Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Sympatric speciation is the opposite of allopatric speciation. In sympatric speciation, a new species is formed in the same territory as that occupied by the parent species. Occurrence in the same territory at the time when speciation occurs is a necessary but not a sufficient condition for the particular speciation to be considered to be sympatric. If the members of the new and original populations basically do not meet in the particular territory, this would not be sympatric speciation. The main difference is that allopatric speciation is accompanied by the formation of internal reproductive barriers between differentiating species in the presence of already existing reproductive barriers, while sympatric speciation occurs in their absence. For example, if a parasite transmitted by direct contact happens to jump over to a different species of host living in the same territory as its original host, the two populations of parasites will continue to exist sympatrically in their range of occurrence. In actual fact, no interactions need occur between the parasite populations on the original and new host species, specifically because of the minimum of physical contact between the members of the two host species, and thus there will be no exchange of genetic information or competition for resources. The two parasite populations can gradually diverge into independent species. However, these will certainly not be sympatric speciation, as internal reproductive barriers would be formed in this case after the formation of external reproductive barriers. Similar cases, when the species live in the same territory but have island ranges at different places, are mostly called microallopatric. In true sympatric speciation, the members of the two populations must constantly meet during the evolution of the new species.
A large fraction of instant speciation, for example polyploid speciation or hybridization speciation, has the character of sympatric speciation. In these types, one-step formation of reproductive isolation barriers precedes, or even causes, phenotypic and therefore also ecological differentiation of a new species. It is understandably rather questionable whether it makes sense to differentiate sympatric and allopatric speciation in cases of instant speciation. However, sympatric speciation also includes gradual ecological speciation, which occurs through the action of disruptive natural selection or ethological speciation. These gradual speciations are accompanied by the accumulation of changes that eventually gradually lead to complete reproductive separation, where constant gene flow between the populations consistently prevents accumulation of differences. The possibility of formation of a new species through gradual sympatric speciation thus continues to be a matter for discussion.