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the population are called individual heterogeneity, and this creates individual
variation. Many studies have demonstrated individual heterogeneity of indi-
vidual survival and reproductions; for example, Clutton-Brock et al. (1982)
demonstrated that lifetime reproductive success of female red deer (
Cervus ela-
phus
) varied from 0 to 13 calves reared per female. Differences in the frequency
of calf mortality between mothers accounted for a larger proportion of vari-
ance in success than differences in fecundity. Bartmann et al. (1992) demon-
strated that overwinter survival of mule deer fawns was a function of the fawn's
weight at the start of the winter, with larger fawns showing better survival.
Individual variation is caused by
genetic
variation, that is, differences
between individuals because of their genome. Individual heterogeneity is the
basis of natural selection; that is, differences between animals is what allows
natural selection to operate. However,
phenotypic
variation is also possible,
where individual heterogeneity is not a result of genetic variation. Animals that
endure poor nutrition during their early development may never be as healthy
and robust as animals that are on a higher nutritional plane, even though both
are genetically identical. Animals with access to more and better resources have
higher reproductive rates, as in the red deer studied by Clutton-Brock et al.
(1982). Thus individual heterogeneity may result from both genetic and phe-
notypic variation. Lomnicki (1988) developed models of resource partitioning
that result in phenotypic variation of individuals.
Another example of individual heterogeneity in reproduction was provided
by Burnham et al. (1996) in northern spotted owls (
Strix occidentalis caurina
).
In the case of northern spotted owls, repeated observations of reproduction
across numerous individuals were used to estimate individual variation with
analysis of variance procedures. The age of the female produced individual het-
erogeneity. This study also demonstrated temporal and spatial variation in owl
fecundity rates.
Undoubtedly, natural selection plays a role in the genetic variation left in a
declining population. Most populations for which we are concerned about
extinction probabilities have suffered a serious decline in numbers. The geno-
types remaining after a severe decline are unlikely to be a random sample of the
original population (Keller et al. 1994). I expect that the genotypes persisting
through a decline are the “survivors,” and would have a much better chance of
persisting than would a random sample from the population before the decline.
Of course, this argument assumes that the processes causing the decline remain
in effect, so that the same natural selection forces continue to operate.
To illustrate individual variation, start with the basic demographic varia-
tion model developed earlier in this chapter. Instead of each animal having
