Speciation

Given the extraordinary diversity of life on the planet, there must be mechanisms for speciation: the formation of two species from one original species. For speciation to occur, two new populations must be formed from one original population and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed.

Biologists have proposed mechanisms by which this could occur that fall into two broad categories. Allopatric speciation, (allo- = "other"; -patric = "homeland") involves geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation (sym- = "same"; -patric = "homeland") open brackets, "allo" equals "other" and "patric" equals "homeland", close brackets, involves geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation, open brackets "sym" equals "same" and "patric" equals "homeland", close brackets, involves speciation occurring within a parent species remaining in one location.

Darwin envisioned this process as a branching event and diagrammed the process in the only illustration found in On the Origin of Species (image below). Compare this illustration to the contemporary diagram of elephant evolution (image below). As one species changes over time, it branches to form more than one new species, repeatedly, as long as the population survives or until the organism becomes extinct.

Darwin's illustration (from On the Origin of Species) showing speciation events leading to biological diversity was first published in the mid-1800s. However, the diagram shows exceptional similarities to the phylogenetic charts that are drawn today to illustrate the relationships between species (such as the elephant tree).

Figure 1: Comparison between Darwin and modern phylogenetic trees.