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of social insect (1859, pp. 202 and 236). Both traits are deleterious to the individuals that have them. Bees that sting intruders to the nest eviscerate themselves; sterile workers have a reproductive success of zero. In each case, Darwin explains the trait’s evolution by pointing out that it is advantageous to the community. Some modern commentators interpret Darwin’s discussion of these traits as anticipating the idea of kin selection, which they view as a type of individual, not group, selection (Ruse, 1980). Others regard kin selection as a kind of group selection and so regard Darwin’s theorizing about barbed stingers and worker sterility as following the same pattern he later used to think about human morality (Sober and Wilson, 1998). For those who prefer the former interpretation, there is an interesting interpretive question: Why did Darwin embrace group selection to account for human morality, but decline to do so in connection with the stinger and the sterility?

Regardless of how one interprets this small handful of examples, it is clear that Darwin invoked group selection hypotheses only rarely. Was this because he thought that group selection occurs more rarely and is a less important cause of evolution than individual selection? In the first edition of the Origin, Darwin (1859, p. 87) does make a general comment about selection’s effect on traits that are good for the group. He says that “in social animals it will adapt the structure of each individual for the benefit of the community; if each in consequence profits by the selected change.” This is not an endorsement of group selection, since traits that are good for the group can evolve by individual selection if they also happen to be good for the individuals who have them. However, in the 6th edition of 1872, Darwin revised this sentence to read: “in social animals it [selection] will adapt the structure of each individual for the benefit of the community; if the community in consequence profits by the selected change (Darwin, 1959, p. 172).” This is an endorsement of the general role played by group selection.

The last facet of Darwin’s concept of natural selection that I want to mention concerns his comment in the Origin (1859, p. 6) that selection is “the main but not the exclusive cause” of evolution. One part of this pronouncement is clearer than the other. The idea that selection is not the exclusive cause of evolution just means that there are other causes. Darwin (1859, pp. 134–139) allows for the Lamarckian mechanism of “use and disuse,” the inheritance by offspring of traits (phenotypes, in modern parlance) because they were acquired by their parents and turned out to be useful. A standard example is the blacksmith’s growing big muscles because of his work and then transmitting these big muscles to his children, who develop those muscles without needing to do what their father did to get them. Darwin also had the idea that descendants retain the traits that their ancestors had, sometimes even though these traits are no longer



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