That genetic mechanisms involving balanced selection pressures play a part in
population dynamics has been advanced by Chitty (1960, 1965). He suggests that, as
the population size increases, a "high density" genotype becomes selected, well
adapted to the social stresses, which high density engenders. This genotype is,
however, less well adapted to the normal selective pressures imposed by adverse
environmental conditions. Severe weather conditions do not alone cause a population
to crash. (Chitty 1957). However, according to Chitty's theory the higher the
population density, and with it the greater the prevalence of high density genotype,
the less severe need be the winter, in order to precipitate a crash.
It is probable that the "high density" genotype will involve a large number of
loci, each making some contribution to the ability of the animal to survive in
crowded conditions. Since the theory also calls for the high density genotype to
be less well suited to adverse environmental conditions, these will tend to select
out those alleles responsible for the high density genotype. Thus, in fact, a system
of balanced selection will exist, and the loci involved will be maintained in a
polymorphic condition.
Although it is unlikely that the Es -1 locus plays a major part in controlling
the population processes, it is interesting to speculate on the possibility. In such
a case the E₁ negative animals constitute Chitty's high density genotype since they
show an increase in frequency when the population is high. That E₁- negative animals
are less well adapted than E₁-positive animals to winter conditions, it is clear from
the results of the preceding chapter.
It may be shown that Chitty's theory appears equally plausible expressed in
reverse. Rapidly increasing populations are generally exposed to fairly limited
selective pressures. For instance, in the first study area described in the previous
chapter, no systematic changes in phenotypic frequencies were observed during the
initial stages of the population increase before the crash. It is probable that
the normal selective pressures are those operative during normal winters. If this
is so, then each year, as the optimum phenotype becomes more common, winter mortality
would be reduced and the population present at the start of each successive breeding
season would increase, with a corresponding great increase in the population level
attained towards the end of the breeding season. Presumably a year would be reached
when social stress would exert strong selective pressure. The population size would
be reduced, during which time the animals best adapted to stress would tend to survive.
If, in fact, these animals were susceptible to severe climatic conditions, the winter
months would tend to reduce the population still further. In the spring the population
density would therefore be much lower than normal; the population would have "crashed."
Chitty (1965) suggests methods by means of which his theory might be checked.
These would consist of setting up a series of artificial populations of the same size,
using either animals from an expanding population (high density genotypes) or a
declining one. His theory would then predict that the first population should continue
to expand, while the second one would not. Further populations of the first type
should prove less well adapted to adverse weather conditions. These predictions are
reversed in the case of the reversed form of Chitty's theory. According to it, an
increasing population is one in which gradual adaptation to winter conditions has taken
place, while the declining populations are the survivors of a density dependent
selection process, and are therefore likely to be well adapted to high population
density.
If suitable parameter for selective pressures and population growth rate could be
estimated from field studies, it ought to be possible to produce a computer
programme to simulate the natural population processes.
