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Abstract

Humans have come to inhabit nearly every terrestrial environment. Their commensals have followed them and have consequently become some of the most abundant and widespread species on Earth, in the process becoming exposed to a variety of abiotic and biotic stressors. For example, the house sparrow was introduced from the UK to North America in the nineteenth century and now millions of birds breed in houses from northern Canada to Tierra del Fuego. Clines in both morphology and colour were rapidly established [1]. But what processes cause these clines? One point of special interest is the extent to which this rapidly established geographical variation is a result of genetic evolution or plastic responses, because this has major implications for how species will respond to—for example—climate change [2].