A new study challenges traditional theories of evolution with findings about the size diversity of species, revealing key drivers of size changes in certain animals over time. The mystery of why Alaskan horses, cryptodiran tortoises and island lizards have declined over time may have been solved by a new study.

A new theoretical study suggests that the size of animals 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.

Research results and environmental factors

The study, which used computer models that simulate evolution, was published in the journal today (Thursday, January 18). Communication BiologyIt determines why some species are becoming increasingly scarce, as the fossil record shows.

Ecosystem modeler Dr. from the University of Reading, who led the study. Shovonlal Roy said: “Just as we adapt to warmer or colder weather depending on where we live, our research shows that animals can grow and shrink in size over long time periods and periods depending on the habitat or environment.

“In places and times where there is strong competition for food and shelter between different species, animal sizes often decrease as species spread and adapt to the distribution of resources and competitors. During the ice age, for example, the number of small horses living in Alaska declined rapidly due to changes in climate and vegetation.”

“Where there is less direct competition, size tends to increase, although animals become really big and small, as was the case with the dinosaurs, making the animals more vulnerable to extinction.

“Changes in environmental factors help explain why the fossil record shows such a confusing mix of size evolution, with some lineages shrinking and others growing larger over time.”

Cope’s rule

The research team carried out their work by questioning the contradictions of fossils regarding the “Cope rule”. Cope’s rule refers to the tendency of certain groups of animals to evolve larger body sizes over thousands and millions of years. The rule is named after 19th-century paleontologist Edward Cope, who is believed to be the first to notice this pattern in fossils. For example, the early ancestors of horses were small, dog-sized animals that increased in size over evolutionary time, eventually giving rise to the modern horse.

However, fossil remains show extremely contradictory trends: an increase in the size of some groups and a decrease in the size of others.

evolutionary pressure

Using computer models simulating evolution, the study found three distinct patterns of body size change that occurred under different conditions:

• Gradual increase in size over time: this occurs where competition between species is determined mostly by relative body size rather than niche differences. For example, various marine animal species (such as invertebrates) have gradually increased in size over millions of years.

• Size increase and subsequent extinction: Here the largest animals die periodically, opening opportunities for other species to replace them and develop even larger bodies, continuing the cycle. Large-bodied predators were most affected by mass extinctions. Very large mammals and birds, such as dinosaurs and giant flying reptiles, are particularly at risk of extinction.
• Gradual reduction in size over time: The simulations also predicted the opposite of Cope’s rule: species decline over time. This occurs when competition is high and there is some overlap in habitat and resource use. As species evolve into different niches, they face evolutionary pressures to reduce their size. Declines have previously been reported in vertebrates, bony fish, cryptodiran turtles, Pleistocene Alaskan horses, and island lizards.