Auburn University, 331 Funchess Hall, Auburn 36849, USA
In the summer of 1939, a group of 40-50 house finches Carpodacus mexicanus collected in southern California was released from a pet store in New York City. In the subsequent 62 years, this introduced population has undergone tremendous expansion, spreading across the eastern U.S. and southeastern Canada and increasing to an estimated 1.3 billion birds. This expansion of ecological range was accompanied by rapid divergence in sexual size dimorphism among new populations. We show that the observed divergence in morphology were caused by population differences in patterns of natural selection acting over the lifespan of both sexes. This represents an apparent paradox of rapid independent evolution of each sex in traits for which there is no sex-biased genetic variance in adults. We show that correlated selection on growth trajectories of males and females in combination with persistent and strongly sex-biased maternal effects can account for the observed adaptive divergence in sexual dimorphism among newly-established populations of the house finch.