A modern definition of “species” focuses on a shared gene pool and reproductive ability. Based on this definition, in order to determine what mutations give rise to new species, we should consider what mutations cause reproductive isolation.
There are three main modes of reproductive isolation that could lead to speciation: allopatric, parapatric, and sympatric speciation. Allopatric speciation occurs when a species splits into two geographically isolated populations in a way that prevents genetic exchange. First, a geographic barrier separates a species; this could be a result of continental movement, human activities, or formation of mountains, islands, and glaciers. Next, genotypic and phenotypic differences begin to arise due to mutations in the separate gene pools, divergence in selective pressures between the two populations, and genetic drift. These differences often gives rise to incipient species – species that may be infertile with each other but do not interbreed due to the geographic barrier. If this barrier were removed, the previously isolated populations could mate and produce low-fitness hybrids, which could become their own species or be eliminated from the population through mate choice discrimination. Over time, speciation progresses to completion. An example of allopatric speciation is the separation of Darwin’s finches in the Galápagos archipelago. The archipelago was colonized from continental South/Central America, in which a breeding population was established. Individual finches eventually colonized neighbouring islands to establish new breeding populations. Ultimately, eleven species of finches in the Galápagos arose from allopatric speciation.
Parapatric speciation is similar to allopatric speciation in that a geographic barrier separates a species, but interbreeding can still occasionally occur. While allopatric speciation results in two distinct geographic populations, parapatric speciation results in a continuous or discontinuous environmental gradient of hybrids, creating distinct species over time. Parapatric speciation usually occurs in marine environments due to the low probability of full geographic barriers. It is the primary mode of speciation in coral-reef fish. Although coral reef fish larvae do not disperse too far from their birthplaces and adults are predominantly sedentary, they can still be separated through the porous coral reefs. In addition, parapatric speciation can occur from the spatial and environmental heterogeneity of coral-reef habitats.
Sympatric speciation is the most rare. It occurs when a new species evolves from an ancestral species in the same geographic population, usually resulting from polyploidy, a trait that describes organisms that inherit more than one homologous set of chromosomes. These polyploids will be infertile with their parental species, consequently becoming reproductively isolated while remaining in the same geographic location. The apple maggot fly Rhagoletis pomonella may currently be undergoing sympatric speciation. The native host of this maggot is the hawthorn, but over the past 200 years, Rhagoletis pomonella have been found in domestic apples, Furthermore, significant allele frequency differences have been observed between hawthorn maggots and apple maggots.
Overall, the three modes of speciation discussed in this article can be summarized into two main events: anagenesis, which occurs when one species gradually accumulates enough genetic changes to become a new species, like in sympatric speciation, and cladogenesis, which occurs when gene pools split to result in two or more new species, like in allopatric and parapatric speciation.
Written by Jessica Chee
References may be found in the journal.