Speciation describes the evolutionary process through which populations of organisms evolve to become new and distinct species. This involves genetic changes that lead to groups becoming reproductively isolated, meaning they can no longer interbreed to produce fertile offspring. While new species often form after populations are geographically separated, speciation can also occur when groups diverge within the same geographic area. This latter process is known as sympatric speciation.
The Nature of Sympatric Speciation
Sympatric speciation occurs when a new species arises from an ancestral species while both inhabit the same geographic region. This process does not rely on a physical barrier to gene flow. Instead, reproductive isolation develops through other mechanisms that prevent interbreeding within the shared environment.
One mechanism is polyploidy, especially common in plants, where individuals acquire one or more additional sets of chromosomes due to errors in cell division. A polyploid individual cannot typically interbreed with the parent population, creating a reproductively isolated group. Disruptive selection is another driver, favoring individuals with extreme traits over those with intermediate ones, which can lead to the divergence of a population into distinct groups adapted to different niches. If a species exploits two different ecological niches, selection pressures can cause divergence.
Habitat differentiation contributes to sympatric speciation when segments of a population begin to use different microhabitats or resources within the same area. This partitioning can reduce gene flow between groups, as individuals become specialized to their preferred habitat or food source. Sexual selection can also play a role, where preferences for specific mating traits, such as coloration or courtship rituals, lead to reproductive isolation. If mating preferences diverge within a population, it can lead to the formation of distinct, reproductively isolated groups.
Sympatric Versus Allopatric Speciation
The distinction between sympatric and allopatric speciation lies primarily in the presence or absence of geographic barriers. Allopatric speciation, meaning “other homeland,” involves the formation of new species when populations are physically separated by an extrinsic barrier. Examples include mountain ranges, rivers, or islands, which prevent gene flow. Over time, these isolated populations evolve independently, accumulating genetic differences due to dissimilar selective pressures and mutations. Eventually, they become reproductively isolated even if they come into contact again.
In contrast, sympatric speciation, meaning “same homeland,” occurs within the same geographic area, without any physical obstruction to gene flow. While allopatric speciation is driven by extrinsic geographic factors, sympatric speciation relies on intrinsic factors such as genetic changes (like polyploidy), ecological divergence, or behavioral differences that reduce interbreeding. Gene flow must be overcome by other means to achieve reproductive isolation.
Illustrative Examples of Sympatric Speciation
An example of sympatric speciation involves cichlid fish in the African Great Lakes, particularly Lake Malawi and the Nicaraguan crater lakes. In Lake Malawi, numerous cichlid species have evolved within the single body of water. Mechanisms like sexual selection, driven by male coloration and female mate choice, contribute to reproductive isolation among these fish. Different species have also specialized in various ecological niches, such as distinct feeding strategies or habitats within the lake, promoting divergence.
The apple maggot fly, Rhagoletis pomonella, provides another example of ongoing sympatric speciation. Historically, this fly laid its eggs exclusively on hawthorn fruits. With the introduction of domesticated apple trees in North America in the mid-1800s, a new population began to infest apples. These apple-infesting flies and the original hawthorn-infesting flies now show partial reproductive isolation. They mate on or near their host plants, and differences in fruit ripening times and odor preferences contribute to their divergence and temporal isolation.
Polyploidy, especially prevalent in plants, is a mechanism where an organism has more than two complete sets of chromosomes due to errors during cell division. For example, modern bread wheat (Triticum aestivum) is a hexaploid, with six sets of chromosomes, resulting from multiple hybridization and polyploidization events involving different ancestral wheat species. Such a change in chromosome number typically makes the new polyploid reproductively incompatible with its diploid parent species, forming a new species.