Sunday, November 18, 2012

Natural selection can happen quickly!

Take an island in the Bahamas with a population of anole lizards (this lovely guy called Anolis sagrei, we’ll call him Andy).

Bahamas location, map: islandbrides.com
Andy the anole (Anolis sagrei), photo: Ianare Sevi


Males like Andy attract females by displaying, and those with larger, brighter dewlaps (that flap under his chin) are usually most successful breeders. However, the brighter display may also attract the attention of predators.

The Predator (Leiocephalus carinatus), photo: Ianare Sevi
Now add a predatory terrestrial lizard called Leiocephalus carinatus that loves to eat anoles, and you can see the results of natural selection in a single generation.

Scientists actually did this: they introduced the larger lizard to 6 tiny Bahamian islands (and studied 6 others with no “invasion” by the large lizard).

They found that:
  • On “invaded” islands, about half of the anoles survived the new threat. Survivors had longer (faster-moving) legs relative to non-survivors, so guess which genes made it to the next generation?

  • You got it. In the next generation, longer legs were more common.

  • Initially, anoles with longer legs could run faster and escape the new predator. So Andy and the other anoles with longer, faster-moving legs contributed more genes to the next generation than males with shorter, slower-moving legs and almost immediately made up a larger portion of the anole population.
  • the lovely female brown anole
    photo: Matt Edmonds
    the dewlap in action, photo: http://floridainvasiveanimals.pbworks.com

  • Andy and his fellow anoles also started spending more time in trees and shrubs, to avoid the big ground lizard.

  • Shorter limbs are better suited to navigating narrow branches and twigs, so anoles with shorter legs survived and bred more successfully there. This move thus enabled the anoles with shorter legs to contribute more to the subsequent generation, and natural selection switched course, almost equally rapidly.

  • Thus, despite (or maybe because of) the shift in behavior (moving up and away from the ground), the genetic make-up and, subsequently, the physical appearance, of the anoles changed rapidly.

  • These surprising findings demonstrate how natural selection functions even within short time periods and switch direction if the difference in success rates between two versions of a gene is sufficiently great.

  • Andy spots a predator - ready to run?
    photo: http://www.the-lizard-lounge.com

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