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Factors responsible for the formation of new species are: (i) Genetic variation between individuals: There is a large amount of variation in the genes within a population. This genetic variation arises from random mutations in the DNA sequence. These mutations can cause a nucleotide base to be inserted, deleted or substituted in the DNA sequence. Mutations in the DNA of a gene coding for a protein can cause changes in the amino acid sequence of the protein. The resulting protein may function differently. (ii) 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. (iii)Genetic drift: It is caused due to 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. (iv)Geographical isolation: It is a major factor in speciation since it interrupts with gene flow. Geographical isolation is caused by various types of barriers such as mountain ranges, rivers and seas. It leads to reproductive isolation due to which there is no flow of genes between separated groups of population which ultimately results in speciation.