With reference to individual fitness explain why we normally observe a 1:1 sex ratio in wild populations. Give three different cases where we might not expect 1:1. You may use two from class. For the last one you must go out and find your own! Do not just name the examples. You need to explain why the case deviates from the expected ratio.
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Most animal populations have a 50: 50 ratio of males to females. This does not have to be so; it is theoretically possible for parents to produce predominantly male offspring or predominantly female offspring. Imagine a population with a male-biased sex ratio, say, $70 \%$ males and $30 \%$ females. Which sex will have an easier time finding a mate? As a result, which sex will probably have higher average fitness? Which parents will have higher fitness- those that produce mostly males or those that produce mostly females? Now imagine the same population with a female-biased sex ratio, and answer the same questions. What sort of selection is probably maintaining the 50: 50 sex ratio seen in most populations?
Katlin K.
Most populations are expected to exhibit a long-term 1:1 sex ratio because eggs always have a 50% chance of being fertilized by an X-bearing sperm and a 50% chance of being fertilized by an Y-bearing sperm. this is the sex ratio that allows the population to grow at its maximal rate. once one sex becomes scarce, there will then be a fitness advantage to a parent that produces more of the rarer sex and the population eventually stabilizes at 1:1. each zygote must be formed by the union of one egg and one sperm, and the representation of males and females remains constant across generations.
Shaiju T.
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