Why do Californian striders fly?

Discerning the adaptive significance of migratory strategies poses significant challenges, not the least of which is measuring migratory capability in natural populations. We take advantage of a visible migratory dimorphism to study variation in migratory capability in the stream-dwelling water strider, Aquarius remigis. Theory predicts loss of migratory capability in this species because streams have been viewed as stable and persistent habitats. As expected, A. remigis lack wings throughout most of North America. However, Californian populations are noted for unexpectedly high frequencies of winged, migratory morphs. To deduce the adaptive significance of this anomalous regional variation, we compare proportion winged among 37 Californian populations. We discover a strong, positive correlation with altitude, but no correlations with latitude, rainfall or stream size. A common garden experiment reveals that both proportion winged and its reaction norm to temperature differ genetically among populations, and a half-sibling experiment demonstrates that wing morph has high heritability, moderate genetic correlations across environments and a significant genotype by environment interaction. These results support the hypothesis that proportion winged and its reaction norm to temperature have diverged genetically in California. We conclude that high migratory capability is an evolutionary adaptation to the unusual harshness and instability of Californian stream habitats, and particularly to the high elevational gradients and extreme seasonal variation characteristic of montane streams.