Species and “strange species” in zoology: Do we need a “unified concept of species”?

Eidonomy is the field of taxonomy, which addresses the “species problem”. The latter has several dimensions. The first is the confusions frequently made between species as an evolutionary unit, a taxon, a taxonomic category and a nomenclatural rank. The second is the reductionist temptation to recognize in nature only one kind of “basic entities” of biodiversity. From a practical viewpoint and for all non-systematist users of specific nomina, any animal organism should be referred to a taxon of nomenclatural rank species, designated by a Latin binomen. These nomina are indispensable for administrative and legal documents, for management and conservation of biodiversity, as well as for all biological research. But this does not imply that all these taxa should be referred to a single taxonomic category, a “unified concept of species”. In nature, several kinds of entities correspond to different “species concepts” or specions that are irreducible one to another. These categories can be defined according to the modalities of their reproduction, i.e., of their gametopoiesis (mode of formation of gametes), of their kinetogenesis (mode of initiation of the development of the ovum) and of the gene flow between individuals. The best known of these categories is that of mayron or “mixiological species concept”, which points to an independent bisexual panmictic entity, constituting a protected gene pool, but others do exist. In order to better understand their differences, it is useful to consider the various patterns of speciation. These can be referred to three main categories: monogeny (change within a single lineage), diplogeny (birth of two specions from a single one) and mixogeny (hybridization between two specions). The splitting in two of a single initial gene pool is a rather long process, which often shows different stages. The latter can be expressed taxonomically through the use of particular eidonomic categories. Speciation can be considered completed only when it has become irreversible in case of new contact between the two stocks after a period of separation. In allopatry, it is difficult or impossible to know if this stage has been reached. This is not a reason for abandoning the mixiological criterion for identifying sympatric or parapatric specions. In allopatry, the method of inference can be used and, in the absence of sufficient information, it is fully possible to use “by default” the “phylogenetic concept of species” or simpson. The situation is further complicated by the fact that some of the basic entities of biodiversity of hybrid origin, the kyons, are not mayrons, as they have particular reproductive modes. Contrary to isolated and “normal” hybrids, such entities may persist in the long term in nature. Their gametopoiesis implies either an ameiosis (mitoses) or a metameiosis (modified meiosis), and their kinetogenesis implies zygogenesis (fertilization), gynogenesis or parthenogenesis. Kyons are of two main categories. The klonons are unisexual female entities in which genetic transmission is clonal, reproduction being often through parthenogenesis, or following other mechanisms with similar results in genetic terms. The kleptons are unisexual or bisexual entities, which depend for their reproduction, at each generation, on a mayron or another klepton. Their metameiosis produces particular gametes, which start their development either by zygogenesis (zygokleptons), by gynogenesis (gynokleptons) or by a combination of both systems (tychokleptons). All these particular cases do not constitute, as it has long been believed, “evolutionary dead ends”. On the contrary, some of these forms are advantaged in some conditions, and may also, in some cases, be at the origin of allopolyploid specions. The latter include both “normal mayrons” or eumayrons (allotetraploid bisexual entities) and heteromayrons (allotriploid bisexual entities with metameiosis). Mayron, klepton, klonon and simpson, and their subcategories, are different taxonomic categories which correspond to a single nomenclatural rank, that of species. The existence of interspecific hybridization in nature is a very common phenomenon in animals, the importance of which has long been underestimated. Reticulous relationships between specions have played a major role in the scenario of evolution. This should be acknowledged by the rejection of the “universal tree of life” model for organic evolution and its replacement by a “network of life” model.