Systematics and the Phylogenetic Revolution Chapter 23

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

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

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 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” Systematics

Phylogenies depict evolutionary relationships 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 Systematics

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

Cladistics Derived characteristic: similarity that is inherited from the most recent common ancestor of an entire group, apomorphy Ancestral: similarity that arose from common ancestor, plesiomorphy 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

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 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 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 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 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 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 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 Cladistics

A simple cladogram is a nested set of clades Plesiomorphies: ancestral states Symplesiomorphies: shared ancestral states 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 Cladistics

Parsimony and Homoplasy Cladistics

A Cladogram; DNA Cladistics

A Cladogram: DNA Cladistics

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

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

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? Systematics and Classification

Monophyletic Group Systematics and Classification

Paraphyletic Group Systematics and Classification

Polyphyletic Group 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 Systematics and Classification