Topic+9.+Population+Structure - Topic 9 Population...

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Topic 9. Population Structure The term “structure” refers to the kinds of individuals within a population. We will classify these according to sex and age (Other ways: size, physiological condition, etc). Topic 9A. Sex Ratio I. Definition: number of males/number of females = M/F Sex ratio at birth the primary sex ratio. It may change over time due to differential survival. II. Fisher’s famous argument for 50:50 (Sir Ronald Fisher one of the founders of statistics, also did biology). In diploid sexual species, males and females contribute equally to next generation: each individual has a mother and a father. If the sex ratio is unbalanced (means more males than females or vice versa), individuals of underrepresented sex leave more descendents on average, so are more valuable. So natural selection favors a shift in allocation toward the rarer sex until sex ratio is 50:50. Fisher’s argument may imply 50:50 at birth, but if mortality for one sex occurs during parental care, adjust that sex to higher values at birth (since we are interested in mature offspring for fitness) III. Test of Fisher theory in the fish Atlantic Silverside by Conover and Van Voorhees. A. This is a species with temperature-dependent sex ratio (no one knows why). Early in the year, low ocean temperatures occur and fish mostly produce females. Later the reverse. Sex ratio average over the year is 50:50, so agrees with Fisher on average. Ability to modify sex ratio genetically based. So far, so good. B. Now see if you can get selection for a new sex ratio. Put the populations in the lab at a constant temperature, low or high all the time, so selecting for old average would be wrong. Experiment lasted 5-6 years. Each population went to a 50:50 SR slide
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IV. Trivers theory for deviations from 50:50 A. Studied polygynous lizard Anolis garmani in Jamaica. It differed in mating frequency as a function of size and sex. Slide Trivers argument is illustrated by the following graph:
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Topic+9.+Population+Structure - Topic 9 Population...

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