The evolutionary advantage of altruism has confused many a researcher. True, altruism—that which confers no obvious reproductive benefits on the giver and may indeed lead to his injury or death,—just doesn't seem to tally with the "nature red in tooth and claw" world of natural selection. And yet it is observed in many species, from ants to humans.
A study drawing on two decades of fieldwork and published this week in the Proceedings of the National Academy of Sciences cites a behavior found in the wild side-blotched lizard as evidence for one evolutionary theory of altruism: Richard Dawkins' "green-beard" hypothesis.
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According to Dawkins's hypothesis, genes are selfish and not bothered by the fitness of the individual bodies that carry them from generation to generation. Altruism between genetically similar individuals who are not related to one another is actually the selfishness of a gene that recognizes itself in a different body and acts to protect the copy that exists in that other body. A "green beard" was Dawkins' metaphor describing how one cooperative organism could recognize its genetic match and join together in a protective alliance that would allow more of their kind to be born.
"A 'greenbeard' is simply a gene that controls three things: a distinct signal that allows the gene to recognize other individuals with the signal, then behaves differently to them, [which is] usually expressed as being nice to them," said Barry Sinervo, lead author of the PNAS report and a biologist at University of California Santa Cruz.
Sinervo and his team studied side-blotched lizards because they could be observed in the wild and they cycle through generations quickly—their average lifespan is only one year. The researchers studied 18 generations of lizards, keeping detailed pedigrees and mapping paternity through DNA testing.
The UC Santa Cruz group discovered three distinct male tribes within the lizard community, each with a unique method for reproductive survival. Orange-throated male lizards are aggressive and build up large amounts of territory. Yellow-throated males are devious and steal females from other males. Finally, blue-throated males survive by joining in pairs to protect their territories.
In some generations, these blue-throated partnerships exemplify mutualism, meaning that both lizards benefit. In others, especially during times when the fierce orange-throated lizard is dominant, the partnership is altruistic, and one blue-throated lizard will often give up his chance at reproducing so that his partner can.
Mapping the genomes of these heroic blue throats, Sinervo and his collaborators found that the partners would share not merely the one gene that controlled the color of their throats, but multiple genes that, they speculate, allowed the two lizards, who were not bound by close kinship ties, to mutually recognize the presence of cooperative genes.
"The genes themselves are actually recognizing a copy of themselves in the other male's genome," said Sinervo. "Blue males end up cooperating by virtue of genes that allow them to self-recognize other males carrying the same genes—at least three genes are required—as well as males that are carrying two blue alleles at the orange-blue-yellow locus. A true blue buddy—or wing-man, if you will."
Rick Grosberg, an evolutionary biologist at the University of California, Davis, called the new report the best illustration of the "green-beard" hypothesis yet.
"Sinervo and colleagues," he said, "have provided important insights into the mechanism by which individuals recognize others that have copies of the same genes underlying cooperation."