The phylogenetic tree, or phylogram, is a graphical representation of phylogenesis, the gradual divergence of species from a common ancestor. In addition to the order of divergence of the individual species, a time scale can also be designated on the phylogenetic tree, permitting dating of the individual events in phylogenesis. For some purposes, it is useful to denote changes in the properties of the studied organisms on the phylogenetic tree, termed anagenesis. If the graph is used to depict not genealogical relationships between organisms, but their mutual similarity, then this is called a phenogram. Mutually unrelated species living in a similar environment and exposed to similar selection pressures (fish, dolphins, sharks, ichthyosaurus) can gradually become more similar (i.e. converge to a similar body structure) and can be placed close to one another on a phenogram (but not on a phylogram).