1) Geographical isolation: Geographical isolation is caused by various types of barriers such as mountain ranges, rivers and seas. In this a group of individuals of a species may become geographically isolated from other members or species. So, both these groups grow in different environment and hence evolve to form a new species.
2) Natural selection: All populations respond to changes in their environment. Individuals will respond in different ways depending on their genes. Those individuals whose genes are best suited to the environment are more likely to survive and pass on their genes to the next generation. This is natural selection. Gradually, favourable genes will start to predominate in the population and less favourable genes will decline.
3) Genetic drift: It is caused by drastic changes in the frequencies of particular genes by chance alone. Genetic drift with changes in the gene flow imposed by isolation mechanism acts as an agent of speciation which ultimately results in evolution.
4) Mutation: A sudden, stable, inheritable change in the genetic material of an organism is termed as mutation. Mutations may also result in speciation.