Fig. 3

Phylogenies indicate both relatedness and historical descent. As a rough analogy, phylogenies can be compared to pedigrees of human families. a shows a simple “phylogeny” of human relationships. You, the reader, are indicated as one terminal node, along with a sibling with whom you share a recent common ancestor (your parent). Also depicted are two of your cousins, each of whom is equally related to you and to your sibling and with whom you share a more distant common ancestor (your grandparent). You are not descended from your sibling or from your cousin (nor are they descended from you)—rather, your relatedness is determined by the pattern of descent and ancestry going back in time. b shows a phylogeny of the contemporary species C, K, Y, and S. Y is not descended from S (nor vice versa), but rather these are sister taxa that are both descended from their common ancestor, P. Similarly, A is the common ancestor of the sister taxa C and K. All of these species share a more distant common ancestor, G. The ancestors A, P, and G are extinct and may be known only from fossils or may be inferred to have existed using genetic or morphological data. c shows the groupings of related species and their shared ancestors, which are known as “clades.” The clades in this tree include the groupings [C + K + A] and [Y + S + P], which are nested within a larger clade [C + K + A + Y + S + P + G]. Both A and P are descended from their common ancestor G, and their descendant clades (i.e., [C + K + A] and [Y + S + P], respectively) are “sister groups” of one another (A and P were themselves sister taxa). Groups that do not include the most recent shared ancestor or that include only some of an ancestor’s descendants are not clades