Speciation Mechanisms: Natural Selection And Genetic Drift
Before the development of the modern theory of evolution, a widely held idea regarding the diversity of life was the "typological" or "essentialist" view. This view held that a species at its core had an unchanging perfect "type" and that any variations on this perfect type were imperfections due to environmental conditions. Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) independently developed the theory of evolution by natural selection, now commonly known as Darwinian evolution.
The theory of Darwinian evolution is based on two main ideas. The first is that heritable traits that confer an advantage to the individual that carries them will become more widespread in a population through natural selection because organisms with these favorable traits will produce more offspring. Since different environments favor different traits, Darwin saw that the process of natural selection would, over time, make two originally similar groups become different from one another, ultimately creating two species from one. This led to the second major idea, which is that all species arise from earlier species, therefore sharing a common ancestor.
When so much change occurs between different groups that they are morphologically distinct or no longer able to interbreed, they may be considered different species; this process is known as speciation. A species as a whole can transform over time into a new species (vertical evolution) or split into more separate populations, each of which may develop into new species (adaptive radiation).
Modern population geneticists recognize that natural selection is not the only factor causing genetic change in a population over time. Genetic drift is the random change in the genetic composition of a small population over time, due to an unequal genetic contribution by individuals to succeeding generations. It is thought that genetic drift can result in new species, especially in small isolated populations.