Read the following text, paying particular attention to the highlighted words.
Natural selection chooses the "fittest" but the fittest what? For Darwin the answer was clear: the fittest individual organisms. Fitness, for Darwin, meant whatever qualities assisted an organism to survive and reproduce. Components of fitness were qualities like fast running legs, keen eyes, abundant, high quality milk. "Fitness" later became a technical term used by mathematical geneticists to mean "whatever is favoured by natural selection". As a trivial consequence of this, it became possible to argue that "survival of the fittest" is a tautology.
Notwithstanding Darwin's emphasis on individual survival and reproduction, other evolutionists have sometimes thought of natural selection as choosing among larger units: groups of individuals, or species. Restraint in aggression, for instance, has been explained as resulting from natural selection between species: those species whose individual members tore each other limb from limb went extinct. At least in this simple, naive form, such "group selectionism" is now discredited. The 1960s and 1970s saw a reversion among theorists, away from group selectionism, back to the Neo-Darwinian rigour of the 1930s. Evolutionary change comes about through gene substitutions in gene pools and these ordinarily result from differences in genetic effects on individual survival and reproduction. Subtle and indirect ways in which genes might influence their survival were also recognized. For example, worker ants are sterile, but they can still affect the representation of copies of their genes in the gene pool, by favouring the reproduction of their close relatives, such as their mother or their reproductive sisters. In a notable theoretical advance, W. D. Hamilton proposed "inclusive fitness" as a generalization of "Darwinian fitness" which took account of such indirect, kinship effects. The phrase "kin selection" is helpfully used to distinguish this important theory from the discredited "group selection" which it superficially, and misleadingly, sometimes seems to resemble.
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