We develop a natural selection model for sex ratio control in a spatially variable environment. Predictions of sex ratio alteration as a function of environmental change are tested in laboratory experiments with two parasitic wasps. Field data from a variety of other organisms also support the model. Finally, we discuss possibilities and difficulties for testing this type of evolutionary model.

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Courtship and mating behaviour of chalcidoid wasps is species-characteristic. Interactions between conspecific males in competition for inseminable females were observed in the laboratory. We have distinguished six types of interaction. Examples of each type are described and discussed. Some speculation is offered about the conditions under which these types are believed to operate in the field.

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