Ecological speciation

Ecological speciation is a form of speciation arising from reproductive isolation that occurs due to an ecological factor that reduces or eliminates gene flow between two populations of a species. Ecological factors can include changes in the environmental conditions in which a species experiences, such as behavioral changes involving predation, predator avoidance, pollinator attraction, and foraging; as well as changes in mate choice due to sexual selection or communication systems. Ecologically-driven reproductive isolation under divergent natural selection leads to the formation of new species. This has been documented in many cases in nature and has been a major focus of research on speciation for the past few decades.^[1]: 179

Ecological speciation has been defined in various ways to identify it as distinct from nonecological forms of speciation.^[2] The evolutionary biologist Dolph Schluter defines it as "the evolution of reproductive isolation between populations or subsets of a single population by adaptation to different environments or ecological niches",^[3] while others believe natural selection is the driving force.^[4][5][6] The key difference between ecological speciation and other kinds of speciation is that it is triggered by divergent natural selection among different habitats, as opposed to other kinds of speciation processes like random genetic drift, the fixation of incompatible mutations in populations experiencing similar selective pressures, or various forms of sexual selection not involving selection on ecologically relevant traits. Ecological speciation can occur either in allopatry, sympatry, or parapatry—the only requirement being that speciation occurs as a result of adaptation to different ecological or micro-ecological conditions.^[6]

Ecological speciation can occur pre-zygotically (barriers to reproduction that occur before the formation of a zygote) or post-zygotically (barriers to reproduction that occur after the formation of a zygote). Examples of pre-zygotic isolation include habitat isolation, isolation via pollinator-pollination systems, and temporal isolation. Examples of post-zygotic isolation involve genetic incompatibilities of hybrids, low fitness hybrids, and sexual selection against hybrids.

Some debate exists over the framework concerning the delineation of whether a speciation event is ecological or nonecological. "The pervasive effect of selection suggests that adaptive evolution and speciation are inseparable, casting doubt on whether speciation is ever nonecological".^[2] However, there are numerous examples of closely related, ecologically similar species (e.g., Albinaria land snails on islands in the Mediterranean,^[7] Batrachoseps salamanders from California,^[8] and certain crickets^[9] and damselflies^[10]), which is a pattern consistent with the possibility of nonecological speciation.^[8][11]