Systematics and the Phylogenetic Revolution Chapter 23 1

Systematics and the Phylogenetic Revolution Chapter 23 1

Introduction • All organisms: – Are composed of one or more cells – Carry out metabolism – Transfer energy with ATP – Encode hereditary information in DNA • Tremendous diversity of life – Bacteria-----whales----sequoia trees • Biologists group organisms based on shared characteristics 2

Systematics • Since fossil records are not complete, scientists rely on other types of evidence to establish the best hypothesis of evolutionary relationships • Systematics: the study of evolutionary relationships • Phylogeny: a hypothesis about patterns of relationship among species 3

Systematics • Darwin envisioned that all species were descended from a single common ancestor • He depicted this history of life as a branching tree. • Now called a cladogram 4

Systematics • Twigs of a tree represent existing species • Joining of twigs and branches reflects the pattern of common ancestry back in time to a single common ancestor • Darwin called this process “descent with modification” 5

Systematics Phylogenies depict evolutionary relationships 6

Systematics • Key to interpreting a phylogeny: look at how recently species share a common ancestor • Similarity may not accurately predict evolutionary relationships – Early systematists relied on the expectation that the greater the time since two species diverged from a common ancestor, more different would be 7

Systematics • Evolution can occur rapidly at one time and slowly at another (punctuated and gradual evolution) 8

Systematics • Oscillating selection: Traits can evolve in one direction, then back the other way • Evolution is not always divergent: convergent evolution – Use similar habitats – Similar environmental pressures • Evolutionary reversal: process in which a species re-evolves the characteristics of an ancestral species 9

Cladistics • Derived characteristic: similarity that is inherited from the most recent common ancestor of an entire group • Ancestral: similarity that arose prior to the common ancestor of the group • In cladistics, only shared derived characters are considered informative about evolutionary relationships • To use the cladistic method character variation must be identified as ancestral or derived 10

Cladistics • Characters can be any aspect of the phenotype – Morphology - Physiology – Behavior - DNA • Characters should exist in recognizable character states – Example: Teeth in amniote vertebrates has two states, present in most mammals and reptiles and absence in birds and turtles 11

Cladistics Examples of ancestral versus derived characters • Presence of hair is a shared derived feature of mammals • Presence of lungs in mammals is an ancestral feature; also present in amphibians and reptiles 12

Cladistics • Determination of ancestral versus derived – First step in a manual cladistic analysis is to polarize the characters (are they ancestral or derived) • Example: polarize “teeth” means to determine presence or absence in the most recent common ancestor 13

Cladistics – Outgroup comparison is used to assign character polarity • A species or group of species not a member of the group under study is designated as the outgroup – Outgroup species do not always exhibit the ancestral condition 14

Cladistics • When the group under study exhibits multiple character states, and one of those states is exhibited by the outgroup, then that state is ancestral and other states are derived • Most reliable if character state is exhibited by several different outgroups 15

Cladistics • Following the character state-outgroup method – Presence of teeth in mammals and reptiles is ancestral – Absence of teeth in birds and turtles is derived 16

Cladistics Construction of a cladogram • Polarize characteristics • Clade: species that share a common ancestor as indicated by the possession of shared derived characters • Clades are evolutionary units and refer to a common ancestor and all descendants • Synapomorphy: a derived character shared by clade members 17

Cladistics • A simple cladogram is a nested set of clades • Plesiomorphies: ancestral states • Symplesiomorphies: shared ancestral states, not informative about phylogenetics. 18

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Cladistics 20

Cladistics • Homoplasy: a shared character state that has not been inherited from a common ancestor – Results from convergent evolution – Results from evolutionary reversal • If there are conflicts among characters, use the principle of parsimony which favors the hypothesis that requires the fewest assumptions 21

Cladistics Parsimony and Homoplasy 22

Cladistics A Cladogram; DNA 23

Cladistics A Cladogram: DNA 24

Other Phylogenetic Methods • Some characters evolve rapidly and principle of parsimony may be misleading • Rate at which some parts of the DNA genome evolve – Mutations in repetition sequences, not deleted by natural selection • Statistical approaches • Molecular clock: rate of evolution of a molecule is constant through time 25

Systematics and Classification • Classification: how we place species and higher groups into the taxonomic hierarchy – Genus, family, class. . • Monophyletic group: includes the most recent common ancestor of the group and all of its descendants (clade) • Paraphyletic group: includes the most recent common ancestor of the group, but not all its descendants 26

Systematics and Classification • Polyphyletic group: does not include the most recent common ancestor of all members of the group • Taxonomic hierarchies are based on shared traits, should reflect evolutionary relationships • Why should you refer to birds as a type of dinosaur? 27

Systematics and Classification Monophyletic Group 28

Systematics and Classification Paraphyletic Group 29

Systematics and Classification Polyphyletic Group 30

Systematics and Classification Old plant classification system 31

Systematics and Classification New plant classification system 32

Systematics and Classification • Phylogenetic species concept (PSC) – Focuses on shared derived characters • Biological species concept (BSC) – Defines species as groups of interbreeding population that are reproductively isolated • Phylogenetic species concept: species should be applied to groups of populations that have been evolving independently of other groups 33

Comparative Biology • Phylogenetics is the basis of all comparative biology • Homologous structures are derived from the same ancestral source (e. g. dolphin flipper and horse leg) • Homoplastic structures are not (e. g. wings of birds and dragonflies): -Parental care • Dinosaurs, birds, crocodiles • Homologous behavior 34

Comparative Biology Parental care in dinosaurs and crocodiles 35

Comparative Biology • Homoplastic convergence: saber teeth – Occurred in different groups of extinct carnivores – Similar body proportions (cat) – Similar predatory lifestyle – Most likely evolved independently at least 3 times 36

Comparative Biology Distribution of saber-toothed mammals 37

Comparative Biology • Homoplastic convergence: plant conducting tubes – Sieve tubes facilitate long-distance transport of food that is essential for the survival of tall plants – Brown algae also have sieve elements – Closest ancestor a single-celled organism 38

Comparative Biology Convergent evolution of conducting tubes 39

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Comparative Biology • Most complex characters evolve through a sequence of evolutionary changes • Modern-day birds – wings, feathers, light bones, breastbone • Initial stages of a character evolved as an adaptation to some environmental selective pressure • First featherlike structure evolved in theropod phylogeny – Insulation or perhaps decoration 41

Comparative Biology 42

Disease Evolution • HIV evolved from a simian (monkey) viral counterpart SIV – First recognized in 1980’s – Current estimate: >39 million people infected; > 3 million die each year – SIV found in 36 species of primates – Does not usually cause illness in monkeys – Around for more than a million years as SIV 43

Disease Evolution 44

Disease Evolution Phylogenetic analysis of HIV and SIV • First: HIV descended from SIV – All strains of HIV are nested within clades of SIV • Second: a number of different strains of HIV exits – Independent transfers from different primate species – Each human strain is more closely related to a strain of SIV than to other HIV strains 45

Disease Evolution • Third: humans have acquired HIV from different host species 46