In vertebrates, sex can be determined either genetically or by the temperature experienced by the embryo, but the evolutionary causes of this variation remain poorly understood. Pen et al. show that a live-bearing lizard at different climatic extremes of its range uses different sex-determining mechanisms - temperature in the lowlands and genotype at higher altitudes. Divergent natural selection on sex determination by altitude is caused by climatic effects on lizard life history and variation in the magnitude of between-year temperature fluctuations.
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