The chapter dealing with speciation will discuss the formation of a new species through interspecific hybridization. However, species are formed in evolution not only through unique hybridization events but, rather, it sometimes occurs that they can exist in nature over long periods of time through interspecific hybridization, appearing anew in each generation, i.e. through hybridogenesis. It should be mentioned, however, that some authors, especially botanists, use the term hybridogenesis to denote any formation of a species through interspecific crossing.
Kleptospecies are an extreme example of real hybridogenesis. The green water frog (Rana esculenta) is a well-known example of such a species (Reyer et al. 2003). This locally very abundant species is formed by hybridization of two species, the marsh frog (R. ridibunda) and the pool frog (R. lessonae)(Fig. XX.8). The genes of both parent species participate in the formation of the somatic tissues of the green water frog; however, the chromosomes originally derived from the pool frog are eliminated in the germinal cell line in the Western part of Europe. In contrast, in the Eastern part of Europe, the green water frog eliminates the chromosomes derived from the marsh frog. Thus, if two green water frogs were to reproduce together (which does not occur very often because males and females rarely exist in the same population), the progeny would have the genotype of the marsh frog in the Western part of Europe. Similarly, if the green water frog were to reproduce with the marsh frog, all the progeny would have the genotype of the marsh frog. Crossing of the green water frog with the pool frog would again yield only the green water frog. A group of all three types of frogs together is sometimes called a synklepton. This situation is very advantageous from the viewpoint of the marsh frog (in the Western part of Europe) because it combines the advantages of both sexual and asexual reproduction. Green water frogs maintain the same amount of intraspecific variability as a sexually reproducing species (so that, for example, it does not so readily submit in the coevolutionary battle with parasites) and, simultaneously, a marsh frog that crosses with the pool frog need not pay the two-fold genetic cost of sex (the cost of meiosis, see XIII.2.3)) as its progeny will pass only their own genes on to the next generation and not the genes of their sexual partners. On the other hand, the pool frog can only lose in this situation as its genes that find themselves in the bodies of green water frogs cannot be passed on to further generations. From its point of view, the green water frogs (in actual fact primarily marsh frogs) “steal” its gametes and frequently also its ecological niche (from which is derived the name of the phenomenon – “klepto” – to steal).
Cases where one species steals gametes (microgametes) from another species are relatively common in nature, but rarely lead to the formation of kleptospecies. In most cases, the microgametes of one species only activate the development of the macrogametes of the other species and its genes do not participate in any way in formation of the bodies (and, of course, also the gametes). This type of parthenogenetic reproduction is called gynogenesis. In the greatest number of cases, parthenogenetic females of a polyploid species that cannot reproduce sexually because of their polyploidy, steal gametes in this way. This situation is, of course, not as advantageous for the female as the formation of kleptospecies. The females avoid the two-fold cost of meiosis and also the two-fold cost of males, but, in parthenogenetic reproduction the genetic polymorphism is constantly reduced in the given line which, in time, can lead to losing out in the co-evolutionary battle with parasites or with sexually reproducing competitors of the same or related species.