Rapid phenotypic evolution following shifts in life cycle complexity.

Life cycle strategies have evolved extensively throughout the history of metazoans. The expression of disparate life stages within a single ontogeny can present conflicts to trait evolution, and therefore may have played a major role in shaping metazoan forms. However, few studies have examined the consequences of adding or subtracting life stages on patterns of trait evolution. By analysing trait evolution in a clade of closely related salamander lineages we show that shifts in the number of life cycle stages are associated with rapid phenotypic evolution. Specifically, salamanders with an aquatic-only (paedomorphic) life cycle have frequently added vertebrae to their trunk skeleton compared with closely related lineages with a complex aquatic-to-terrestrial (biphasic) life cycle. The rate of vertebral column evolution is also substantially lower in biphasic lineages, which may reflect the functional compromise of a complex cycle. This study demonstrates that the consequences of life cycle evolution can be detected at very fine scales of divergence. Rapid evolutionary responses can result from shifts in selective regimes following changes in life cycle complexity.