(a) The process by which new species develop from the existing species is known as speciation. The factors which could lead to speciation are
(i) Geographical isolation of population caused by various types of barriers such as mountain ranges, rivers and seas. This leads to reproductive isolation because of which there is no flow of genes between separated groups of population.
(ii)Genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.
(iii) Variations caused in individuals because of natural selection.
(b) According to Darwin's theory of natural selection, the individuals who are most suitable and fit are successful in struggle for existence for food, space, mate. etc. Their offsprings are also better developed and adapted to the environment. Whereas one who are less adapted to the environment may die. Thus, nature selects better adapted organisms. This is called natural selection of the well adapted, better developed individuals of species.
