Speciation

The two phases of speciation are reproductive isolation and genetic divergence.

In reproductive isolation, a few members from the original population become reproductively isolated and thus can no longer interbreed. In genetic divergence, populations accumulate genetic differences with time because of natural selection or genetic drift.  

Causes

The main and common reason behind all speciation events is that the diverging species can no longer recognize other pre-existing members as its potential mate. Even sometimes, due to failure in prezygotic isolation

• Natural selection drives variation in population. It selects those traits that enhance survival and reproduction in the changed environment. With time, these changes lead to differences in population, causing speciation.
• Geographic isolation is another common cause of speciation. Barriers like mountains and large water bodies prevent members of a species from interbreeding. With time, genetic differences accumulate due to different factors leading to speciation.
• Reproductive isolation is caused due to some reproductive barriers that prevent copulation. These barriers can be prezygotic, such as mating behaviors, timing, or physical incompatibility, or postzygotic, like hybrid offspring being infertile.
• Mutation and genetic drift in a small population are major causes of speciation and random changes in gene frequencies. It leads to species being unable to interbreed.
• Sexual Selection also results in speciation. Let us suppose individuals within a population begin to prefer specific traits in mates (like coloration, song, or courtship behaviors). In that case, these preferences can lead to reproductive isolation and, eventually, speciation.

Speciation is of different types based on the cause that drives the event:

1. Allopatric Speciation

It occurs because of geographic isolation. Natural barriers such as mountains and large water bodies like oceans and rivers prevent a section of the parent population from interbreeding. Over time, the isolated population evolves differently, leading to speciation.

Example: Darwin’s finches on the Galápagos Islands  

2. Sympatric Speciation

It happens when some members of the species, despite being in close contact, stop interbreeding due to processes like polyploidy (where an organism develops extra sets of chromosomes leading to genetic divergence) or for changes in habitat preferences or mating behaviors within the same geographic area.

Example: Although convincing evidence is rare, the apple maggot (Rhagoletis pomonella) that originally appeared from hawthorn species has undergone sympatric speciation and is now capable of reproducing eggs in the apple instead of hawthorns.

3. Peripatric Speciation

Also known as the founder effect, it happens when small groups break off from a larger group due to edge-selection pressure. It is similar to allopatric speciation, with the difference being in peripatric speciation, one group is much smaller than the other. This population, being located in the periphery, experiences selective pressure and thus drifts from the parent population, ultimately forming a new group.   

Example: Evolution of the London Underground mosquito (Culex molestus).

4. Parapatric Speciation

It occurs when a few individuals within a species stop interbreeding as they become partially isolated due to some differences in the same environment. Similar to allopatric and peripatric speciation, the difference in habitat is the cause of parapatric speciation.

Example: Evolution of buffalo grass (Bouteloua dactyloides) that can grow in soil contaminated by heavy metals.