A
phylogenetic network or
reticulation is any
graph used to visualize evolutionary relationships (either
abstractly or explicitly) between
nucleotide sequences,
genes,
chromosomes,
genomes, or
species. They are employed when reticulate events such as
hybridization,
horizontal gene transfer,
recombination, or
gene duplication and loss are believed to be involved. They differ from
phylogenetic trees by the explicit modeling, by means of the addition of hybrid nodes (nodes with two parents) instead of only tree nodes (nodes with only one parent).
Phylogenetic trees are a subset of
phylogenetic networks. Phylogenetic networks can be inferred and visualised with software such as
SplitsTree and, more recently,
Dendroscope. A standard format for representing phylogenetic networks is a variant of
Newick format which is extended to support networks as well as trees.