Even if two individuals have identical phenotypes, their future longevity and reproduction will generally be different. Chance processes unique to the individual make this outcome likely. For example, chance effects may lead to death of some individuals, but not others, mates for some, but not for others, and different foraging sucess for different individuals. These outcomes are likely regardless of the phenotypic similarities of the individuals involved. This variation is called within-individual variation (Chesson 1978, 1991) or demographic stochasticity (May 1973). Why "within-individual?" The idea is that the variation is unique to the individual, but is unpredictable, given the phenotype. Depending on the event in question, e.g. foraging success, the variation is a continuing phenomenon for the indvidual. It is unpredictable each time an individual embarks on foraging---its success changes unpredictably, even though the long-run average for an individual may be predictable given its phenotype and the environment. Over short spans of time, due to within-individual variability, we expect different individual's to have different foraging success even when they have the same phenotype, and occupy the same environment, but over the long run, individuals with same phenotype and same environment would be expected to have similar aggregate foraging success, and thus not differ from one another due to this cause. If they share the same environment, they would only differ in long-run aggregrate foraging success due to differences in phenotype, or between-individual variation. Events like death only happen once, but it general, even perfect knowledge of the environment and the individual's phenotype does not predict the time of death. This uncertainty therefore is classed as within-individual variability. It cannot easily be distinguished from between-individual variation in the absence of knowledge of the phenotype, but if phenotypes are known, within-individual variation, even in one-off events like death, can be separated from between-individual variation.
Chesson, P. 1991.
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May, R. M. (1973).
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