Editing Allopatric speciation (section)

=== Reproductive isolation ===
{{Main|Reproductive isolation}}
Reproductive isolation acts as the primary mechanism driving genetic divergence in allopatry<ref name="Theory and speciation">{{Citation |title=Theory and speciation | author=Michael Turelli| author2=Nicholas H. Barton| author3=Jerry A. Coyne | journal=Trends in Ecology & Evolution | year=2001 | volume=16 | issue=7 | pages=330–343 | doi=10.1016/s0169-5347(01)02177-2| pmid=11403865}}</ref> and can be amplified by [[Divergent evolution|divergent selection]].<ref name="Rice&Hostert">{{Citation |title=Laboratory Experiments on Speciation: What Have We Learned in 40 Years? | author=William R. Rice | author2=Ellen E. Hostert | journal=Evolution | year=1993 | volume=47 | issue=6 | pages=1637–1653 | doi=10.1111/j.1558-5646.1993.tb01257.x| pmid=28568007 | jstor=2410209 | s2cid=42100751 }}</ref> Pre-zygotic and post-zygotic isolation are often the most cited mechanisms for allopatric speciation, and as such, it is difficult to determine which form evolved first in an allopatric speciation event.<ref name="Theory and speciation"/> Pre-zygotic simply implies the presence of a barrier prior to any act of fertilization (such as an environmental barrier dividing two populations), while post-zygotic implies the prevention of successful inter-population crossing after fertilization (such as the production of an infertile [[Hybrid (biology)|hybrid]]). Since species pairs who diverged in allopatry often exhibit pre- and post-zygotic isolation mechanisms, investigation of the earliest stages in the life cycle of the species can indicate whether or not divergence occurred due to a pre-zygotic or post-zygotic factor. However, establishing the specific mechanism may not be accurate, as a species pair continually diverges over time. For example, if a plant experiences a [[Polyploid|chromosome duplication event]], reproduction will occur, but [[Sterility (physiology)|sterile]] hybrids will result—functioning as a form of post-zygotic isolation. Subsequently, the newly formed species pair may experience pre-zygotic barriers to reproduction as selection, acting on each species independently, will ultimately lead to genetic changes making hybrids impossible. From the researcher's perspective, the current isolating mechanism may not reflect the past isolating mechanism.<ref name="Theory and speciation"/>