A network in mathematics is a graph in which all nodes are connected via edges. In biology, not only topological relationship but edge lengths are also important. When this biological or evolutionary feature is stressed, we call it phylogenetic network. If there is no reticulated structure, a network is also a tree. However, when various biological phenomena such as parallel changes, recombinations, gene conversions, horizontal gene transfers, hybridizations, and domain fusions occurred, we may have a reticulated evolutionary history. A reticulated phylogenetic network is useful for describing these complex phylogenetic structure.
Saitou, N. (2013). Network. In Brenner’s Encyclopedia of Genetics: Second Edition (pp. 28–29). Elsevier Inc. https://doi.org/10.1016/B978-0-12-374984-0.01028-7