A species can be defined as a population that is reproductively isolated or and exclusive group of organisms. Boundaries between species are maintained by barriers to gene flow.1 However, natural hybridization can occur between species when individuals of different species are able to crossbreed.2 Individuals of those two different offspring must be distinguishable on one or more heritable characters1. In areas where hybridization occurs, there may a "hybrid zone," with parent species, F1 hybrids, and multiple generation hybrids.1

Introgression occurs when genes are transferred between species.1 Introgression usually occurs through backcrossing between F1 hybrids and the parent species.2 During introgression, alleles transfer from on species to a second divergent species. However, in order to recognize there are two distinct gene pools, some portion of each of the hybridizing species gene pools must remain intact. It is the genes that define those distinct gene pools that compromise the species boundary.1

  1. Harrison, R. G. & Larson, E. L. "Hybridization, Introgression, and the Nature of Species Boundaries." Journal of Heredity (2014): 795-801. Web. 30 March 2015.
  2. Twyford, A. D. & Ennos, R. A. "Next-generation hybridization and introgression." Heredity (2012): 179-189. Web. 30 March 2015.

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