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Figure%201.%20A%20Click to Enlarge ImageThe process known as group selection was once a central part of evolutionary theory. It seemed obvious that evolution would often favor traits that benefit groups—colonies, flocks, populations, entire species—rather than individual organisms. For example, groups that exercise restraint over their reproductive rate might be supposed to have an advantage over those that overpopulate their territory and quickly exhaust some critical resource. Later theorists recognized a flaw in this reasoning: The evolution of traits that involve sharing or cooperation could be undermined by "cheaters"—individuals who gain the benefits of group membership without contributing to the common welfare. After the 1960s, most biologists avoided explanations based on group selection and tried to describe all evolutionary events in terms of selection at the level of the individual. However, this extreme view gives misleading interpretations of many important biological phenomena. Now a more nuanced theory, generally known as multilevel selection theory, acknowledges competing selective forces within and between groups.

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