What is the only source of new genetic variation in a population?

The only source of new genetic variation in a population is mutation. Mutations are changes in the DNA sequence of an organism, which can occur due to various factors such as errors in DNA replication, exposure to environmental agents, or spontaneous chemical changes. When these mutations occur in a germ cell, they can be passed on to the next generation, thus introducing new alleles into the gene pool.

Other processes such as gene flow, natural selection, and genetic drift can alter the frequency or distribution of existing genetic variations within a population, but they do not create new genetic variants. Gene flow introduces new alleles into a population through the migration of individuals, while natural selection acts on existing variations to favor those that are advantageous for survival and reproduction. Genetic drift refers to random changes in allele frequencies, particularly in small populations, but again, it does not generate new genetic material. Therefore, mutation is indeed the essential mechanism that provides the raw material for evolution by generating new genetic diversity.