Photo shows Lialis burtonis. The enigmatic pygopodid lizards adaptively radiated into at least 50 species of lizards that are endemic to Australia and New Guinea. Unlike conventional limbed lizards, their locomotor abilities are mainly determined by their relative tail lengths and body girths. Credit: Bryan Jennings, UC Riverside.

A new evolutionary theory suggests changes in body size, rather than competition for food and habitat, may drive the earliest physical differences between species.

In a study published in the Evolutionary Journal of the Linnean Society , UC Riverside evolutionary biologist Bryan Jennings describes how physical traits such as beak size, tail length and body proportions may begin to diverge when a population evolves a different body size, even before closely related species begin competing for resources.

The theory addresses a gap Jennings identified in the prevailing framework for adaptive radiation, the evolutionary process in which a common ancestor gives rise to multiple…