The evolutionary history of a group of species is called it phylogeny and a phylogenetic tree is a graphical summary of this history. An evolutionary tree describes the patter, and in some cases the timing, of events that occurred as species diversified. It also records the sequence in which lineages appeared and documents which organisms are more closely or distantly related. They are shared, derived traits: any group that includes an ancestor and all of its descendants is called a monophyletic group (or clade or lineage) This means all synapomorphies are homologous traits but not all homologous traits are synapomorphies. Check out figure 4. 3 and its associating paragraph on page 114: problems in reconstructing phylogenies, not all similar traits are homologous. Some morphological similarities evolve independently in different lineages due to convergent evolution: these occur when natural selection favours similar structures as solutions to problems by similar environments.