Sex and Sexual Selection
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© 1997
David H.A. Fitch
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Adaptations:  Sexual selection and the evolution of sex

I.  Sexual selection

A.  Darwin:  "...the advantage which certain individuals have over others of the same sex and species solely in respect of reproduction"

B.  Intrasexual selection
1.  Arises from competition among males for access to females or mating sites
2.  Often results in larger body sizes or weaponry that may be advantageous in battle

C.  Intersexual
1.  Females are usually more selective in mate choice than males (why?)
2.  Female preference can be a complex trait
a.  Preference may serve to increase her fitness (examples?)
b.  Fitness may not be affected by her preference (if not, why should she have a preference?)

II.  Evolution of recombination and sex

A.  Variation exists for rate of recombination and mode of reproduction, so these are traits that are subject to selection

B.  Group selection models
1.  Recombination enhances adaptibility of populations to changing environments:  Allowing allelic combinations that are adaptive allows faster response to environmental change
2.  Asexual populations have higher extinction rates:  Obligate parthenogens are generally closely related to sexual species, implying only recently derived parthenogenetic lineages still survive
3.  However, group selection models also suffer from:
a.  The much more efficient individual selection for reduced recombination (or asexuality) to maintain advantageous allelic combinations in stable (or slowly changing) environments
b.  Each parthenogen might generate the same number of progeny that a sexual pair of individuals would, doubling the intrinsic rate of parthenogenetic reproduction

C.  Non-group selection models
1.  Short-term environmental fluctuations or a mosaic of microenvironments would select for the more diverse progeny of a sexual genotype over the uniform progeny of asexual genotypes--
However, this model suffers from the possibility that mosaic or short-term environmental patterns might favor multi-locus allelic combinations; recombination would thus be disadvantageous because it would break up such combinations
2.  Inbreeding may result in disadvantageous traits ("inbreeding depression") such as low fecundity, resulting in selection for traits that reduce the opportunity for inbreeding

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Exercises

  1. If females are generally the "choosy" ones, how might you explain some of the exceptions (like sea horses) where males are the ones who are choosy?
     

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 Examples of
Adaptations  Defining
Adaptation  Levels of
Selection  Optimal
Models  Tradeoffs  Sexual
Selection
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[Examples of Adaptations] [Defining Adaptation] [Levels of Selection] [Optimal Models] [Tradeoffs] [Sexual Selection]

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