Search for a command to run...
The genetic plasticity of populations implies that the numbers of animals, which is what the population ecologist studies, may be a reflection of change in genetic composition either in space or in time. Alternatively the number of animals may itself be a cause of genetic change. These genetic aspects of ecology will be more important when genetic plasticity is more characteristic than phenotypic plasticity. In relation to changing environment genetic plasticity can only be adaptive when the length of the generation is less than the time the environment takes to change. Both types of plasticity exist throughout the animal and plant kingdoms. In some groups of animals in which phenotypic plasticity is common, as in fishes, it is extremely difficult to establish the extent to which genetic plasticity exists. Adaptation through genetic plasticity depends upon a continuous source of genetic variability. The source is mutation and in addition, in sexual organisms, recombination of genes. Sex also introduces the possibility of stored variability which is another source of variability available to the species. The sources of stored variability are, dominance, canalizing selection, linked polygenes in which crossing over occurs and heterosis. A balance between flexibility and stability of genetic composition is attained by a variety of genetic mechanisms of which chromosomal inversions and alternation of sexual and asexual generations are two. From an ecological point of view the significance of change in genetical 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. The rate of increase 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. This sort of selection has resulted in characteristic clutch sizes in birds and sterile castes in the social insects. 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. Concerning the influence of numbers of animals on their genetic composition, there is experimental evidence that some genotypes are favored in a crowd and others are favored when sparse. Further 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.