Body size change, not competition, may trigger speciation in Australian legless lizards

Biologist Brian Jennings of the University of California, Riverside, proposes that early divergence of species may be driven by simple changes in body size rather than by competition for food or territory. The study was published in the Evolutionary Journal of the Linnean Society, as reported by pravda.ru pravda.rupravda.rupravda.ruOpen source ↗. For decades, evolutionary biology has relied on the idea that species split and evolve only when they begin to compete for the same resources.

The work builds on the concept of allometry, the change in body proportions as overall size changes. When an organism grows or shrinks over evolutionary time, its body parts do not scale uniformly. It was long assumed that traits change proportionally with size, as if a photograph were simply enlarged.

Pygopodid lizards and museum data

To test this, Jennings studied pygopodids, Australian legless lizards whose tail can be 4.5 times longer than the body. He analyzed 1,755 museum specimens covering 31 species, comparing snout length, tail length and body girth.

The results were unexpected. In species that diverged relatively recently, less than two million years ago, proportions changed highly unevenly. Instead of orderly geometric scaling, the data showed abrupt "dissociative" changes: tails or body girth shifted differently from what classical theory predicts pravda.rupravda.rupravda.ruOpen source ↗.

Two stages and niche scaling

Jennings describes two stages of the process. The first is dissociative scaling: when overall body size changes, internal proportions "break" and produce new, sometimes extreme combinations of traits. The second is readaptation: only after a long time does natural selection refine these chance changes, making them functional and useful.

The main conclusion is that a struggle for resources is not needed to start evolutionary divergence. New body proportions arising from a chance change in size may give an animal new physical capabilities, such as entering a different burrow or feeding in a different way. Jennings calls this "niche scaling".

"Competition between species is not a prerequisite for initial divergence," the scientist said. In his view, a change in body size gives a population a head start in occupying a new habitat, and natural selection consolidates the differences afterwards pravda.rupravda.rupravda.ruOpen source ↗. The hypothesis does not reject the role of natural selection but supplements it, addressing what happens before competition for survival enters the process.