The mating habits of a small lizard are challenging a central tenet of evolutionary theory. One of the best explanations for why animals cooperate is that they help each other because they are closely related, a concept known as kin selection. It now appears that this isn't always the case. Male side-blotched lizards cooperate with other males who are genetically similar, but are not related. It's a subtle but extraordinarily significant difference. Because the behaviour can't be explained by kin selection, biologists may have to radically redefine the theory, or invoke a new form of evolutionary selection to explain why the lizards act this way. It even suggests a new mechanism by which "selfish" genes propagate themselves.
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