sexualselectionoutline - Sexual Dimorphism Sexual Selection...

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Sexual Selection Introduction To Evolution Sexual Dimorphism ! Males and females of the same species are often very different in appearance. These differences are often referred to as sexual dimorphisms . ! Why are these common in nature?
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Asymmetries in Sexual Reproduction ! Orangutans are antisocial. In general the males and females only tolerate each other during mating. ! After mating, the female has a pregnancy (8 months) and a baby for about 3 years. ! The male is potentially off to do this again in just minutes. In 90% of mammals the males make no investment in parental care.
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Asymmetries in Fish ! Consider the female cod may produce more than 1 million eggs, but each egg is fairly large and contains some stored food material for the developing embryo. ! The sperm contains only genetic information and enough energy to get it to the egg. Therefore, the male can always potentially produce more gametes and mate more often than the female. Even though the female is typically not investing in parental care, she is investing more in production of gametes. A Generalization ! Males and females have different limitations in reproduction, so selection works differently in males and females. ! Females are limited by the number of gametes they can produce and/or the number of young they can raise, but not by access to males. ! Males are limited almost entirely by the access to females. How many females a male can convince to mate with him is the limiting resource for male reproduction. Testing This Idea ! Bateman (1948) tested this idea in fruit flies and made the following observations: Reproductive success in males: ! Increased directly with the population of females. ! Showed extreme variation with some males leaving many offspring and others leaving none. Reproductive Success in females: ! Was not increased with a larger population of males. ! Did not vary widely, most females reproduced and there were few big winners or losers. The Behavioral Consequences of Asymmetric Limits on Fitness ! This asymmetry leads to very different strategies for males and females in reproduction. Males will tend to be competitive for the access to females, male-male competition. Females will tend to be choosy.
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Intrasexual Selection ! Male-male competition leads to winners and losers, if the females mate with the winners, the main type of selection will be between male for traits that allow them to succeed at male-male competition. ! This type of selection is called intrasexual selection . Intrasexual selection is the competition between members of the same sex (usually males) for access to members of the opposite sex. Intersexual Selection ! Female choice leads to selection for characteristics that females find attractive. ! This type selection is called
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This note was uploaded on 04/16/2008 for the course BIO 300 taught by Professor Johnson during the Spring '08 term at Central Washington University.

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sexualselectionoutline - Sexual Dimorphism Sexual Selection...

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