In the Light of Evolution Volume III: Two Centuries of Darwin (2009) / Chapter Skim
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10 Sexual Selection and Mating Systems--Stephen M. Shuster
10 Sexual Selection and Mating Systems--Stephen M. Shuster
Pages 191-212
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From page 191... ...
Analyses of mating systems since Darwin have emphasized either the genetic relationships between male and female mating elements, usually among plants, or the numbers of mates males and females may obtain, usually among animals. Combining these schemes yields a quantitative methodology that emphasizes measurement of the sex difference in the variance in relative fitness, as well as phenotypic and genetic correlations underlying reproductive traits that may arise among breeding pairs.
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From page 192... ...
. A second goal of this chapter, then, is to explain how disparate existing approaches to mating system analysis can be combined to yield a quantitative methodology that emphasizes measurement of the sex difference in the variance in relative fitness, as well as genetic correlations underlying reproductive traits arising from the spatial and temporal distributions of fertilizations.
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From page 193... ...
TWO PERSPECTIVES ON SEXUAL SELECTION Darwin (1859/1964) argued convincingly that male combat and female mate choice were the contexts in which sexual differences appeared.
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From page 194... ...
This condition increases the fraction of unsuccessful males overall because some of the males who successfully mate ultimately fail to sire offspring. in contrast, when mating success and fertilization success among males are uncorrelated, multiple mating by females always decreases the intensity of sexual selection because it increases the total number of males contributing to each generation.
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From page 195... ...
, and, with increasing resolution of genetic markers, population genetic approaches to plant mating systems and their theoretical foundations have become increasingly sophisticated. selfing, self-incompatibility, dioecy and gynodioecy all appear to have evolved by remarkably simple means (Charlesworth and Charlesworth, 1978, 1987; Charlesworth, 2006)
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From page 196... ...
however, although male and female fitness may vary within each of these contexts in plants, the extreme variance in male fitness seen in animals appears to be constrained by limited opportunities for pollen dispersal, by inconsistent success of particular pollen tubes among different stigmas, or by large within-individual variation in seed size and seed set, often due to pollen limitation (Ashman et al., 2004; Knight et al., 2005)
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From page 197... ...
, these authors presumed to measure the degree to which the social and ecological environment allows males to monopolize females as mates. emlen and oring considered ePP highest when resources or females were spatially clumped and female receptivity was asynchronous, and lowest when resources were uniformly distributed and female receptivities were synchronous.
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From page 198... ...
. in this example, we assume that females mate once and their receptivity is maximally asynchronous; i.e., individual females mate sequentially across the breeding season.
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From page 199... ...
m, mean mating success; Vm, variance in mating success when all males are included in parameter estimates ( A) and when the zero class of males (hatched bars)
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From page 200... ...
during the breeding season fail to distinguish between males who mate and males who do not, and overestimate the intensity of sexual selection that exists over the entire breeding season. shown are two scenarios illustrating a hypothetical breeding season in which 5 females mate with 5 males on territorial patches (rows)
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From page 201... ...
(2002) quantified the sex difference in "Bateman gradients," in which females had a stronger positive association between mate numbers and fertility than males, and changes in sex ratio influenced the slopes of these relationships, as predicted by emlen and oring (1977)
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From page 202... ...
. This methodology separates the effects of natural selection acting on traits possessed by focal individuals from the effects of social selection on these same traits, caused by interactions focal individuals may have with potential rivals or with potential mates themselves.
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From page 203... ...
. however, because mate numbers and offspring numbers are the currency by which I is measured, the outcomes of chance on mating success and fertilization success are already incorporated into these estimates of selection opportunities.
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From page 204... ...
Although the emlen-oring estimates of the environmental potential for polygamy (ePP) and the operational sex ratio (osr)
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From page 205... ...
if multiple males inseminate the same female, these numbers may also represent the fraction of the total fertilizations with a given female that a male obtains, providing a specific means for considering the effects of postcopulatory sexual selection. The more precise paternity data are, the more detailed such analyses can be, and fractions of clutches instead of individual matings can be substituted into the matrix.
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From page 206... ...
, the total number of receptive females, Nj, divided by the total number of males, Kt, equals Rt, the interval sex ratio, or the average number of females mated by each male within the jth interval (upper horizontal rectangle, Fig.
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From page 207... ...
, the total number of receptive females, Ni, divided by the total number of intervals containing at least one female, T, equals Ni/T, the number of females mated by the male in the ith territory, averaged across all T intervals (left vertical rectangle, Fig.
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From page 208... ...
When only the spatial and/or temporal distributions of sexually receptive individuals are available, yet another method for estimating ΔI involves calculating the mean spatial crowding, m* and the mean temporal crowding, t*
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From page 209... ...
When female receptivity is maximally asynchronous within the breeding season, t* approaches zero, whereas when female receptivities overlap within a single interval, the value of t*
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From page 210... ...
. here, not only do males and females have approximately similar variance in mating success and so tend to show little sexual dimorphism, but when individuals are limited in their mobility or are restricted to patchy habitats, they may also be simultaneously or sequentially hermaphroditic.
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From page 211... ...
Dioecy could be favored when pollen limitation seldom occurs, as when plants are relatively clumped in space and temporally synchro nous in their flowering, but could also exist when spatially clumped flow ers open asynchronously and allow plants to attract pollinators over long durations and gain considerable fitness through pollen. sexual selection is considered relatively weak among plants because of approximately equivalent fitness variance within each sex (shuster and Wade, 2003)
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From page 212... ...
As if to invite collaborative research, the population genetic tools, both theoretical and empirical, that characterize research in plant mating systems are less well developed for animals, while the spatiotemporal data and quantitative genetic methods for measuring selection that characterize research in animal mating systems are less well developed for plants. each discipline has much to offer the other and much exciting work remains to be done.
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