6 Sexual selection - BIO 370 Kirkpatrick & Hall, Fall...

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BIO 370 Kirkpatrick & Hall, Fall 2004 p. 1 6. Sexual selection Darwin realized some traits don’t make sense under natural selection They are often: • Sexually dimorphic (limited to one sex, typically males) • Expressed only in breeding season • Not expressed in immature individuals Examples: Birds of paradise, stag beetles Darwin proposed his theory of sexual selection: Selection caused by competition between individuals of the same sex (usually males) for access to mates Transmitting genes to next generation requires survival and reproduction Traits can evolve if they increase mating success even if they decrease survival Why is it usually males that are sexually selected? Operational sex ratio: Relative number of males and females available to breed Competition for mates is more severe in the more numerous sex Usually fewer females are available to breed because they put more energy and time into reproduction and are unavailable to breed for a period after they lay eggs or have been fertilized Exceptions in which there is more sexual selection on females: Sex role reversal Examples:
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6 Sexual selection - BIO 370 Kirkpatrick & Hall, Fall...

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