Reticulate evolution

Reticulate evolution, or network evolution, describes the origination of a lineage through the partial merging of two ancestor lineages, leading to relationships better described by a phylogenetic network than a bifurcating tree. Reticulate patterns can be found in the phylogenetic reconstructions of biodiversity lineages obtained by comparing the characteristics of organisms. Reticulation processes can potentially be convergent and divergent at the same time. Reticulate evolution indicates the lack of independence between two evolutionary lineages. Reticulation affects survival, fitness and speciation rates of species.   The adjective reticulate stems from the Latin words reticulatus “having a net-like pattern” and reticulum “little net”, according to the Etymology Dictionary. Reticulate evolution, or network evolution, describes the origination of a lineage through the partial merging of two ancestor lineages, leading to relationships better described by a phylogenetic network than a bifurcating tree. Reticulate patterns can be found in the phylogenetic reconstructions of biodiversity lineages obtained by comparing the characteristics of organisms. Reticulation processes can potentially be convergent and divergent at the same time. Reticulate evolution indicates the lack of independence between two evolutionary lineages. Reticulation affects survival, fitness and speciation rates of species.   The adjective reticulate stems from the Latin words reticulatus “having a net-like pattern” and reticulum “little net”, according to the Etymology Dictionary. Reticulate evolution can happen between lineages separated only for a short time, for example through hybrid speciation in a species complex. Nevertheless, it also takes place over larger evolutionary distances, as exemplified by the presence of organelles of bacterial origin in eukaryotic cells. Reticulation occurs at various levels: at a chromosomal level, meiotic recombination causes evolution to be reticulate; at a species level, reticulation arises through hybrid speciation and horizontal gene transfer; and at a population level, sexual recombination causes reticulation. Since the nineteenth century, scientists from different disciplines have studied how reticulate evolution is induced. Researchers succeeded to increasingly identify these mechanisms and processes. Reticulate evolution is found to be driven by symbiosis, symbiogenesis (endosymbiosis), lateral gene transfer, hybridization and infectious heredity. Symbiosis is a close and long-term biological interaction between two different biological organisms. Often, both of the organisms involved develop new features upon the interaction with the other organism. This may lead to the development of new, distinct organisms. The alterations in genetic material upon symbiosis can occur via germline transmission or lateral transmission. Therefore, the interaction between different organisms can drive evolution of one or both organisms. Symbiogenesis (endosymbiosis) is a special form of symbiosis whereby an organism lives inside another, different organism. Symbiogenesis is thought to be very important in the origin and evolution of eukaryotes. Eukaryotic organelles, such as mitochondria, have been theorized to have been originated from cell-invaded bacteria living inside another cell. Lateral gene transfer, or horizontal gene transfer, is the movement of genetic material between unicellular and/or multicellular organisms without a parent-offspring relationship. The horizontal transfer of genes results in new genes, which could give new functions to the recipient and thus could drive evolution. In the neo-Darwinian paradigm, one of the assumed definition of a species is that of Mayr’s, which defines species based upon sexual compatibility. Mayr’s definition therefore suggests that individuals that can produce fertile offspring must belong to the same species. However, in hybridisation, two organisms produce offspring while being distinct species. During hybridisation the characteristics of these two different species are combined yielding a new organism, called a hybrid, thus driving evolution. Infectious agents, such as viruses, can infect the cells of host organisms. Viruses infect cells of other organisms in order to enable their own reproduction. Hereto, many viruses can insert copies of their genetic material into the host genome, potentially altering the phenotype of the host cell. When these viruses insert their genetic material in the genome of germ line cells, the modified host genome will be passed onto the offspring, yielding genetically differentiated organisms. Therefore, infectious heredity plays an important role in evolution, for example in the formation of the female placenta.