This is important because one cannot discuss speciation without first specifying criteria for recognizing species (Wiens 2004).
For example, under these criteria, sympatric speciation can only be detected if there are no major changes in geographic distributions and/or subsequent allopatric divergence after the initial sympatric speciation [ 21].
Additionally, the last criterion poses allopatric speciation as the null model, which effectively limits the verification of sympatric speciation to unique isolated terrestrial and freshwater settings (e.g., [ 22, 23]) where any contribution to historic allopatry can be confidently excluded.
These populations therefore satisfy the first criterion of parallel speciation established by Schluter and Nagel [ 27].
According to current criteria for rickettsia speciation, this new rickettsia qualifies as Candidatus Rickettsia tasmanensis, named after the location of its detection.
Each node on the optimized taxon-area cladograms was examined for evidence of speciation by vicariance or dispersal following criteria of [12], [14].
Elmer and Meyer [ 77] outline four "gold standard" criteria for the hypothesis of sympatric speciation: 1) sympatric distributions, 2) a reciprocally monophyletic relationship between the taxa (sister species) with respect to others in the complex, 3) reproductive isolation and, 4) a setting where allopatric divergence is unlikely.
Our software offers a unique and comprehensive combination of functions: it includes losses in the optimization criterion, does not require estimates of speciation times and reports all optimal event histories.
Furthermore, gradual change, even through speciation, means that species will not be categorically distinct (criterion 6), which for Ellis (2001) is an additional reason to conclude that species are not natural kinds.
