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Evolution produces two distinct but related patterns, both evident in nature. First is the nested pattern of similarities found among species on present-day Earth. The second is the historical pattern of evolution recorded by fossils. Life, in its simplest form, originated more than 3.5 billion years ago. Darwin recognized that the species he observed must be the modified descendants of earlier ones. Distinct populations of an ancestral species separate and diverge through time, again and again, giving rise to multiple descendant species. This history of descent with branching is called phylogeny,and is much like the genealogy that records our own family histories. 23.1 READING A PHYLOGENETIC TREE Chapter 22 introduced the concept of speciation, the set of processes by which physically, physiologically, or ecologically isolated populations diverge from one another to the point where they can no longer produce fertile offspring. As species proliferate, their evolutionary relationships to one another unfold in a treelike pattern, with individual species at the twig tips and their closest relatives connected to them at the nearest fork in the branch, called a node. A node thus represents the most recent common ancestor of two descendant species. Phylogenetic trees provide hypotheses of evolutionary relationships.Phylogenetics is one of two related disciplines within systematics, the study of evolutionary relationships among organisms. The other is taxonomy, the classification of organisms. The aim of taxonomy is to recognize and name groups of individuals as species, and, subsequently, to group closely related species into the more inclusive taxonomic group of the genus, and so on up through the taxonomic ranks—species, genus, family, order, class, phylum, kingdom, domain. Phylogenetics, on the other hand, aims to discover the pattern of evolutionary relatedness among groups of species or other groups by comparing their anatomical or molecular features,and to depict these relationships as a phylogenetic tree. A phylogenetic tree is a hypothesis about the evolutionary history, or phylogeny, of the species. Phylogenetic trees are hypotheses because theyrepresent the best model, or explanation, of the relatedness of organisms on the basis of all the existing data. The search for sister groups lies at the heart of phylogenetics.Two species, or groups of species, are considered to be closest relatives if they share a common ancestor not shared by any other species or group. Groups that are more closely related to each other than either of them is to any other group, like lungfish and tetrapods, are called sister groups. Simply put, phylogenetic hypotheses amount to determining sister-group relationships because the simplest phylogenetic question we can ask is whichtwo of any three species (or other groups) are more closely related to each other than either is to the third. In this light, we can see that a phylogenetic tree is simply a set of sister-group relationships; adding a species to the tree entails finding its sister group in the tree.