sexual dimorphismSexual Dimorphism – Why?
• If phenotypic traits shaped by natural selection, then…..
– What about traits that differ between males and females?
• Morphological differences between sexes, if a trait is adaptive, then why wouldn’t both sexes have it?
• Tough to explain using the natural selection model.
Lecture 12
Sexual Selection
The cichlid fish, Lamprologus callipterus
• Endemic to Lake Tanganyika
• Extreme sexual size dimorphism
• Males average 13 times heavier than females, may exceed 27 x in some cases.
Sexual Selection
• Evolution of traits based on differences in mating (reproductive) success among individuals.
• Darwin first found sexual selection paradoxical.
– The Descent of Man, and Selection in Relation to Sex (Darwin, 1871).
• Mating success may actually trade off with other components of fitness.
– e.g. if a trait increases lifetime reproductive success, it may be favored even if it lowers survival probability.
Sexual Selection
• Sexual selection can occur in two ways:
– Some traits may increase the ability to compete with individuals of the same sex for access to mates (intrasexual).
– Some traits increase the ability to attract individuals of the opposite sex (intersexual).
Why does sexual selection exist?
• Early in the evolution of sexual organisms, gametes were of equal size = isogamous.
• Gradually, two different types of gametes (anisogamous) evolved
– large gametes (greater energy stores, higher survival probability)
– small gametes (more produced, greater motility).
• Females produce few large gametes and males produce many small gametes
Sexual Selection
• Conflict between the interests of the sex with fewer, larger gametes (expensive), and the sex with many, small gametes (cheap).
• Thus, reproductive “strategies” of the sexes are different.
• Males can mate with many females
– Often suffer little reduction in lifetime fitness if a poor quality female is selected.
• All of a female’s eggs can be fertilized by a single or a few males.
– her fitness can be significantly lowered by poor choice of males
Why does sexual selection exist?
• Bottom line: eggs are expensive, sperm is cheap (relatively – it is not free!)
• Females - limiting resource for males
• Males not usually a limiting resource for females (But not always!!!)
• Variance in reproductive success (= number of offspring) is greater among males* than among females (Bateman’s 1st Principle).
• Variance in mating success is a measure of the intensity of sexual selection.
Bateman’s 1st Principle
• Variance in male* reproductive success (# of offspring) is greater than in females
Bateman’s 2nd Principle
• Males* exhibit a greater variance in mating success (number of mates)
Bateman’s 3rd Principle
• Slope of the regression relating reproductive success to mating success is steeper for males*. In other words, maximizing mating opportunity is key for males (quantity). Maximizing mate “quality” is key for females.
Rough-Skinned Newts
See example in the text book.
Discuss
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