Modified Hennigian species concept:
A species concept based on the criterion of "reproductive community" alone does not satisfy the demands of strict phylogenetic systematics (Hennig, 1950 against Mayr). Hennig includes the cohesion (gene flow) to define species. The initial concept of Hennig was revitalized by Willmann (1985; 1986).
The modified Hennigian species concept based on reproductive isolation and cohesion through gene flow, because he was interested in the delimitation of species in time (1966). someone support the importance of the cohesion in the delimitation of species; others support the reproductive isolation (Mayr, 1957). The cohesion and reproductive isolation can be applied to the "most inclusive Mendelian population"; but the factor more important is the isolation (reproductive gap).
Agamotaxa are taxa consisting of uniparental organisms originate in a way similar to bisexual species, each agamotaxa is isolated reproductively from all others. The phylogenetics relationships of agamotaxas are different to bisexual populations, the terminals would be organisms individual.
A concept appropriate for phylogenetic systematics must consider the historical dimension of species. The monophyletic groups consist of a stem species and all their descendants (Hennig). Species is viewed as a temporal series of populations connecting two speciation events. If we assume monophyly in the Hennigian concept; we assume that the stem species cannot survive speciation, and a species comprise the entire branch segment between two speciation events.
A species concept based on reproductive isolation attempts to describe natural entities (Willmann, 1991). The BSC (Biological species concept) and HSC (Hennigian species concept) is not character related and they is identical if absolute isolation is adopted. In allopatric populations, the breeding experiments in a artificial environment determines whether it belong to the same species (Wiley, 1981). the species concepts based on characters does not inferring species boundaries.
Potential and Actual Interbreeding:
The potentiality of interbreeding of the populations is a important factor for the stability of the BSC. However; the HSC modified (Willmann) only is related to reproductive isolation.
The criterion of absolute isolation is not exclude any arbitrariness (Key, 1981; Willmann, 1985). The use from several criteria can be incorrect. The isolation criterion does not interfere with any kind of biological research.
Survival of the Stem Species:
The populations becomes species only relative to their next kin. Speciation creates a pair of news species, and it eliminate the stem species (Willmann, 1989). If we does not assume the dissolution of the stem species, we does denies that species have boundaries in time.