Chapter 26 Phylogeny and the Tree of Life 1 Chapter 24 Phylogeny and the Tree of Life Investigating the Tree of Life species their evolutionary relationships To set the stage for surveying life s diversity biologists trace phylogeny the evolutionary history of a species or group of To construct phylogenies biologists utilize systematics a discipline focused on classifying organisms and determining Systematists use data ranging from fossils to molecules and genes to infer evolutionary relationships Concept 26 1 Phylogenies show evolutionary relationships Organisms share many characteristics because of common ancestry An organism is likely to share many of its genes metabolic pathways and structural proteins with its close relatives Binomial Nomenclature names To avoid ambiguity when communicating about their research biologists refer to organism by Latin scientific The two part format of the scientific name commonly called a binomial was instituted in the 18th The first part of a binomial is the name of the genus to which the species belongs century by Carolus Linnaeus The first letter is always capitalized The second part call the specific epithet is unique for each species within the genus The entire binomial is italicized Hierarchical Classification In addition to naming species Linnaeus also grouped them into a hierarchy of increasingly inclusive categories The first grouping is built into the binomial species that appear to be closely related are grouped into the same genus The Linnaean System places related genera in the same family families into orders orders into classes classes into phyla phyla into kingdoms and more recently kingdoms into domains The named taxonomic unit at any level of the hierarchy is called a taxon Linking Classification and Phylogeny The evolutionary history of a group of organisms can be represented in a branching diagram called a phylogenetic tree Branching pattern often matches how taxonomists have classified groups of organisms nested within more Misclassification might be that over the course of evolution a species has lost a key feature shared by its close inclusive groups relatives If DNA or other new evidence indicates that such a mistake has occurred the organism may be reclassified to accurately reflect its evolutionary history Such difficulties in aligning Linnaean classification with phylogeny have led some systematists to propose that classification be based entirely on evolutionary relationships A system called PhyloCode only names groups that include a common ancestor and all of its descendants Chapter 26 Phylogeny and the Tree of Life 2 A phylogenetic tree represents a hypothesis about evolutionary relationships These relationships often are depicted as a series of dichotomies or two way branch points Each branch point represents the divergence of two evolutionary lineages from a common ancestor Sister taxa are groups of organisms that share an immediate common ancestor and are each other s closest Rooted means that a branch point within the tree represents the most recent common ancestor of all taxa in relatives the tree Basal Taxon refers to a lineage that diverges early in the history of a group and lies on a branch that originates near the common ancestor of the group Polytomy is a branch from which more than two descendant groups emerge A polytomy signifies that evolutionary relationships among the taxa are not yet clear What We can and cannot learn from phylogenetic trees Phylogenetic trees are intended to show patterns of descent not phenotypic similarity Although closely related organisms often resemble one another due to their common ancestry they may not if their lineages have evolved at different rates or faced very different environmental The sequence of branching in a tree does not necessarily indicate the actual absolute ages of the particular conditions species Do not assume that the length of a tree branch indicates when a species evolved or how much change occurred in each lineage We should not assume that a taxon on a phylogenetic tree evolved from a taxon next to it Applying Phylogenies From a phylogeny of maize base on DNA data researchers have been able to identify two species of wild grasses that may be maize s closest living relatives Concept 26 1 Summary Phylogenies show evolutionary relationships Linnaeus s binomial classification system gives organisms two part names a genus plus a specific epithet In the Linnaean system species are grouped in increasingly broad taxa Related genera are placed in the same family families in orders orders in classes classes in phyla phyla in kingdoms and kingdoms in domains Systematists depict evolutionary relationships as branching phylogenetic trees Many systematists propose that classification be based entirely on evolutionary relationships Unless branch lengths are proportional to time or amount of genetic change a phylogenetic tree indicates only patterns Much information can be learned about a species from its evolutionary history hence phylogenies are useful in a range of descent of applications Concept 26 2 Phylogenies are inferred from morphological and molecular data Morphological and molecular homologies It is important to focus on features that result from common ancestry because only such features reflect evolutionary relationships Chapter 26 Phylogeny and the Tree of Life 3 Phenotypic and genetic similarities due to shared ancestry are called homologies Bones in the forelimbs of mammals descend from a common ancestor with the same bone structure Genes or other DNA sequences are homologous if they are descended from sequences carried by a common ancestor Organisms that share very similar morphologies or similar DNA sequences are likely to be more closely related than organism with vastly different structures or sequences In some cases the morphological divergence between related species can be great and their genetic divergence small or vice versa Sorting homology from analogy In constructing a phylogeny the similarities are due to convergent evolution called analogy rather than to shared ancestry homology Convergent evolution occurs when similar environmental pressures and natural selection produce similar analogous adaptations in organisms from different evolutionary lineages Analogous structures that arose independently are called homoplasies The more elements that are similar in two complex structures the more likely it is that they evolved from a If genes of