Examples: White tigers and white leopards are common in the populations. The former must have given rise to Siberian tiger and the latter to snow leopard by migration. Similarly, origin of bird’s wing from a reptilian foreleg must have occurred due to megamutation, but it may not have been an advantageous character in the formative stage but was still maintained in the population and became advantageous later. Polymorphism is very common in insects, such as butterflies, aphids and hoppers (winged and wingless forms), web-spinners and social insects. Polymorphism permits the species to exploit different types of ecological conditions by different forms. Widely distributed species, living in mosaic of environmental conditions, usually possess polymorphism. K-selection and R-selection: and R-selection: These two categories are made based on the density of population and reproductive rate at which the natural selection operates. K-selection operates in stable environmental conditions in which species live in saturated population densities. These populations show sigmoid growth curves and live near the carrying capacity. Selection favours those individuals that have enhanced competitive ability at high population densities near the carrying capacity. Such populations have slow growth rate and prevail in non-seasonal tropics. R-selection operates on populations having rapid growth rate but low adaptiveness. They can rapidly exploit new environment, such as a burnt out forest or brief summer in mountains or in areas of uncertain environmental stress, e.g. storm, drought, fire etc. Such populations are good pioneers and can rapidly exploit a new environment with their high biotic potential and no selection pressure. They show J-shaped growth curves, as in the case of seasonal insects.

## Natural selection in microorganisms

Microorganisms reproduce in three ways and natural selection may be different accordingly. 1. In self-reproducing individuals: In self-reproducing individuals: If the process of self-reproduction is perfect, there is no room for natural selection. But the process is never complete and mutants appear from time to time. If these mutants reproduce less efficiently, they are eliminated but if they reproduce more efficiently, they can maintain themselves side by side, causing diversification in the population. For example, protozoa reproducing by binary fission, Hydra by budding and coelenterates by strobilation. 2. In self-fertilizing individuals: In self-fertilizing individuals: Hermaphrodite or parthenogenic individuals usually pass the same genotype to their offspring, which are called “pure races”. Unless there are mutations, the genotype of each individual will remain unchanged. When several such races are exposed to natural selection, some of them are favoured while others are eliminated, which will alter the ratio accordingly. For example, flatworms, lower insects and other invertebrates reproduce by self-fertilization. 3. In cross-fertilizing individuals: In cross-fertilizing individuals: In cross-fertilising microorganisms no two individuals are alike genetically and every population possesses genes in different frequencies. Natural selection may increase the frequencies of some genes and decrease the frequency of others. Since new genotypes are constantly produced by cross-fertilisation, selection has an opportunity to exercise its effect. The character becomes heterozygous and dormant and survives for some generations, without being expressed and exposed to natural selection. During this period environment may change and the character may become advantageous. This also produces heterozygosity in the population.

## VARIATIONS

In evolutionary terms, differences between closely related organisms are termed as variations. Thus differences between individuals of the same species, subspecies or race can be called variations and not differences between two genera, families or classes.