Body size changes may trigger early evolutionary divergence

Bryan Jennings, an evolutionary biologist at the University of California Riverside, proposed a new theory suggesting that changes in body size can initiate the physical differences between species before competition for resources occurs.

He published this theory in the Evolutionary Journal of the Linnean Society to address a gap in the current understanding of adaptive radiation.

This process describes how a common ancestor produces multiple related species that occupy different ecological niches, such as the Darwin's finches on the Galápagos Islands. Jennings argues that traditional models focus on competition as the primary driver of trait changes, such as beak shape. He proposes a two-stage process instead. In the first stage, a population evolves a different body size, which causes traits like tail length or body proportions to decouple from their original developmental patterns.

He calls this disassociational scaling. This initial change can create a functional deficit in a trait, which natural selection then refines over time in a second stage he calls re-adaptation. To test this, Jennings analyzed data from pygopodid lizards, which are snakelike lizards found in Australia and New Guinea. He measured more than 1,000 preserved specimens across several museums and at the Museum of Comparative Zoology at Harvard University.

His findings suggest that these initial changes in proportions could allow a population to exploit new ecological resources, a process he terms niche scaling. Jennings notes that while competition still plays a role in refining these traits, it may not be the starting point for the initial divergence.