How speciation can proceed in the face of gene flow between potentially interbreeding populations, including among parents and newly formed hybrid species (Andrew et al., 2012; Renaut et al., 2013), is especially revealing of the challenges associated with habitat specialization under parapatry. Gene flow can dilute the adaptive differences that selection generates between divergent populations; this may be especially problematic when one population is significantly larger than the other, as in the case of a newly divergent population found adjacent to an ancestral population, or in the presence of asymmetric gene flow (Ellstrand, 2014).
Phylogeographic analyses of the 30[degrees]S south-east Pacific biogeographic transition zone establish the occurrence of a sharp genetic discontinuity in the kelp Lessonia nigrescens: Vicariance or parapatry? Mol.
Recently, Turner and Mallet (1996) suggested that mimicry rings in Heliconius did not originate via "biotic drift" in allopatry and in conjunction with Pleistocene climatic-vegetational fluctuations (as proposed by Brown et al., 1974), but rather by mutation and random drift in parapatry. Novel color patterns then spread to become predominant in local areas and later over wide areas by the migration of clines.
Miller (1964: 256) believed that "Two species may exist in close proximity and their ranges may even interdigitate, but they maintain a relationship of contiguous allopatry and do not form mixed-species populations." Today, "contiguous allopatry," in which otherwise sympatric species are separated ecologically, is known as parapatry (e.g.