Phylogeny

Intro: Why study evolutionary relationships? Legless lizards and snakes look like they could be considered the same species By studying evolutionary relationships, we know that they are not that closely related

Intro: Why study evolutionary relationships? It turns out that legless lizards evolved independently in several different groups. They look like snakes because they developed similar adaptations in response to similar environments

Figure 20.2 ANCESTRAL LIZARD (with limbs) Eastern glass lizard Monitor lizard Snakes Geckos No limbs Iguanas No limbs

Phylogeny is the evolutionary history of a species or group of related species We study evolutionary relationships because it helps us to classify organisms more accurately. Scientists reconstruct and interpret phylogenies using systematics Systematics: a discipline focused on classifying organisms and determining evolutionary relationships Taxonomy is the ordered division and naming of organisms

Binomial Nomenclature In the 18th century, Carolus Linnaeus published a system of taxonomy based on resemblances Two key features of his system remain useful today: 1.Binomial nomenclature 2.Linnaean system of hierarchical classification

Binomial Nomenclature

The system of naming species that uses two terms to denote a species First word: the genus Genus: a group to which a species belongs Second word: the specific epithet Specific epithet: the species Both parts together name the species (not the specific epithet alone) Binomial nomenclature prevents confusion caused by common names Homo sapiens Latinized, italicized, Genus is capitalized Binomial Nomenclature

Linnaean System of Hierarchical Classification Groups species in increasingly broad categories The taxonomic groups from narrow to broad are species, genus, family, order, class, phylum, kingdom, and domain Taxon: any level of the hierarchy

Figure 20.3 Species: Panthera pardus Kingdom: Animalia Domain: Archaea Domain: Bacteria Domain: Eukarya Genus: Panthera Order: Carnivora Family: Felidae Class: Mammalia Phylum: Chordata

Phylogenic trees link hierarchical classification and phylogeny The Linnaean system of hierarchical classification tells us nothing about evolutionary relationships Sometimes taxonomists place species within a genus to which it is not most closely related Systematists have proposed that classification be based entirely on evolutionary relationships

Phylogenic trees link hierarchical classification and phylogeny Systematists have proposed that classification be based entirely on evolutionary relationships Phylogenic trees represent hypotheses about evolutionary relationships Phylogenic tree: a branching diagram showing inferred evolutionary relationships between species

Figure 20.4 Panthera pardus (leopard) Species Order FamilyGenus Taxidea taxus (American badger) Canis latrans (coyote) Lutra lutra (European otter) Canis lupus (gray wolf) Panthera Taxidea Canis Lutra Felidae Mustelidae Carnivora Canidae 12

Branch points: represents the divergence of two evolutionary lineages from a common ancestor Reading Phylogenic Trees

Figure Branch point: where lineages diverge This branch point represents the common ancestor of taxa A−G. This branch point forms a polytomy: an unresolved pattern of divergence. ANCESTRAL LINEAGE Sister taxa Basal taxon Taxon A Taxon B Taxon C Taxon D Taxon E Taxon F Taxon G

Branch points: represents the divergence of two evolutionary lineages from a common ancestor Sister taxa: groups of organisms that share an intermediate common ancestor and are each other’s closest relatives Reading Phylogenic Trees

Figure Branch point: where lineages diverge This branch point represents the common ancestor of taxa A−G. This branch point forms a polytomy: an unresolved pattern of divergence. ANCESTRAL LINEAGE Sister taxa Basal taxon Taxon A Taxon B Taxon C Taxon D Taxon E Taxon F Taxon G

Branch points: represents the divergence of two evolutionary lineages from a common ancestor Sister taxa: groups of organisms that share an intermediate common ancestor and are each other’s closest relatives Basal taxon: a lineage that diverges early in the history of a group and lies on a branch that orignates near the common ancestor Reading Phylogenic Trees

Figure Branch point: where lineages diverge This branch point represents the common ancestor of taxa A−G. This branch point forms a polytomy: an unresolved pattern of divergence. ANCESTRAL LINEAGE Sister taxa Basal taxon Taxon A Taxon B Taxon C Taxon D Taxon E Taxon F Taxon G

Branch points: represents the divergence of two evolutionary lineages from a common ancestor Sister taxa: groups of organisms that share an intermediate common ancestor and are each other’s closest relatives Basal taxon: a lineage that diverges early in the history of a group and lies on a branch that orignates near the common ancestor Polytomy: a branch point from which more than two descendant groups emerge Reading Phylogenic Trees

Figure Branch point: where lineages diverge This branch point represents the common ancestor of taxa A−G. This branch point forms a polytomy: an unresolved pattern of divergence. ANCESTRAL LINEAGE Sister taxa Basal taxon Taxon A Taxon B Taxon C Taxon D Taxon E Taxon F Taxon G

Branch points: represents the divergence of two evolutionary lineages from a common ancestor Sister taxa: groups of organisms that share an intermediate common ancestor and are each other’s closest relatives Basal taxon: a lineage that diverges early in the history of a group and lies on a branch that originates near the common ancestor Polytomy: a branch point from which more than two descendant groups emerge Rooted phylogenic tree: contains a branch point (furthest to the left) representing the most recent common ancestor of all taxa in the tree Reading Phylogenic Trees

Figure Branch point: where lineages diverge This branch point represents the common ancestor of taxa A−G. This branch point forms a polytomy: an unresolved pattern of divergence. ANCESTRAL LINEAGE Sister taxa Basal taxon Taxon A Taxon B Taxon C Taxon D Taxon E Taxon F Taxon G

What We Can and Cannot Learn from Phylogenetic Trees Phylogenetic trees show patterns of descent, not phenotypic similarity Phylogenetic trees do not generally indicate when a species evolved or how much change occurred in a lineage It should not be assumed that a taxon evolved from the taxon next to it

Phylogenies are inferred from morphological and molecular data To infer phylogeny, systematists gather information about morphologies, genes, and biochemistry of living organisms The similarities used to infer phylogenies must result from shared ancestry

Morphological and Molecular Homologies Homologies: phenotypic and genetic similarities due to shared ancestry Organisms with similar morphologies or DNA sequences are likely to be more closely related than organisms with different structures or sequences

Sorting Homology from Analogy When constructing a phylogeny, systematists need to distinguish whether a similarity is the result of homology or analogy Homology is similarity due to shared ancestry Analogy is similarity due to convergent evolution

Convergent evolution occurs when similar environmental pressures and natural selection produce similar (analogous) adaptations in organisms from different evolutionary lineages

Figure 20.7 Convergent Evolution Analogous structures or molecular sequences that evolved independently are also called homoplasies

Evaluating Molecular Homologies Molecular homologies are determined based on the degree of similarity in nucleotide sequence between taxa Systematists use computer programs when analyzing comparable DNA segments from different organisms Shared bases in nucleotide sequences that are otherwise very dissimilar are called molecular homoplasies

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