How does a single species give rise to two or more different species? This concept, called speciation, requires that a single population of organisms divide into two or more populations that no longer interbreed. Without interbreeding, there is no gene flow between the populations, and these populations may then evolve separately into distinct species. There are many definitions for what a species is, but we will begin with biologist Ernst Mayr's 1940 definition, in which he states that species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups. We will extend this to say that if two individuals of different species do mate and produce offspring, that those offspring would not be fertile.
In the accompanying animation, we examine two mechanisms of establishing a barrier to gene flow, leading to speciation.
The key to one species diverging into two or more different species is establishing a barrier to gene flow. That is, two or more populations of a species must be prevented from sharing the same pool of genes.
If the barrier is geographic (allopatric speciation), over time the populations may acquire their own, unique genetic changes. After enough changes have been introduced, the two populations may no longer be able to interbreed even if they became reunited geographically. In this way, geographic isolation may lead to reproductive isolation and the formation of new species. Allopatric speciation is thought to be the main mechanism of speciation among most groups of organisms.
In the case of sympatric speciation, two populations become reproductively isolated while they live in the same geographic location. These events are considered to be less common than allopatric speciation. However, many new plant species do emerge through sympatric speciation, by simply duplicating their sets of chromosomes and becoming polyploid. Plants with different numbers of chromosomes cannot mate with their parent species to produce fertile offspring and are therefore distinct species.