The Genetic Factor in Population Ecology

Abstract

Two questions are discussed. How does genotype influence numbers and how do numbers influence genotype? From an ecological point of view the significance of change in the genetic composition of a population is its effect on the rate of increase (r), that is on birth rate and death rate. The tendency of natural selection is to maximize r. This does not mean that natural selection will tend to increase birth rate to the absolute maximum possible and to reduce death rate to the absolute minimum possible, but it will tend to maximize the difference between them. Life histories have been patterned by natural selection to this end. r can be increased by a variety of alterations to the life history pattern. The ones which will be useful will depend upon the environment the animal lives in. An increase in r may be achieved through natural selection by a decrease in the birth rate. This can occur in one sort of situation only and that is when progeny and parents remain together as a family, at least during the rearing stage. Selection for low birth rate could, in theory, occur in non-family organisms under special circumstances in which the offspring of the one or a few parents tended to be reared in isolation from those of other parents. Although selection cannot favor a low rate of increase, it may favor the development of mechanisms which cause a cessation of increase when a certain critical density is reached. Some forms of territoriality may serve this end. The survival value of a genotype is a function not only of the number of other genotypes around it but also of their kind. These findings in experimental populations may be important in resolving the causes of oscillations in numbers in some experimental and possibly in some natural populations; and secondly they suggest a role for genetic plasticity in reducing the chance of extinction. In relation to changing environment genotypic plasticity is only adaptive when the length of generation is less than the time within which the environment changes. The mechanisms of adaption through genetic plasticity are discussed.