by Whit Gibbons

September 23, 2007

Charles Darwin's ideas of natural selection, survival of the fittest, and evolution, which means "change," are elegant in their simplicity. Why anyone interested in understanding the way the world works would try to challenge the easily understandable basic concepts of the process is not clear. Following is a simplistic example of evolution, showing how change in a lizard species could hypothetically occur in a natural manner. Natural selection operates because some members of a population have a trait that makes them more likely to survive than others.

In the simple example, imagine a species of lizards living on an island in which most individuals are bright yellow but a rare form of the same species is dull green. The color is caused by genes that produce either green or yellow, but because the green individuals are so rare, the species would be considered to be a population of yellow lizards. The lizards live peacefully on the island for centuries. Eventually, a type of hawk flies over from the mainland, and over time the bird predator begins to inhabit the island.

The hawks discover that when flying over the thick, dark green vegetation of the island they see bright yellow lizards. If a green lizard and a yellow one are sitting side-by-side when a hawk flies over looking for a lizard meal, the yellow lizard will stand out like a beacon on the green bush and will be the one to become a meal for the hawk. The green lizard will escape detection and be more likely to survive, consequently proving to be "fit" because the green genes are the ones passed on to the next generation. As the natural process in which hawks select and remove the yellow lizards continues, proportionately more green genes are inherited than are yellow ones. Eventually, the hawks will practically eliminate the yellow lizards while green lizards survive, to the point that the island lizards have evolved to become a population of green lizards. Basic evolution has prevailed.

But evolutionary processes can be far more complicated. One concept that has been particularly difficult to understand when using the simple explanation has been that of the frequently observed phenomenon of "helper" birds. Helpers are non-reproductive birds that help feed another couple's young. For example, red-cockaded woodpecker colonies often have young males that help feed the young but do not themselves mate with the females. The observation of helper birds seemed to contradict the evolutionary principle that the most biologically fit individuals were those that passed their genetic material on to the next generation.

Helpers appear to be altruistic, engaging in a charitable act with no expectation of a reward. But most evolutionary biologists will tell you that strict altruism does not exist as a permanent and persistent trait in any species (including humans) because natural selection would ultimately not favor individuals who invest in making it easier for someone else to survive without a benefit to themselves.

But about a half-century ago an English biologist named William Hamilton proposed a theory that explained how seemingly altruistic acts fit into the evolutionary scheme. Being a helper fits quite well into an individual passing on his genetic line, if the helpful acts are directed toward close kin. Siblings on average share half the genes inherited from their parents, so assisting the parents in successfully reproducing offspring that are genetically close to itself, an individual increases the chances that some of its own genes are being passed along. The same principle applies to a lesser degree with cousins, which also share genetic material. The overall phenomenon has been called "kin selection," meaning that natural selection is operating but that genes are being passed along by close kin rather than by the individual directly.

Evolutionary biology is challenging because when scientists unravel one mystery of life, such as why helper birds fit Darwin's ideas, another phenomenon calls for explanation. A recent study in Australia addressed the inexplicable observation in which helper birds did not appear to "help." That is, nestlings did not grow faster or get larger whether or not helpers were present to assist the parents with feeding.

Next week: a blue wren provides the answer.

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