Blackwell Publishing

The units of selection - Does natural selection ever favor groups, rather than individuals?


Can individual selection within the group be overcome by selection between groups?

The question is highly important, both conceptually and historically. It is important historically because vague group selectionist thinking - particularly in the form of statements like 'adaptation X exists for the good of the species' - was once common. It is now more usual (though by no means universal) for biologists to believe that group selection is a weak and unimportant process.

There are no definite examples of adaptations that need to be interpreted in terms of group advantage: Williams (1966) argued that the characters that Wynne-Edwards had suggested evolved to regulate population size can all be explained as adaptations that benefit individuals. Individuals generally reproduce at the maximum rate they can; the only obvious exceptions concern genetically related individuals, and can be explained by kin selection. Moreover, living things have characteristics that contradict the theory of group selection. The 50:50 sex ratio, which we discussed in the tutorial on Adaptive Explanation, is a case in point. In polygynous species, it is inefficient for the population to produce 50% males, most of whom are not needed. The widespread existence of the 50:50 sex ratio suggests that group selection has been ineffective on this trait.

However, the matter has not been settled finally, and group selection probably operates sometimes. The process is theoretically possible and has been demonstrated in a laboratory with Wade's experiments on flour beetles, Tribolium castaneum (shown opposite). Wade artificially selected for groups that had shown a low and a high fecundity: not surprisingly the reproductive rate in the "low" lines decreased more than in the "high" lines. This decrease is due to group selection: while the beetle types with high fecundity were increasing within each colony, Wade's selection for low fecundity more than outweighed individual selection and average fecundity declined.

Moreover, group selection can have evolutionary consequences even if it never over-rides individual selection. This idea is uncontroversial: the "group selection controversy" in evolutionary biology was concerned with whether group selection could ever cause individuals to sacrifice their own reproductive interests to those of its group.

The conclusion of the controversy was that in nature group selection was rarely likely to over-ride individual selection, and establish individually disadvantageous behavior.

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