Ring Species
Parapatric speciation can occur across a set of neighboring populations in which each can interbreed with closer populations but end populations are too distantly related to interbreed. This population distribution of a species is referred to as a ring species. When an ancestral population extends its range around a geographical barrier and differentiates (despite low-level gene flow), reproductive isolation occurs between terminal populations. This results in technically continuous populations that undergo parapatric speciation; fertility is negatively correlated with distance between populations, and terminal populations between ring species become distinct sister species.
Read more about this topic: Parapatric Speciation
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