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Coexisting lizards challenge what we know about natural selection

In a world where change is often considered the only constant, many species surprisingly remain the same over long spans of time. Traditional biology explains this phenomenon by suggesting that natural selection favors moderate characteristics. This means that creatures with more extreme features might not fare as well as their more average counterparts. 

However, a new study led by Washington University in St. Louis is shedding new light on our understanding of evolution in species that live in close proximity to each other. 

According to the research, these species occupy distinct “fitness peaks,” which can best be described as part of a wider community “fitness surface” or landscape. This study fundamentally alters our perception of how species relate and remain distinct over time.

Lizards and natural selection

The experts closely observed the survival of various lizard species over time. Their results challenge the conventional wisdom that species undergo little change due to natural selection favoring moderate characteristics. 

“If species are adapted to their environment, and the environment doesn’t change, then you wouldn’t expect the species to change. However, when scientists have gone out and studied natural selection, they rarely find evidence of such stabilizing selection,” explained study co-author Professor Jonathan Loso. 

“Given this disconnect, we set out to study natural selection on the organisms we know so well, Anolis lizards, to measure selection over several years and try to understand what’s going on.”

Focus of the study

The research was focused on Anolis lizards living on a small island within the Fairchild Tropical Botanical Garden near Miami. This site was identified by James Stroud of Georgia Tech, who was working as a postdoctoral researcher in Losos’ lab at WashU at the time.

Stroud and his team caught, tagged, and measured thousands of individual lizards, closely monitoring their survival and physical traits over 2.5 years (two to three generations of these lizards). This data allowed for a detailed analysis of survival in relation to physical traits such as leg length.

Coexisting lizard species 

“What is special about this study is that we simultaneously measured natural selection on four coexisting species, something that has rarely been accomplished,” said Losos, who also serves as the director of the Living Earth Collaborative. 

“By coincidence, just as our paper was published, another group published a similar study on Darwin’s famous finches of the Galapagos Islands.”

What the researchers learned 

In the Florida lizards, Losos and Stroud found that the stabilizing form of natural selection – that which maintains a species’ average features – was extremely rare. In fact, natural selection varied massively through time. 

Some years, lizards with longer legs would survive better, and other years, lizards with shorter legs fared better. For other times, there was no clear pattern at all.

“The most fascinating result is that natural selection was extremely variable through time,” said Stroud. “We often saw that selection would completely flip in direction from one year to the next. When combined into a long-term pattern, however, all this variation effectively canceled itself out: species remained remarkably similar across the entire time period.”

Implications of the study

Professor Losos elaborated on the significance of these insights. He emphasized that even though species might appear unchanged, evolution is consistently at work beneath the surface. 

The variability in selection was emphasized as a particularly intriguing outcome of the study, underscoring the complex and dynamic nature of evolutionary processes.

“Evolution can and does happen – it’s this ongoing process, but it doesn’t necessarily mean things are constantly changing in the long run,” said Stroud. “Now we know that even if animals appear to be staying the same, evolution is still happening.”

The study is published in the Proceedings of the National Academy of Sciences.

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