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Survival of the fittest: Research team finds that population density is key to evolution

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Posted August 28, 2019

A multiuniversity research team including a Florida State scientist has found that a lack of predators creates an environment that is key to evolutionary changes found in guppies.

Florida State University Robert O. Lawton Professor of Biological Science Joseph Travis and a team of researchers have been studying guppies for more than a decade. The team has been studying how this tiny tropical fish offers insight into key factors of evolution and found that population density drives the evolutionary changes found in guppies.

Their study was published in American Naturalist.

Male guppies are very colourful compared to their dull brown female partners. Image credit: Silvana Gericke via Wikimedia (CC BY-SA 3.0)

In 2006, the team transplanted guppies from the bottom of four different streams to the top of those streams. The quality of life for the guppies in the bottom of the streams versus the top was quite different.

“Downstream, your life is nasty, brutish and short,” Travis said. “You mature as fast as you can and reproduce. Upstream, you can expect to live longer and perhaps take your time with life.”

Over the years, guppies have become a textbook example of rapid evolution, with scientists believing the differences between upstream and downstream guppies were a direct response to their different risks of predation. However, Travis and his colleagues found that this was not the case — something else was driving the rapid evolution of upstream guppies.

“In these upstream communities, they’re dealing with a crowded world,” Travis said. “Their evolution turns out to be a response to that crowded world.”

The team was able to make this conclusion because they tracked every aspect of the life of more than 40,000 individual guppies over the past decade, specifically noting how fast they matured and how large they grew. They also carefully tracked ecological changes in these streams.

Travis and his colleague David Reznick from University of California, Riverside developed the idea for the study several years ago.

“He and I designed the experiment so that we could find out what really drives guppy evolution and eliminate all of the competing explanations,” Travis said.

This is natural selection in its purest form. With a higher population, there is less food available. The guppies evolved to handle those changes. One such change involved having fewer offspring.

“You don’t want to waste your reproductive effort on offspring that won’t live,” Travis said.

Travis said these results could be applicable to other animals because this is a pattern seen often in nature.

“Whenever a top predator is removed from an ecosystem, the species that used to be limited by that predator soon find themselves in a very different world,” Travis said. “This happens when we overfish marine ecosystems, when we exterminate wolves and when our attempts at insect control inadvertently remove the insect predators of the pests we were trying to eliminate. In guppies, the results are more pronounced because they evolve so quickly and the team was able to study the process in a controlled experimental fashion.”

Source: Florida State University

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