Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

In rare cases, two originally independent lines can merge to form a single line, which can then branch apart. Fundamentally, there are two basic mechanisms of syngenesis, i.e. the formation of a new phylogenetic line by the merging of two older lines: symbiogenesis and interspecies hybridization (Fig. XXIII.2). Symbiogenesis refers to the formation of a new species of organism by the integration of two unrelated organisms that live for some time in some form of symbiosis, most probably parasitism or mutualism, into a single organism. If both symbionts begin to reproduce together in a coordinated manner, i.e. so that each daughter organism of symbiotic origin begins to inherit from its parents only the genetic material of both symbionts, the evolutionary fates of the two original species become so interconnected that they sooner or later merge into a single species. Evolutionary dissolution of one species in another species, for example a microscopic parasite or mutualist in its macroscopic host, is sometimes term the Cheshire cat effect (unfortunutly, this term is also used for at least two unrelated phenomena). The relevant literary sources state that, under suitable conditions, a Cheshire cat can gradually disappear and, in the last stage, only its smile remains and, after a certain time, this also disappears. If both symbionts that form a common symbiotic organism produce independent progeny and a new symbiotic organism is formed each time (or at least frequently) through new integration of both symbionts that grew from embryos produced by two unrelated and independent individuals, both species will most probably preserve their species identity (Fig. XXIII.3). This is the frequent case of the symbiosis of fungi and vascular plants. The best known opposite case is the formation eukaryotes, occurring through gradual integration of the members of several unrelated lines of prokaryotic organisms. Completely unrelated phylogenetic lines of organisms can merge through symbiogenesis.
Two lines can also merge through interspecies hybridization, i.e. accidental crossing of the members of two different species. However, in contrast to symbiogenesis, this mechanism can occur only in closely related species with sexual reproduction. The effect of symbiogenesis and hybridization on the topology of the phylogenetic tree is the same; in both cases, the cladogenesis scheme can have a recticular structure instead of a tree structure at some places.