Evolution

Sewall Green Wright was an American geneticist known for his influential work on evolutionary theory. The theory of random genetic drift was proposed by him. Genetic drift or allelic drift is the change in the frequency of a gene variant (allele) in a population due to random sampling. The effect of genetic drift is larger in small populations, and smaller in large populations.

Oparin and Haldane proposed that the first form of life originated from pre-existing non-living organic molecules like RNA and protein and that formation of life was proceded by chemical evolution.

The Devonian Period is known as the age of fishes. It is famous for the thousands of species of fish that developed in Devonian sea. The Devonian Period of Palaeozoic Era lasted from million years ago to million years ago.

Population may sometimes be reduced to low numbers by predation, diseases, adverse period of climatic changes. Earth quakes, floods, etc. The few surviving individuals may constitute a random genetic sample of the original population. The resultant alterations and loss of genetic variability is known as bottleneck effect.

Adaptive radiation (divergent evolution) is the evolution from one species of animals or plants of a number of different forms. As the original population increases in size it spreads out from its centre of origin to exploit new habitats and food sources. In time, this results in a number of populations each adapted to its particular habitat: eventually these populations will differ from each other sufficiently to become new species.