection

Tim Connallon, Monash University, Clayton, Victoria, Australia
Matthew D Hall, Monash University, Clayton, Victoria, Australia

Published online: December 2018

DOI: 10.1002/9780470015902.a0028171

Full Article on Wiley Online Library

Abstract
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Abstract

Sexually antagonistic selection (SAS) occurs when the direction of natural selection on a trait, or a combination of traits, differs between the sexes. For example, the different roles of females and males in reproduction, along with different interactions between each sex and the environment, can generate selection for larger body size in one sex and smaller body size in the other – a pattern of selection that may eventually lead to the evolution of sexual size dimorphism. SAS has been documented in several animal and plant populations and is thought to constrain adaptation and reduce population fitness. Recent research has emphasised that the intensity of SAS depends, in part, on environmental conditions of the population, which may vary over time or across each species' geographic range. Theory predicts that SAS should be more common in stable compared to changing environments. These predictions have some experimental support, though the general manner with which environmental changes affect SAS remains an open question.

Key Concepts

Sexually antagonistic selection arises when the direction of natural selection differs between female and male traits.
Sexually antagonistic selection favours the evolution of sex differences (sexual dimorphism).
Conditions of the social environment, mating environment and habitat interact to influence the pattern of selection on female and male traits, including the potential for sexually antagonistic selection.
Genetic correlations between the sexes constrain the evolution of sex differences and can lead to persistent sexually antagonistic selection.
Environmental change can reduce or eliminate sexually antagonistic selection.

Keywords: sexual dimorphism; genetic variation; genetic correlation; adaptive evolution; natural selection; sexual selection; sexual conflict

Article Title:	Environmental Changes and Sexually Antagonistic Selection
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	Figure 1. Sexually antagonistic selection (SAS) in a single trait. The solid blue and red curves show the frequency distributions for the trait within females and males, respectively; ${\overline{z}}_{f}$ and ${\overline{z}}_{m}$ refer to the average of the female and male trait distributions. The broken curves show female (blue) and male (red) relative fitness as a function of trait expression, with the optimum for each sex corresponding to the maximum of the function (O_f and O_m are the female and male trait expression optima). Since the female trait mean is above the female optimum, and the male mean is below the male optimum, directional selection on this trait is sexually antagonistic.
	Figure 2. Sex‐specific directional selection in two dimensions of trait variability. SAS is absent in panel a, whereas SAS is present on panel b. Within each panel, the dashed arrows show the selection gradients for each trait and sex (horizontal arrows depict the selection gradients for ‘trait 1’; vertical arrows depict selection gradients for ‘trait 2’). The solid arrows show the net directions of selection, as represented by vectors of selection gradients (β_f = (β_f_,1, β_f_,2) and β_m = (β_m_,1, β_m_,2) for females and males, respectively, which each point towards a trait optimum: O_f or O_m). In Panel b, the angle between the vectors, θ, is less than 90°, which denotes a partial alignment between female and male directional selection (and, thus, SAS is absent). In Panel b, the angle between the vectors, θ, is greater than 90°, which denotes a misalignment between female and male directional selection (and, thus, SAS is present).
	Figure 3. Genetic correlations and the emergence of SAS during adaptation. The broken arrows show the initial vectors of directional selection towards the female and male optima. The average trait expression is initially the same in each sex ( ${\overline{z}}_{0}$ in both sexes at time ‘0’). The solid arrows show the vectors of selection after some time has passed and the population has partially adapted to the environment. Note that SAS is initially absent in both figure panels, as the initial angle between female and male selection gradient vectors is less than 90°. Panel a: A strong genetic correlation leads to rapid and correlated evolutionary divergence of female and male traits when direction of selection in each sex is aligned (i.e. from the initial condition where SAS is absent). However, adaptation of the population causes selection to become sexually antagonistic: following t generations of evolution, selection vectors point in opposing directions. Panel b: When there is no genetic correlation between the sexes (r_mf = 0), evolutionary divergence of each sex can proceed towards its optimum, without modifying the angle between female and male selection vectors (in the case shown, SAS is initially absent and remains absent). This example assumes that the two different traits genetically vary independently of one another.