What is the unit of evolution, the level of life upon which natural selection acts? A geneticist would say the gene; Charles Darwin saw it in the unique populations on the Galapagos. On Friday, Leticia Aviles, associate professor of zoology at the University of British Columbia, singled out the individual as dividing the cellular from the group level. ?But what an individual is depends on one's frame of reference,? she said, and the level at which natural selection acts remains an unresolved issue. Added Sean Rice, an associate professor of ecology and evolutionary and biology at Yale University, in his commentary, ?Levels at which selection acts is not a philosophical question, but is a property of the biological system. You can't correctly represent evolution if you don't recognize the right level of selection.? Aviles offered an example of selection at the population level in social spiders: when the sex ratio...

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