There are four basic mechanisms by which biological evolution takes place. These include mutation, migration, genetic drift, and natural selection. Each of these four mechanisms are capable of altering the frequencies of genes in a population and as a result, they all are capable of driving descent with modification.
Mechanism 1: Mutation. A mutation is a change in the DNA sequence of a cell's genome. Mutations can result in various implications for the organism—they can have no effect, they can have a beneficial effect, or they can have a detrimental effect. But the important thing to keep in mind is that mutations are random and occur independent of the organisms' needs. The occurrence of a mutation is unrelated to how useful or harmful the mutation would be to the organism. From an evolutionary perspective, not all mutations matter. The ones that do are those mutations that are passed on to offspring—mutations that are heritable. Mutations that are not inherited are referred to as somatic mutations.
Mechanism 2: Migration. Migration, also known as gene flow, is the movement of genes between subpopulations of a species. In nature, a species is often divided into multiple local subpopulations. The individuals within each subpopulation usually mate at random but might mate less often with individuals from other subpopulations due to geographic distance or other ecological barriers.
When individuals from different subpopulations move easily from one subpopulation to another, genes flow freely among the subpopulations and the remain genetically similar. But when individuals from the different subpopulations have difficulty moving between subpopulations, gene flow is restricted. This may in the subpopulations becoming genetically quite different.
Mechanism 3: Genetic Drift. Genetic drift is the random fluctuation of gene frequencies in a population. Genetic drift concerns changes that are driven merely by random chance occurrences, not by any other mechanism such as natural selection, migration or mutation. Genetic drift is most important in small populations, where the loss of genetic diversity is more likely due to their having fewer individuals with which to maintain genetic diversity.
Genetic drift is controversial because it creates a conceptual problem when thinking about natural selection and other evolutionary processes. Since genetic drift is a purely random process and natural selection is non-random, it creates difficulty for scientists to identify when natural selection is driving evolutionary change and when that change is simply random.
Mechanism 4: Natural selection. Natural selection is the differential reproduction of genetically varied individuals in a population that results in individuals whose fitness is greater leaving more offspring in the next generation than individuals of lesser fitness.