Evolution occurs, gradually throughout time; genetic material is shuffled and recombined to produce stronger offspring that are better adapted to the environment. But how exactly are these positive traits retained in a population, or for that matter, how are any traits chosen to remain or disappear from a population? What are the mechanisms that affect evolutionary change?
The main mechanism is natural selection but there are two other ones: genetic drift and gene flow. Natural selection is basically the retaining of traits that are beneficial for an organism’s survival in an environment. Genetic drift is slightly more complicated and usually describes a small population while natural selection describes a large population. But it describes the gene frequencies due to random change that does not include migration, mutation and natural selection.
Contrary to natural selection, this type of random appearance of certain genes has no relevance to whether or not they convey positive benefits to the population. Genetic drift influences the disappearance of certain traits from population which may explain how two populations, who were at one point, could diverge into two separate populations with such different traits of the same genes.
Gene flow describes how genes from one population end up in another population, usually of the same species. How this occurs is mainly through migration of individuals from one population to another and then breeding. The flow of genetic material is thus in two directions, since immigration adds genetic variety to the new population, while detracts genetic material from the population the individual has emigrated from.
The mechanisms of evolutionary change determine and describe how certain traits in populations remain, disappear or get transferred between populations. This explains how divergent life forms appeared over time as well as helps us understand where we came from: who our ancestors could have been and what life was like then.