CHAPTER 26 PHYLOGENY AND THE TREE OF LIFE Connecting Classification to Phylogeny

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1 CHAPTER 26 PHYLOGENY AND THE TREE OF LIFE Connecting Classification to Phylogeny To trace phylogeny or the evolutionary history of life, biologists use evidence from paleontology, molecular data, comparative anatomy, and other approaches. Tracing phylogeny is one of the main goals of systematics, the study of biological diversity in an evolutionary context.

2 1. Taxonomy employs a hierarchical system of classification The Linnean system, first formally proposed by Linneaus in Systema naturae in the 18th century, has two main characteristics. Each species has a two-part name. Species are organized hierarchically into broader and broader groups of organisms.

3 Under the binomial system, each species is assigned a two-part latinized name, a binomial. The genus ( general type ), is the closest group to which a species belongs. The second part, the specific epithet, refers to one species within each genus. The first letter of the genus is capitalized and both names are italicized and latinized. For example, Linnaeus assigned to humans the scientific name Homo sapiens, which means wise man, perhaps in a show of optimism.

4 A hierachical classification will group species into broader taxonomic categories. Species that appear to be closely related are grouped into the same genus. For example, the leopard, Panthera pardus, belongs to a genus that includes the African lion (Panthera leo) and the tiger (Panthera tigris). Biology s taxonomic scheme formalizes our tendency to group related objects.

5 More inclusive Genera are grouped into progressively broader categories: family, order, class, phylum, kingdom and domain. Most specific Fig. 25.7

6 Each taxonomic level is more comprehensive than the previous one. 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. Example: Pinus is a taxon at the genus level, the generic name for various species of pine trees. Mammalia, a taxon at the class level, includes all the many orders of mammals.

7 Phylogenetic trees reflect the hierarchical classification of taxonomic groups nested within more inclusive/genetically related groups. Fig. 25.8

8 2. Modern phylogenetic systematics is based on cladistic analysis A phylogeny is determined by a variety of evidence including fossils, molecular data, anatomy, and other features. A cladogram is constructed from a series of dichotomies.

9 These dichotomous branching diagrams can include more taxa. The sequence of branching symbolizes historical chronology. The last ancestor common to both the cat and dog families lived longer ago than the last common ancestor shared by leopards and domestic cats.

10 Each branch or clade can be nested within larger clades. A clade consists of an ancestral species and all its descendents, a monophyletic group. Groups that do not fit this definition are unacceptable in cladistics. Fig. 25.9

11 Horse s Ancestral Horse Zebra s Asses Hippos Ancestral Odd-Toed Ungulate B Lineage includes only horselike animals I Lineage includes the hippos, but share a common ancestor with the horse lineage

12 Genus Homo Ancestor to Hominids and Pongids Chimps Bipedal Loss of tails Gorillas Genus Australopithecine Genus Pongida Knucklewalking e Monkey Lineage s Tailed Ancestor to Monkey and Apes See, people did not evolve from monkeys. Monkeys are paraphyletic to us.

13 Humans Bipedal Chimp s Gorillas Genus Australopithecine Genus Pongida e Knucklewalking Note: Knuckle-walkers inherited their trait from F the common ancestor for that trait. Bipedality developed in a separate lineage from knuckle-walking. Thus, while both groups lack tails, bipedality and knuckle-walking developed from separate lineages. Ancestor to Hominids and Pongids Loss of tails Well, I ll be a monkey s uncle!

14 Determining which similarities between species are relevant to grouping the species in a clade is a challenge. Similarities are based on shared ancestry or homology, not those that are based on convergent evolution or analogy. Dolphins and sharks are both streamlined aquatic vertebrates, but their resemblance is due to analogous adaptations.

15 The more homologous parts that two species share, the more closely related they are. Also, the more complex two structures are, the less likely that they evolved independently. human and chimpanzee skulls are composed not of a single bone, but a fusion of multiple bones that match almost perfectly. It is highly improbable that such complex structures matching in so many details could have separate origins.

16 A shared derived character is unique to a particular clade. Example: thumbs are shared by all primates A shared primitive character is found not only in the clade being analyzed, but older clades too. Example: Primates have fur, just like other mammals, but since primates have thumbs, thumbs are a more recent derived character than is fur.

17 Shared derived characters are useful in establishing a phylogeny, but shared primitive characters are not. The status of a character as analogous versus homologous or shared versus primitive may depend on the level at which the analysis is being performed.

18 A key step in cladistic analysis is outgroup comparison, which is used to differentiate shared primitive characters from shared derived ones. To do this we need to identify an outgroup: a species or group of species that is closely related to the species that we are studying, but is less closely related than any study-group members are to each other.

19 Analyzing the distribution of homologies enables us to identify the sequence in which derived characters evolved during vertebrate phylogeny. Fig

20 A cladogram presents the chronological sequence of branching during the evolutionary history of a set of organisms.

21 Parsimony - Systematists can never sure of finding the best tree in a large data set They narrow possibilities by applying the principles of maximum parsimony and maximum likelihood

22 Maximum parsimony assumes that the tree that requires the fewest evolutionary events (appearances of shared derived characters) is the most likely. Principle of Maximum Liklihood given rules about how DNA changes over time, a tree can be found that reflects the most likely sequence of evolutionary events Pearson Education, Inc.

23 TECHNIQUE Applying Parsimony to Molecular Systemics Species Species Species 1 Three phylogenetic hypotheses:

24 Figure 26.15b Species Species Species Three phylogenetic hypotheses: TECHNIQUE 2 Site Species C T A T Species C T T C Species A G A C Ancestral sequence A G T T

25 TECHNIQUE 3 1/C 1/C 1/C 1/C 1/C 4 2/T 3/A 3/A 2/T 4/C 3/A Species Species Species Ancestral sequence C T A T C T T C A G A C A G T T 4/C 3/A 4/C 4/C 2/T 4/C 2/T 2/T 3/A RESULTS 6 events 7 events 7 events

26 So, that makes us all related, and therefore, family!