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Modes of Speciation

Author(s): Tadzia GrandPre, PhD, Nancy Moreno, PhD, and Lisa Marie Meffert, PhD

Sympatric Speciation

In sympatric speciation, reproductive isolation is said to arise within a single, freely, and randomly interbreeding population in the absence of any spatial segregation. Thus, gene flow is initially restricted by biological features of organisms rather than by geography or distance. Because even low levels of gene exchange can swamp out the build up of genetic differentiation that is required for speciation, the occurrence of sympatric speciation is highly debated and controversial. Models of sympatric speciation propose various evolutionary forces and processes as the driving force behind the genetic divergence required for reproductive isolation in sympatry, including diversifying selection (a form of natural selection) and polyploidy (when multiple, duplicate copies of the genome are present within individuals). 

When populations inhabit environments with multiple resources and microhabitats, some individuals may possess traits or characteristics that allow them to use one of the resources and/or microhabitats more efficiently. Over time, diversifying selection can cause the population to split into genetically distinct groups that are adapted to discreet niches or the use of different resources within the environment. If selection pressures are strong enough to overcome gene exchange in the population, speciation can occur in sympatry.

An example of sympatric speciation resulting from diversifying selection may be found in flies of the genus Rhagoletis. These flies exhibit strong fidelity to the host plants in which they mate and leave their offspring to develop. Until the mid-nineteenth century, Rhagoletis in the northeastern United States used hawthorns exclusively as their host plant. However, when apples were introduced in some areas approximately 150 years ago, a new "race" of Rhagoletis appeared that inhabits apples rather than hawthorns. Because hawthorns and apples are often found in the same geographic area, the hawthorn and apple-maggot flies can exist in trees that are only yards apart. Many consider that the hawthorn flies are the parent species of the apple flies and that the speciation event was initiated by genetic variations that caused some members of the original population to be attracted to new hosts. This argues for sympatric speciation in Rhagoletis. However, this claim has come under debate. For example, some argue that the hawthorn and apple-maggot flies descended from distinct, independent lineages, through allopatric speciation.