When thinking about sociality and how it evolved, ants are some of the first examples to spring to mind. Not only does each individual share resources, there is an entire caste of sterile females who never breed at all. The existence of sterile castes in ants and other eusocial insects is curious, because a mutation which removes the ability of the workers to breed seems obviously deleterious: no breeding, no fitness. The mechanism which brought this sterility about had historically been under some debate. Opinions were split between kin selection (improving the fitness of other, genetically related individuals improves your fitness as well), or some form of parental modification in which the queen actively suppressed the development of her daughters. To settle this debate, researcher Joan M. Herbers reasoned that studying the sex ratios of a colony would provide the data to conclude one way or another.
The premise of her idea is that each evolutionary mechanism would effect the sex ratio in different ways. Supposing the queen is in total control, and suppresses her workers, the sex ratio would reflect the optimum fitness ratio for the queen’s genes: a even distribution of males and females. However, if kin selection is the mechanism behind the development of sterile worker castes, the sex ratio would be approximately three females to every one male. This is because ants, like all hymenopterans, have a haplodiploid sex determination system. Instead of combinations of X and Y chromosomes determining gender, male ants emerge from unfertilized eggs, and are haploid, while females emerge from fertilized eggs, and are diploid. Assuming the queen mates once, that means that each of her daughters will share an average of 75% of her genetic material with her sisters, but only about 25% with her brothers. Because sisters share more genes with each other than with their brothers, kin selection results in strong selection towards a 3-1 female to male sex ratio.
Armed with this knowledge, Herber started gathering sex selection data for the ant species Leptothorax longispinosus, controlling for other variables that can alter sex ratios. What she found was that queens actually do produce an even ratio of male and female children. However, the measured worker sex-ratios were always around 3-1. The answer to this puzzling contradiction? Far from being developmentally and behaviorally suppressed by the queen, workers were actively killing and eating their infant brothers, enforcing the sex ratio in their favor. Thus, Herber concluded that kin selection is the primary mechanism of eusocial evolution in the species.
The full article can be found here: http://www.jstor.org/stable/2408712?seq=2
-Adam
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