4.5: Group selection

A alternative to inclusive fitness (kin selection) is the concept of group selection. In this scenario, small groups of organisms of the same species are effectively acting as single (perhaps colonial) organism. It is the reproductive success of the group, rather than the individuals within the group, compared to other groups that is the basis of selection. In certain situations, groups that display cooperative and altruistic traits may have a selective advantage over groups that do not. It has been claimed that group and kin selection are mathematically equivalent, even though one occurs between population groups and the other within a population group.^[191] The costs of a trait must be offset by the benefits, but now the key factor is membership in a group that tend to be closely related to one another. The life cycle of the bacterium Myxococcus xanthus provides an example of this type of behavior. When environmental conditions are harsh, the cells aggregate into dense, 100 μm diameter “fruiting bodies”, each containing ~100,000 stress resistant spores. When the environment improves, and nutrients become available, the spores are released en mass and return to active life. They move and feed in a cooperative manner through the release of digestive enzymes that, because they are acting in a quorum mode, can reach high functional levels.^[192] A well-coordinated group is expected to have a significant reproductive advantage over a more anarchic collection of individuals.

While their functional roles are different, analogous types of behaviors are seen in flocks of birds, schools (or shoals) of fish, swarms of bees, blooms of algae, and groups of slime mold cells (→).^[193] Each of these examples represents a cooperative strategy by which organisms gain a reproductive advantage over those that do not display the behavior. While the original behavior is likely the result of kin selection, in the wild it is possible that different groups (communities) are in competition with one another, and the group(s) that produces the most offspring, that is, the most reproductively successful group will come to dominate.

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