In a small population, an individual may carry a gene for a particular trait. If this individual is the only carrier for that trait and does not reproduce, the gene is no longer present in the population. The loss of a gene in a population due to a chance event is known as genetic drift. Genetic drift can be caused by natural disasters as well.
A mutation is a change in the DNA of an organism. It can be caused by environmental factors or by random chance. Mutations allow natural selection to occur by creating diversity within a species and adding new genes to the gene pool.
When members of two populations meet and are able to breed, gene flow can result. For instance, two populations of zebras are divided by a river. Their gene pools are not shared because they are reproductively isolated. However, if this river were to become crossable due to drought, these two populations of zebra could breed. This would introduce new genes into the population.
Another mechanism of evolution is recombination. If you recall, genes are carried on chromosomes within a cell. During the process of meiosis, chromosomes can cross over and exchange pieces with one another. This can add new traits to the population.
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