This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 closely related species that occupy different ecological niches. Darwin's finches of the Galápagos Islands are one of the best-known examples. Jennings, an associate professional researcher in the Department of Evolution, Ecology, and Organismal Biology, said traditional explanations of adaptive radiation emphasize competition between species after they come into contact.
"Competition for resources can drive increasingly pronounced differences in traits such as the size and shape of finches' beaks, a process known as character displacement," he said. Jennings proposes, however, that an earlier process can initiate that divergence. "When one population evolves a different body size, the traits associated with that body size can become decoupled from their ancestral developmental patterns," he said.
"That initial divergence likely occurs before competition between species becomes important." Jennings' theory focuses on scaling, the relationship between an organism's overall size and the size and shape of its individual traits. As an organism grows, different traits generally do not grow at the same rate. A human baby's head, for example, is relatively large compared with its body and becomes proportionally smaller as the individual matures.
In lizards, Jennings found that body girth shows this pattern as well, whereas other traits, such as tail length, can become proportionally larger as body size increases. Jennings initially hypothesized that when a population evolves a different body size, associated traits would change proportionally. His subsequent analysis of a larger data set led him to a different conclusion.
"Rather than simple geometric scaling, the data showed evidence of a more extreme decoupling between trait development and body-size evolution," he said. "I call this broader process disassociational scaling. Under this model, a population that evolves a larger or smaller body size can experience a substantial change in the proportions of its traits." According to his model, this initial change can produce a functional deficit in a trait—for example, a tail not working as well as it could.
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