Friday, November 6, 2009

Group Selection

Can evolution select attributes "for the good of the group," even if they're bad for individuals? That is the essential notion of group selection, which has been a highly controversial area of evolutionary theory. In fact, some evolutionary biologists insist it never happens.

David Sloan Wilson of Binghamton University disagrees, in an interesting series that he calls Truth and Reconciliation in Group Selection. Originally at Huffington Post, he's re-posting the series in his new blog location at ScienceBlogs.

The author of the very readable Evolution for Everyone: How Darwin's Theory Can Change the Way We Think About Our Lives, Wilson is a long-standing proponent of group selection in some circumstances. But his chosen title evokes his contention that opposition to even the possibility of group selection has become a kind of dogma, in stark contrast to the ideal of scientific hypothesis testing.

From the time Darwin introduced it until the 1960s, Wilson says, group selection was taken seriously. But then W.D. Hamilton devised a model that explained why evolution would favor acts that benefit close relatives. As long as the benefit to the relatives, multiplied by the fraction of shared genes, exceeds the cost to the individual, he argued, an action would make propagation of the genes more likely.

But a successful model based on kinship doesn't rule out the possibility of other successful explanations.

Clearly, group selection requires that the group advantage mathematically outweigh the cost to the individual. So the details matter; the existence of one model in which individual needs triumph, such as John Maynard Smith's influential "haystack model," doesn't prove that group needs would not triumph under other circumstances. Nonetheless, Wilson says, the field adopted the kinship model as the only way that evolution could produce altruistic behavior.

Wilson and others have since created models in which group selection works. A theory that encompasses both kinship and other drivers of group selection was developed by the science writer George Price, as described in this 2008 article from Current Biology. Hamilton himself accepted this more general framework for balancing group and individual needs.

Experiments have also demonstrated the effect. Wilson recalls an experiment where researchers artificially selected chickens for greater egg production, according to two different criteria. They found that choosing the individual hens in a cage that laid the most eggs resulted, over generations, in less egg production than selecting cages that together laid the most eggs.

There is no question that selection "for the good of the group" has been carelessly used to motivate "just-so stories" for all sorts of traits. But it seems equally clear that there are examples of altruistic group behavior that does not derive from shared genes. Even the special teamwork that characterizes social insects, for example, still occurs in species like termites that don't have the highly-shared genes that we know from bees.

A comprehensive understanding of how evolution works in complex situations is critical to many areas of biology, such as the inference of biological function from evolutionary constraint. It is troubling to read Wilson's description of a field so mired in prejudice and tradition that it fails to fairly evaluate and incorporate valid new evidence.

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