Unlocking the Mystery: New Evolution Theory Explains Why Animals Shrink

 

A recent study might have unraveled the mystery behind why Alaskan horses, cryptodiran turtles, and island lizards shrank over time.

The new research suggests that the size of animals over time is influenced by two crucial ecological factors: the intensity of competition for resources between species and the risk of extinction from the environment.

The study, led by Dr. Shovonlal Roy, an ecosystem modeller from the University of Reading, used computer models to simulate evolution. The findings, published in Communications Biology on Thursday, 18 January, shed light on why certain species gradually become smaller, as indicated by fossil records.

Dr. Roy explained, "Similar to how we adapt to hot or cold weather depending on where we live, our research shows that animal size can change over long periods based on the habitat or environment."

"In places and times where there's intense competition between different species for food and shelter, animals tend to get smaller as they spread out and adapt to the distribution of resources and competitors. For instance, small horses in Alaska during the Ice Age rapidly shrank due to changes in climate and vegetation."

"On the other hand, where direct competition is less, sizes tend to get bigger. However, being really big and few in number can make animals more vulnerable to extinction, as seen with dinosaurs. Changes in ecological factors help explain the confusing mix of size evolution patterns seen in fossil records."

The study challenged the contradictions posed by fossil evidence to "Cope’s rule," which refers to the tendency for certain animal groups to evolve larger body sizes over thousands and millions of years. Despite Cope's rule, fossil evidence shows conflicting trends, with increased size in some groups but decreased size in others.

The research team, using computer models simulating evolution, identified three distinct patterns of body-size change under different conditions:

1. **Gradual size increase over time:** This occurs when competition between species is determined mostly by their relative body sizes rather than niche differences. For example, various genera of marine animal species gradually increased in size over millions of years.

2. **Size increase followed by extinctions:** In this scenario, the largest animals frequently go extinct, creating opportunities for other species to evolve even bigger bodies. Large-bodied apex predators are particularly vulnerable to mass extinctions.

3. **Gradual size decrease over time:** The simulations predicted a contrary pattern to Cope's rule – species shrinking over time. This happens when competition is high, and there is a degree of overlap in habitat and resource use. As species evolve into distinct niches, they face evolutionary pressure to reduce in size. Previous reports have noted a decline in size for vertebrates, bony fish, cryptodiran turtles, Alaskan Pleistocene horses, and island lizards.

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