Lizards Help Explain Survival of the Not-So-Fittest
for National Geographic News
|November 24, 2004|
Glance at a crowd at just about any big sporting event and you'll notice that humans are a diverse bunch. Not only the fittest have survived.
Natural selection depends as much on behavior and environmental conditions as it does on physical prowess, as demonstrated by two studies of lizards in tomorrow's issue of the journal Nature.
"People have a very simple view of how natural selection shapes traits," said Barry Sinervo, an evolutionary biologist who was not involved in the studies. A biologist at the University of California, Santa Cruz, Sinervo said the studies demonstrate the complexity of evolution.
"When behavior or physiology complicates what traits are involved, the results of natural selection are often thwarted or attenuated when compared to our naive perspective," he added.
In one study biologist Jean-Francois Le Galliard and colleagues show how environmental conditions like the abundance of food govern which traits evolve.
Based at the Centre for Ecological and Evolutionary Synthesis at the University of Oslo in Norway, Le Galliard's team found that in food-rich environments, physical prowess is of little advantage to lizard survival. But in environments with scarce food, lizards with high endurance had a better chance of making it to adulthood.
In the other study biologist Jonathan Losos and colleagues shed light on the unpredictable role of behavior in shaping the course of evolution.
"Animals are incredibly dynamic organisms, and the interplay between behavior and environmental change is a complex one which will be hard to predict without extensive knowledge of the organism," said Losos, who teaches at Washington University in St. Louis, Missouri.
Le Galliard and his colleagues wanted to understand why several species, including humans, have a wide range of running abilities, even though the theory of natural selection suggests that weak runners ought to be weeded out from the gene pool.
Le Galliard's team compared the running abilities of the common lizard (Lacerta vivipara) from environments with both ample and limited food supplies. They found that endurance at birth is of little consequence among lizards with plenty to eat, but it is a factor where food is scarce.
In environments where food was ample, lizards that were born with weak endurance caught up with the high-endurance lizards after a few months, the researchers found. In areas of scarce food, though, endurance is often a life-or-death factor.
Le Galliard and his colleagues are uncertain how the low-endurance lizards in the food-rich environment were able to catch up with the high-endurance lizards. But they suggest it is related to a trade off between endurance and some other trait.
"We found that individuals that caught up quickly after birth also grew faster, suggesting that they allocated resources [to] traits enhancing endurance," Le Galliard said.
The finding challenges the strong link between running ability and Darwinian fitness by showing that running ability is part of a more complex set of fitness traits that determine which individuals survive, according to Le Galliard.
"I think it is a really nice demonstration of the role of the environment in confounding the potential role of selection," Sinervo said.
Losos and his colleagues wanted to settle a debate over whether species can prevent natural selection by changing their own behavior.
To find out, the team introduced the large, ground-dwelling predatory curly-tailed lizard (Leiocephalus carinatus) to six tiny islands in the Caribbean occupied by the small, ground-dwelling brown anole lizard (Anolis sagrei).
(Losos dismisses any concerns that the introductions would throw off the tiny islands' natural balance. The curly-tails are native to adjacent, larger islands and occasionally colonize the small islands naturally. The introductions thus mimick a natural process, he said. And since hurricanes periodically roll through the area and completely clear off the islands, there are no long-term impacts from these introductions, he added.)
The curly-tails will eat any brown anoles they can catch and fit into their mouths. This led Losos' team to speculate that the brown anoles would do one of two things. They might move into the trees and out of reach of the curly-tails, or they might begin to "select" for larger, harder-to-eat bodiesmeaning that smaller anoles would be eaten, with the result that future generations of anoles would be larger than previous generations.
According to the results of the study, both happened. The brown anoles began to spend more time up in the trees. And since they also spent some time on the ground, successive generations had larger bodies and longer legs.
"Our interpretation is that the lizards exhibited a behavioral change to avoid the curly tails, but the change was not sufficient to get them out of harm's way," Losos said.
His team predicts that in the future the brown anoles will move higher and spend more time in the trees in order to avoid the curly tails. This behavioral response should force brown anoles to evolve shorter, more nimble limbs in order to survive on the small branches, he said.
Sinervo said the Losos team's study shows that natural selection resulting from "traits in which behavior is operating are even less straightforward than the physiological avenues demonstrated by Le Galliard."
Losos' research was supported in part by a grant from the National Geographic Society's Committee for Research and Exploration.
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