Evolution is caused by action of forces on Natural Selection of Variation. Reproductive Isolation keeps the species distinct. Therefore, the elemental forces of Organic Evolution are:
- Natural Selection
Sources of Organic Variation
Variation arises in an individual member of a population, and if favourable, spreads into the population through differential reproduction by the action of natural selection. Variations may occur by
1. Mutation: It is a sudden genetic change. It may be a change in a single gene or may affect many genes.
2. Genetic recombination: It occurs in sexually reproducing organisms at every reproduction. The chromosomes and thus genes of the parents mix at random during zygote formation. That is why offspring of same parents are different from each other as they have different combinations of parental genes.
3. Gene flow: It is when there is chance mixing of genes of closely related species through sexual reproduction.
4. Genetic drift: It occurs in small populations when a part breaks off from a large population. Only representative genes of the large population are present which undergo change at a right time and the small population may evolve into a new sub-species or species.
Natural selection is considered to be responsible for differential reproduction of genes, which means that more of favourable genes get reproduced in a population.
Once new species arise from the parental species due to the effect of variation and natural selection, reproductive barriers prevent the two species from exchanging genes through reproduction. Thus, two related species cannot mate with each other and remain distinct.
Ecological Isolation: The two species are unable to mate as they live in geographically different areas.
Seasonal Isolation: Mating is prevented because the reproductive organs mature at different times.
Ethological (Behavioural): The songs in birds of two species or the colouration isolation of two fishes are so different that female of one species is able to recognize only the male of its own species.
Mechanical Isolation: The male and female organs for mating differ in different species and prevent their union.
Physiological Isolation: The sperms of one species are not able to survive in the female tract of another species.
Zygotic and Developmental Isolation: If the hybrid zygote survives it dies during development.
Hybrid Sterility: Mule, the offspring of a female horse and male donkey is a good example. It leads a normal life but is sterile and cannot reproduce.