Systematics and the Phylogenetic Revolution Chapter 23

1. Systematics and the Phylogenetic Revolution Chapter 23

2. 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

3. 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

4. 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

5. 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”

6. Systematics Phylogenies depict evolutionary relationships

7. 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

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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

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

20. Cladistics

21. 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

22. Cladistics Parsimony and Homoplasy

23. Cladistics A Cladogram; DNA

24. Cladistics A Cladogram: DNA

25. 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

26. 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

27. 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?

28. Systematics and Classification Monophyletic Group

29. Systematics and Classification Paraphyletic Group

30. Systematics and Classification Polyphyletic Group

31. 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