Chapter 26 Phylogeny and the Tree of Life
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Chapter 26 Phylogeny and the Tree of Life Lecture Outline Overview: Investigating the Tree of Life Evolutionary biology is about both process and pattern. o The processes of evolution are natural selection and other mechanisms that change the genetic composition of populations. o These processes lead to evolutionary patterns, the products that evolution has produced over time. Phylogeny is the evolutionary history of a species or group of species. To reconstruct phylogeny, scientists use systematics, an analytical approach to classifying the diversity and determining the evolutionary relationships of living and extinct organisms. o Evidence used to reconstruct phylogenies can be obtained from the fossil record and from morphological, biochemical, and genetic similarities between organisms. Scientists are working to construct a universal tree of all life, which will be refined as new data are collected. Concept 26.1 Phylogenies show evolutionary relationships. Organisms share homologous characteristics due to common descent. o Organisms share genes, metabolic pathways, and structural pathways with their close relatives. The scientific discipline of taxonomy determines how organisms are named and classified. Taxonomy employs a hierarchical system of classification. The Linnaean system, first formally proposed by Carolus Linnaeus in the 18th century, has two main characteristics. 1. Each species has a two-part name. 2. Species are organized hierarchically into broader and broader groups of organisms. Under the binomial system, each species is assigned a two-part Latinized name, a binomial. The first part of the name, the genus, is the closest group to which a species belongs. o The second part, the specific epithet, refers to one species within each genus. o The first letter of the genus name is capitalized, and both the genus and species names are italicized and Latinized. o For example, Linnaeus assigned to humans the optimistic scientific name Homo sapiens, which means “wise man.” A hierarchical classification groups species into increasingly inclusive taxonomic categories. Lecture Outline for Campbell/Reece Biology, 9th Edition, © Pearson Education, Inc. 26-1 Species that appear to be closely related are grouped into the same genus. o For example, the leopard, Panthera pardus, belongs to a genus that includes the African lion (Panthera leo) and the tiger (Panthera tigris). Genera are grouped into progressively broader categories: family, order, class, phylum, kingdom, and domain. Each taxonomic level is more comprehensive than the previous one. o As an example, all species of cats are mammals, but not all mammals are cats. The named taxonomic unit at any level is called a taxon. o For example, Panthera is a taxon at the genus level, and Mammalia is a taxon at the class level that includes all of the many orders of mammals. Higher classification levels are not defined by some measurable characteristic, such as the reproductive isolation that separates biological species. As a result, the larger categories are not comparable between lineages. o An order of snails does not necessarily exhibit the same degree of morphological or genetic diversity as an order of mammals. Classification and phylogeny are linked. The evolutionary history of a group of organisms can be represented in a diagram called a phylogenetic tree. The branching of the tree may match the hierarchical classification of groups nested within more inclusive groups. o In other cases, similarities between organisms may have led taxonomists to place a species in other than the group to which it is most closely related. ○ Such species may be renamed so that their classification accurately reflects evolutionary history. The difficulty of aligning Linnaean classification with phylogeny has led systematists to propose that classification be based entirely on evolutionary relationships. Phylocode only names groups that include a common ancestor and all its descendents. o If this system were adopted, most taxonomic names would be unchanged. However, ranks such as family, order, and class would be abandoned. o Some commonly recognized groups would become part of other groups previously of the same rank. o Because birds descended from a taxon of reptiles, Aves (a Linnaean class) would be considered a subgroup of the Reptilia (another Linnaean class). A phylogenetic tree represents a hypothesis about evolutionary relationships. Evolutionary relationships are often represented as a series of dichotomies, or two-way branch points. o 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 relatives. A rooted tree includes the most recent common ancestor to all taxa in the tree. The term basal taxon refers to a lineage that diverges early in the history of a group, lying on a branch that originates near the common ancestor of the group. Lecture Outline for Campbell/Reece Biology, 9th Edition, © Pearson Education, Inc. 26-2 A polytomy, or branch point from which more than two descendent groups emerge, indicates our limited understanding of certain evolutionary relationships. Three key points about phylogenetic trees should be emphasized. 1. Phylogenetic trees are intended to show patterns of descent, not phenotypic similarity. Closely related organisms may resemble one another due to common ancestry, but may not if their lineages have evolved at different rates or faced very different environmental conditions. 2. The sequence of branching in a tree reflects patterns of descent and does not indicate the absolute ages of particular species. 3. A taxon in a phylogenetic tree did not evolve from an adjacent taxon. Rather, both taxa evolved from a common ancestor. A species’ phylogeny can provide useful information. o From a phylogeny of corn based on DNA data, researchers have identified two closely related species of wild grasses that may serve as “reservoirs” of beneficial alleles. These alleles may be transferred to cultivated corn by plant breeding or genetic engineering. o Phylogenetic trees played a role in demonstrating that “whale meat” sold in Japan was illegally harvested from protected species. Concept 26.2 Phylogenies are inferred from morphological and molecular data. Phenotypic and genetic similarities due to shared ancestry are called homologies. Organisms that share similar morphologies or DNA sequences are likely to be more closely related than organisms without such similarities. Morphological divergence between closely related species can be small or great, because morphological diversity may be controlled by relatively few genetic differences. o Consider the Hawaiian silversword plants, which vary from tall, twiggy trees to dense, ground- hugging shrubs. These phenotypic differences are based on small molecular divergences that arose over the last 5 million years, when the oldest of the Hawaiian Islands formed. Similarity due to convergent evolution is called analogy. o When two organisms from different evolutionary lineages experience similar environmental pressures, natural selection may result in convergent evolution. o For example, marsupial and eutherian moles are very similar in external appearance. However, they last shared a common ancestor 140 million years ago, when marsupial and eutherian mammals diverged. o This common ancestor was not mole-like. Analogous similarities evolved independently in these two mole lineages as they adapted to similar lifestyles. Distinguishing homology from analogy is critical in the reconstruction of phylogeny. o For example, both birds and bats have adaptations that allow them to fly. o A close examination of a bat’s wing shows a greater similarity to a cat’s forelimb than to a bird’s wing. o Fossil evidence also documents that bat and bird wings arose independently from walking forelimbs of different ancestors. o Thus, a bat’s wing is homologous to other mammalian forelimbs but is analogous in function to a bird’s wing. Analogous structures that have evolved independently are also called homoplasies. Lecture Outline for Campbell/Reece Biology, 9th Edition, © Pearson Education, Inc. 26-3 In general, the more elements that are similar in two complex structures, the more likely it is that they evolved from a common ancestor. o For example, the skulls of an adult human and an adult chimpanzee are formed by the fusion of many bones. o The two skulls match almost perfectly, bone for bone. o It is highly unlikely that such complex structures have separate origins. o More likely, the genes involved in the development of both skulls were inherited from a common ancestor. The same argument applies to comparing genes, which are sequences of nucleotides. o If the genes in two organisms have very similar nucleotide sequences, it is highly likely that the genes are homologous. It may be difficult to carry out molecular comparisons of nucleic acids. The first step in molecular comparisons is to align nucleic acid sequences from the two species being studied. o In closely related species, sequences may differ at only one or a few sites. o Distantly related species may have many differences or sequences of different length. Over evolutionary time, insertions and deletions may accumulate, altering the lengths of the gene sequences. Deletions or insertions may shift the remaining sequences, making it difficult to recognize closely matching nucleotide sequences.