Sympatric and parapatric speciation refer to the origin of biological species in the absence of complete geographic isolation between the diverging taxa. Until recently, most biologists believed that geographic isolation was almost universal in the development of species, i.e. most species originated in allopatry. However, new empirical and theoretical studies have shown that speciation may occur despite the diverging populations having adjacent or overlapping geographic ranges and despite on-going gene flow. Attention in speciation studies has shifted to the mechanisms responsible for reducing gene flow, regardless of the extent of geographic range overlap.

Key Concepts:

• The generation of new species has historically been classified according to geographic context, based on the overlap of different populations.
  
• Geographic overlap is usually thought to influence gene flow between populations.
  
• Of all possible cases, diversification in sympatry has been considered the most unlikely.
  
• Theoretical models and examples from nature have shown sympatric speciation to be possible, but rare. A sympatric stage following allopatric divergence may be more common.
  
• Divergence in the presence of gene flow may be common, especially if disruptive selection and/or assortative mating are strong.
  
• A modern view of speciation focuses on processes that generate divergence, rather than a strict geographical classification.
  
• Speciation is usually the result of complex interactions between different genetic, environmental and geographic processes, and must be understood in terms of this more complicated reality.