demographic variation between individuals, even when those individuals are in the
peak of health and the environment is entirely favorable.We have seen that the demographic behavior of a population in a constant environ-
ment is broadly predictable when it contains several hundred individuals. The larger
the population the tighter the correspondency between actual rate of increase and
the expected deterministic rate of increase. However, individual variation is by no
means the most important source of variation in r. Year-to-year variation in envir-
onmental conditions has a more profound effect and, unlike the effect of individual
variation, does not decline with increasing population size. It is called environmental
stochasticity.
The most important source of environmental variation is yearly fluctuation in weather.
Weather has a direct effect on the demography of plants, invertebrates, and cold-
blooded vertebrates. Their rates of growth are often a direct function of temperature
as measured in degree-days. Wildlife is largely buffered against the direct effect of
temperature and humidity, the influence being indirect through food supply.
We denote as Var(r)ethe variance in rcaused by a fluctuating environment. It can
be measured as the actual year-to-year variance in rexhibited by a population whose
size is large enough to swamp the effect of variance in rdue to individual variation.
We recommend that such a population should contain at least 5000 individuals. Even
so, Var(r)ewill be overestimated because its measurement contains a further com-
ponent of variance introduced by the sampling variation generated in estimating the
year-to-year rates of increase.
The major influence of environmental variation on the probability of extinction is
its interaction with the effect of individual variation. Thus it becomes progressively
more important with decreasing population size, even though its average effect on r
is independent of population size.In the next few sections we examine some of the ways in which genetic malfunction
may contribute to the extinction of a population. But first we provide a brief intro-
duction to population genetics for those who have not studied it previously. Those
who have can skip to Section 17.4.A chromosome may be thought of as a long string of segments, called loci, each locus
containing a gene in paired form. The two elements of that pair, one contributed by
the individual’s mother and the other by its father, are called alleles and they can be
the same or different. The chromosomes of vertebrates and vascular plants contain
around 100,000 loci.
Suppose the gene pool of a population contains only two alleles for locusA. These
will be referred to as A 1 and A 2. Any individual in that population will thus have one
of three combinations of alleles at that locus: A 1 A 1 or A 1 A 2 or A 2 A 2. If the first or third
combination obtains, the individual is homozygousat that locus, if the second het-
erozygous. The proportions of the three combinations in the population as a whole
are called genotypic frequencies. Which will be the most common depends on the
frequencies (proportions) of the two alleles in the population as a whole. Suppose
the frequency of the A 1 allele is p=0.1 and therefore thatA 2 is q=0.9 (because the
sum of a complete set of proportions must equal 1), then the frequencies of the three
genotypes will be:CONSERVATION IN THEORY 291
17.2.2The effect
of environmental
variation
17.3 Genetic problems contributing to risk of extinction
17.3.1
Heterozygosity