New Evolution Theory Explains Why Animals Shrink Over Time – SciTechDaily

A recent study uncovers the factors influencing animal size changes over time, identifying three evolutionary patterns based on competition and environmental pressures, providing clarity on the inconsistencies in fossil records. Credit: SciTechDaily.com

New research reveals key factors behind the changing sizes of certain animals over time, challenging traditional evolutionary theories with its findings on species size variations.

The mystery behind why Alaskan horses, cryptodiran turtles, and island lizards shrunk over time may have been solved in a new study.

The new theoretical research proposes that animal size over time depends on two key ecological factors: the intensity of direct competition for resources between species, and the risk of extinction from the environment.

Using computer models simulating evolution, the study, published today (Thursday, January 18) in the journal Communications Biology, identifies why some species gradually get smaller, as indicated by fossil records.

Dr. Shovonlal Roy, an ecosystem modeler from the University of Reading who led the research, said: Just like how we try to adapt to hot or cold weather depending on where we live, our research shows animal size can get bigger or smaller over long periods depending on the habitat or environment.

In places and times where theres lots of competition between different species for food and shelter, animal sizes often get smaller as the species spread out and adapt to the distribution of resources and competitors. For example, small horses that lived in Alaska during the Ice Age rapidly shrank due to changes in the climate and vegetation.

Where direct competition is less, sizes tend to get bigger, even though being really big and few in number can make animals more vulnerable to dying out such as what happened with the dinosaurs.

Changes in ecological factors help explain why fossil records shows such confusing mixes of size evolution patterns, with some lineages shrinking over time and others growing.

The research team carried out their study by challenging the contradictions fossil evidence posed to Copes rule. Copes rule refers to the tendency for certain animal groups to evolve larger body sizes over thousands and millions of years. The rule is named after Edward Cope, a 19th-century paleontologist who was credited to have first noticed this pattern in the fossil record. For example, early horse ancestors were small dog-sized animals that increased in size over evolutionary time, ultimately producing the modern horse.

However, fossil evidence shows remarkably conflicting trends, with increased size in some groups but decreased size in others.

Using computer models simulating evolution, the study identified three distinct patterns of body-size change emerging under different conditions:

Reference: Ecological determinants of Copes rule and its inverse 18 January 2024, Communications Biology. DOI: 10.1038/s42003-023-05375-z