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Lizard’s evolution keeps ancestors close

UC DAVIS / DUKE (US) — A devastating 2004 hurricane that wiped out a Caribbean lizard population offered an unprecedented opportunity to put an evolutionary theory known as the “founder effect” to the test.

The founder effect describes the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. The extent to which it contributes to evolution has been up for debate since the early 1940s, when German evolutionary biologist Ernst Mayr first outlined it.

Some scientists argue that the founder effect is pivotally important in how a species evolves, but others say it is a bit player on the evolutionary stage, quickly overwhelmed by the forces of natural selection.

httpv://www.youtube.com/watch?v=fAgxpB9fn50

In a new paper published in Science Express, scientists from the University of California, Davis; Duke University, and Harvard University suggest that both sides are right.

Complicating the debate has been the dearth of data from nature: Founder events are rarely observed.

“Founder effects are very hard to study,” says Thomas Schoener, a professor of evolution and ecology at UC Davis and a co-author of the study. “One must be in exactly the right place at the right time to observe the founder event—and then fortunate enough to be able to follow a population through time.”

That opportunity presented itself in September 2004, when Hurricane Frances submerged several small, low-lying islands near Great Abaco, Bahamas. Before the hurricane, the islands supported populations of a Caribbean lizard, the brown anole, Anolis sagrei. After the hurricane, seven of the islands were thoroughly searched but none of the lizards were found.

In May 2005, researchers randomly selected one male and one female brown anole from lizards collected on a nearby larger island to found new anole populations on the seven small islands. During the next four years, the researchers repeatedly sampled lizards from the source island, from the seven experimental founder islands, and from 12 nearby islands that served as a control.

All the lizard populations adapted to their environment, yet retained characteristics from their founders. For instance, lizard limb length correlates with the average diameter of vegetation on an island. Because the founder islands had smaller vegetation than the source island, the length of lizard limbs decreased, as expected, due to natural selection.

But islands containing lizards with the largest limbs at the beginning of the study still had the lizards with the longest limbs at the end of the study. “Natural selection drives them all down, while the founder effect keeps the order the same,” Schoener says. “So they’re both right, in a sense.”

If natural selection had overpowered the founder effect, lizards’ limbs would have converged at the same length, regardless of how long-legged the founders were. Instead, limb length decreased roughly in parallel, signifying the persistence of the founder effect.

“Our study is an entirely unique approach to a question of longstanding importance for evolutionary biology regarding the founder effect: Will it persist in the face of the strong selection that would often exist in the colonized environment?” asks Schoener.

“The answer we found is that founder effects can leave a persistent signal as generations replace one another over time, even as populations adapt to new conditions. Our study of these fundamental evolutionary principles affects our general understanding of how the biological world works.”

The study was funded by the National Science Foundation.

More news from UC Davis: http://www.news.ucdavis.edu/

More news from Duke: http://today.duke.edu/

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4 Comments

  1. Roy Niles

    Why would you expect the limbs to get shorter if they worked fairly well as they were, and that evolution, even if by adaptive mutation, would occur in a short period of years instead of in a least a decade or more.
    These are multi celled and versatile creatures, not bacteria. Also, the pre-adaptive functions of any population are developed over time, and if you start over with only two individuals, the largest reservoir of optional building blocks is just not there.

  2. Rob N

    I agree with Roy. I don’t know what the life cycle of the anole lizards is, but I wouldn’t end the experiment until at least 50 generations have gone by (or limb length, etc. has stabilized). Maybe limb length can only change by some % each generation, regardless of how big the founders are. Also, having 3 or 4 founder pairs per island would be interesting, and probably beneficial in avoiding in-breeding problems. Admittedly, results are easier to interpret with just one founding pair.

    On the plus side, whoever’s studying lizards on the beaches of isolated Caribbean islands has got to be loving their job.

  3. Kk

    I agree with both of them how can you get shorter limbs if They are long too start with. Plus the evolutionary process. The hurricane cause that much damage how can the process be excluded in which limb is longer or shorter. This is because you start with 2 brown lizards and they produce the offspring so the biologist come back and study the results on how they are developing.

    It’s just how 2 brown lizards are study for the evolutionary nation of the hurricane and different islands.
    Studying the lizards is what the biologist are paid to do. Evolution is common to natural selection .
    Limbs can’t change I don’t think. Limbs stay one way forever.

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