The study of the sex ratio is one of the brightest jewels in evolutionary biology's crown. At issue are why do most sexual organisms invest equal amounts of resources in producing male and female offspring? And how can one explain the relatively rare exceptions? The first ques- tion can be answered in a number of equivalent ways. Consider a typical plant or animal with diploid males and females that are equally costly for their mothers or parents to rear. Now suppose that one sex is rarer than the other. Because every individual has both a mother and a father, any gene that caused its bearer preferentially to produce the rarer sex would leave more copies of itself than would its alternatives, and so it would spread through the population until its advantage was nullified. Only when there are equal numbers of the two sexes does selection on sex ratio modifiers cease, and hence that is the "evolutionary stable strategy" we should observe. This simple argument can be extended to more complex genetic systems such as haplodiploidy and to predict equality of investment, rather than numbers, when sons and daughters are not equally expensive to produce.
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