Body Size, Not Competition, May Spark the First Divergence Between Species, Theory Proposes

In 2023, evolutionary biologist Bryan Jennings spent 40 days traveling Australia, moving between six museums and measuring more than 1,000 preserved lizards. He recorded body length, body girth and tail length for each specimen. The work, followed by a stop at Harvard's Museum of Comparative Zoology to measure more animals, became the evidence behind a new proposal about how species first begin to differ from one another.

Jennings, an associate professional researcher in the Department of Evolution, Ecology, and Organismal Biology at UC Riverside, published the theory in the Evolutionary Journal of the Linnean Society. His claim is that changes in body size, rather than competition for food and habitat, may drive the earliest physical differences between species. Traits such as beak size, tail length and body proportions, he argues, can start to diverge when a population evolves a different body size, even before closely related species compete for resources.

The idea targets what Jennings sees as a gap in the prevailing account of adaptive radiation, in which a common ancestor gives rise to several closely related species that fill different ecological niches. Darwin's finches in the Galápagos are the best-known example. Traditional explanations stress competition after species come into contact. As Jennings put it, competition can drive increasingly pronounced differences in traits like finch beak size and shape, a process called character displacement. His point is that an earlier process may set the divergence in motion.

The theory rests on scaling, the relationship between an organism's overall size and the size and shape of its individual traits. Traits generally do not grow at the same rate. A human baby's head is large relative to its body and becomes proportionally smaller with maturity. In lizards, Jennings found body girth follows that pattern, while tail length can become proportionally larger as body size increases.

He began with the hypothesis that when a population evolves a different body size, its associated traits would change proportionally. A larger data set pointed elsewhere. The data showed a more extreme decoupling between trait development and body-size evolution than simple geometric scaling would predict. He calls the broader process disassociational scaling: a population that evolves a larger or smaller body can undergo a substantial change in trait proportions, with geometric scaling being only one form of it.

In his model, that first change can leave a trait with a functional deficit, such as a tail that works less well than it could. Natural selection then acts over longer stretches of time to fine-tune the trait and restore or improve its function. Jennings calls this second stage re-adaptation. Competition, in this sequence, arrives later, refining differences that already exist. In his words, competition between species is not necessary for the initial divergence.

The lizards came from his doctoral research. He developed the theory in 2020 while revisiting data on pygopodids, snakelike lizards found mainly in Australia and New Guinea, with elongated bodies and greatly reduced limbs. Tail length and body girth matter for locomotion and escaping predators, which made them useful for studying form and function. During the COVID-19 pandemic he also reread influential work on geometric scaling, including that of Stephen Jay Gould, and wondered whether geometric scaling could describe the earliest stage of divergence. The expanded data set supported the re-adaptation stage, but the initial-stage evidence led him to replace geometric scaling with disassociational scaling as the model's first step.

Jennings suggests the mechanism could bear on how adaptive radiation begins. If body-size changes substantially alter trait proportions, some resulting combinations might let a population exploit a different ecological resource. Within adaptive radiation he calls this niche scaling, and it would allow populations to develop niche-matched differences before they ever live together and compete. He says the theory complements established findings and does not dispute the role of competition and natural selection.

He claims the theory more fully accounts for beak evolution in Darwin's finches and antler evolution in the extinct Irish elk. But the open question is breadth: Jennings says further studies must determine how widespread disassociational scaling and niche scaling are across groups of organisms.