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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.
Photo Credit: Bryan Jennings / University of California, Riverside
Scientific Frontline: Extended "At a Glance" Summary: Disassociational Scaling and Adaptive Radiation
The Core Concept: A new evolutionary theory posits that the initial physical divergence between species is driven by evolutionary changes in body size rather than early competition for resources.
Key Distinction/Mechanism: Traditional models emphasize character displacement, where competition drives trait differences. In contrast, this theory highlights disassociational scaling, where an evolutionary shift in body size causes traits, such as tail length or body girth, to decouple from ancestral developmental patterns prior to interspecies competition.
Origin/History: UC Riverside evolutionary biologist Bryan Jennings developed the theory in 2020 while reevaluating geometric scaling research by Stephen Jay Gould and his own doctoral data on Australian pygopodid lizards (such as Delma fraseri and Lialis burtonis). It was validated by data from over 1,000 museum specimens in 2023 and published in the Evolutionary Journal of the Linnean Society.
Major Frameworks/Components:
- Adaptive Radiation: The evolutionary process in which a common ancestor diversifies into multiple species occupying distinct ecological niches.
- Disassociational Scaling: A process where trait development unlocks from overall body size, resulting in significant changes to trait proportions.
- Re-adaptation: A secondary evolutionary stage where natural selection refines and restores functionality to traits that lost efficiency due to disassociational scaling.
- Niche Scaling: The pathway by which disassociational scaling creates novel trait combinations, allowing a population to exploit new resources and niches before encountering competition.
Branches of Science: Evolutionary Biology, Evolutionary Morphology, Organismal Biology, Evolutionary Ecology, and Herpetology.
Future Application: This theoretical framework offers predictive models for species adaptations, aiding researchers in mapping evolutionary trees and deciphering the fossil record of rapid diversification events, such as those of Darwin's finches or the extinct Irish elk.
Why It Matters: This theory reorders the established sequence of adaptive radiation, shifting the primary catalyst for initial physical divergence from external species interactions to internal scaling mechanisms.
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| Delma fraseri, a Pygopodid lizard. Photo Credit: Bryan Jennings / University of California, 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 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.”
From Body Size to Trait Divergence
Jennings’s 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 dataset 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. Natural selection can then act over longer evolutionary periods to fine-tune the trait and restore or improve its functionality. Jennings calls this second stage readaptation.
The distinction is important, he explained, because it shifts the proposed sequence of events. Rather than competition being responsible for the initial divergence of traits, competition can occur later, during the process of refining differences that have already emerged.
“In other words, competition between species is not necessary for the initial divergence,” he said.
Evidence from Australian Lizards
Jennings developed his theory in 2020 while revisiting data from his doctoral research on pygopodid lizards, a group of snakelike lizards found primarily in Australia and New Guinea.
Pygopodids have elongated bodies and greatly reduced limbs. Two of their traits—tail length and body girth—are important for locomotion and escaping predators, making them useful traits for studying the relationship between body form and function.
During the COVID-19 pandemic, Jennings also revisited influential work on geometric scaling and evolutionary form, including research by evolutionary biologist Stephen Jay Gould, leading him to wonder if geometric scaling could describe the earliest stages of evolutionary divergence—when one population splits into two and one of them evolves a different body size.
To test the idea more rigorously, he obtained funding to conduct research in Australia. During a forty-day trip in 2023, he visited six museums and collected measurements from more than 1,000 preserved lizard specimens, including body length, body girth, and tail length. He followed this with a visit to the Museum of Comparative Zoology at Harvard University to measure lizards in its collection.
The expanded dataset allowed him to examine differences between populations and among closely related species. His analysis provided support for his model’s second stage, readaptation. Evidence from the initial stage, however, led him to revise his original hypothesis.
“The data suggested that the initial divergence was often more extreme than geometric scaling alone would predict,” he said. “This led me to replace geometric scaling as the first step of the model with disassociational scaling, which encompasses several ways the development of a trait can become unlocked from changes in overall body size, with geometric scaling being just one form of this broader category.”
Possible Pathway to New Ecological Niches
Jennings’s proposed mechanism could have implications for understanding how adaptive radiation begins.
“If changes in body size produce substantial changes in the proportions of traits, those changes could sometimes result in trait combinations that allow a population to exploit a different ecological resource,” he said. “I call this proposed process ‘niche scaling’ when it occurs in the context of adaptive radiation.”
Under this scenario, populations could begin developing differences that correspond to different ecological niches before they live together and compete for the same resources.
“Competition could subsequently refine those differences through natural selection,” Jennings said. “This differs from the conventional sequence in which competition is viewed as a major driver of the initial divergence in ecological traits.”
Jennings said his theory is intended to complement, rather than replace, established findings about adaptive radiation. He does not dispute the role of competition and natural selection in shaping species differences.
“Instead, I’m proposing that an important part of the story occurs earlier—when one population first begins diverging from another,” he said.
Jennings emphasized that additional studies will be needed to determine how widespread these mechanisms are in nature.
“My theory more fully accounts for beak evolution in the iconic Darwin’s finches and antler evolution in the enigmatic, extinct Irish elk,” he said. “We still need to determine how widespread disassociational scaling and niche scaling are across different groups of organisms.”
Published in journal: Evolutionary Journal of the Linnean Society
Title: A scaling theory of trait evolution
Authors: W Bryan Jennings
Source/Credit: University of California, Riverside | Iqbal Pittalwala
Edited by: Scientific Frontline
Reference Number: ebio110326_01
