From an evolutionary point of view, the achievement of species status by a group of populations is an ongoing process (except for rare cases of instantaneous speciation), during which isolated populations acquire traits and adaptations that minimize gene flow between them. However, depending on the group, the ways and timing of the gaining of reproductive isolation may be different. In such a complex group for systematics as small vipers (genus Vipera, subgenus Pelias), there are a number of problematic situations for species delimitation: the ability to form hybrids and hybrid populations between species, adaptive hybridization and hybrid speciation, and possible traces of reversed speciation, which indicate that complete and irreversible reproductive isolation may not be achieved at all. The effectiveness of reproductive isolation largely depends on external factors, mostly related to the range spatial structure, ecology of species in areas of sympatry, as well as climatic conditions and their changes, landscape dynamics, etc. The only species concept that allows to describe new species in the absence of reproductive isolation, recognizes the possibility of hybrid and reverse speciation — de Queiroz unified species concept — is practically unsuitable due to the need to consider as species a large number of isolated populations. Therefore, the solution is a conservative approach in practical taxonomy, which takes into account as a species criterion the ecological divergence of species, the possibility of their sympatric existence, which is usually achieved at 5% divergence of the mitochondrial cytochrome b gene. Stabilized hybrid populations occupying distinct ecological niche different from those of parental species also deserve recognition as species of hybrid origin. Not only species deserves to be protected, but also a distinguishable and diverged group of populations — Evolutionary Significant Unit (ESU) or subspecies in traditional taxonomy.
Cytoplasmic incompatibility between Old and New World populations of a tramp ant