Step 4. Examine your cladogram. Each node shows a presumed common ancestor. Find themonophyletic groups. A monophyletic group is a group of species that includes an ancestral species andall of its descendants. Thus in the following cladogram, C, D and their common ancestor form amonophyletic group. B, C, D and their common ancestor are another monophyletic group. If you tried togroup B, C and their common ancestor together they would be considered paraphyletic. A paraphyleticgroup is a group of species that includes an ancestral species but not all of its descendants. In this caseyou are excluding D, which shares a common ancestor with B and C.ABDCommon ancestorpf C and DCommon ancestorof B, C and DCommon ancestorof A, B, C and DAn important thing to realize is that although the nodes represent an ancestral population it does not mean thatthere was a single species that directly gave rise to species C and D. Rather, tracing back from D toward thecommon ancestor of C and D, there could be several other species along the lineage (sequence of ancestor-descendant populations). For example, imagine that species D is a chimpanzee and C is a gorilla. The cladogramdoes not suggest that there was a single species that evolved into chimps and gorillas. What it does mean is thatthere was an ancestral population that gave rise to two distinct lineages. One lineage eventually gave rise tochimps and the other to gorillas. The lines of a cladogram represent the lineages and time. The deeper into thecladogram you go, the further back in time you go. Thus the common ancestor of B, C and D occurred before thecommon ancestor of C and D. Remember that cladistics is concerned with the timing of divergence. In thiscladogram, C and D are the two most closely related species because they shared a common ancestor morerecently than any other clade. ADEF

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Systematics involves the scientific study of biological diversities and their evolutionary history.…

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