Another aspect of behavioural ecology that has benefited from molecular data and that, like mating systems, is linked to reproductive behaviour, is the way in which parents manipulate the sex ratio of their offspring. In 1930 the biologist and statistician R.A. Fisher wrote an influential book on evolutionary genetics in which he addressed, among many other things, the importance of sex ratios (Fisher, 1930). Fisher maintained that a sex ratio should remain stable if the production of males and females provides equal fitness, per unit of effort, for the individuals that are controlling sex ratios. If, on the other hand, greater fitness can be obtained by producing an excess of one sex, then either males or females will be favoured, at least until the time when there is no longer an advantage to biasing the sex ratio.
Research into adaptive sex ratios really got under way in the 1970s after Trivers and Willard (1973) wrote a seminal paper in which they resurrected the argument that parents may manipulate the sex ratio of their offspring for adaptive reasons. Over the years considerable support for this has come from a wide range of taxonomic groups, but until recently investigations were mainly limited to species in which males and females were easily distinguished on the basis of external morphology. In many species we are now able to use sex-specific markers to identify the sexes of morphologically indistinguishable adults and juveniles. In addition, by genotyping tissue from eggs we can sometimes use molecular data to calculate the primary sex ratio (that found in eggs) of many species. This allows us to compare the primary and secondary sex ratio (that found in hatchlings) of a population, which is sometimes a necessary distinction to make before we can determine whether or not a secondary sex ratio has been influenced by disproportionate egg mortality in either males or females, as opposed to adaptive parental behaviour.
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