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The perpetuation of an interbreeding population, its capacity to evolve or to extend its range, depends upon its gene pool, the aggregate of genes of which every individual bears a selection but none the whole range of them. In contemporary ornithology the term cooperative breeders is applied to a group consisting of a reproductive pair with one or more nonbreeding helpers. In a wider sense, all the members of an interbreeding population, or deme, form a single cooperatively breeding group, providing its progeny with

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mates, preventing debilitating inbreeding by exchanging genes. The number and quality of an individual's offspringits fitness depends upon the quality of the partner in reproduction that the species can provide for him or her.

Interindividual selection helps to maintain, or to improve, the anatomical and physiological quality of a species by removing, by agents the most diverse, defective or substandard individuals. It is adequate to account for the extinction of races or species. In the absence of a natural catastrophe that wipes out a whole population at a stroke, or a drastic climatic or ecological change that extinguishes it, individuals may be eliminated one by one, by predation, disease, or other natural agents, if not by humans, until the last member of a species vanishes from Earth. But individual selection cannot account for the evolution of a race, which depends upon changes in the composition of its gene pool, a process in which many interbreeding individuals participate but to which none can contribute alone.

The foregoing considerations apply to all organisms, plants as well as animals, that can reproduce only by the union of two individuals, or of their sexual cells, their gametes. In certain special situations, it is more obvious that evolution depends upon coordinated genetic changes in interacting individuals. The first is in the field of social relations. Animals attract sexual partners by signs or signals, which may be visual, vocal, olfactory, or a combination of these. If a mutation in the appearance, sounds, or odors of one sex is not supplemented by a complementary mutation in the preference or reaction of the opposite sex, the former will fail to mate and leave progeny, with the result that its mutation will disappear from the gene pool. The gorgeous plumage of many male birds, which since Darwin has been attributed to sexual selection, could not have developed if the preferences of females had not evolved in the direction of the changes of the males' attire. Cooperation of a male and female in rearing their young, the two sexes playing complementary rather than identical rolesas in many birds and fewer mammalscould hardly have been perfected without coordinated evolution in an interbreeding population. The calls or other

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signals by which animals alert their companions to approaching danger would be meaningless if signal and response did not evolve together in a group of related animals. Releaser and innate releasing mechanism, social interactions the most diverse, point to the natural selection of groups as well as of individuals.

In recent decades ornithologists have been discovering an increasing number of avian species that breed in closely knit groups of parents and their self-supporting offspring, who aid their elders in defending the territory, feeding and protecting their younger siblings, and often, too, in building the nest and incubating the eggs. Sometimes the family is joined by individuals less closely related. Some of the species in which cooperative breeding is widespread can breed successfully as unassisted pairs; others cannot. Among the latter are White-winged Choughs in Australia, Yellow-billed Shrikes and apparently also White-browed Sparrow-Weavers in Africa, in all of which pairs without helpers raise so few young, or suffer such high mortality, that their species would become extinct in the absence of cooperative breeding (Skutch 1987). In these cases it is