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Cladistics And Systematics

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Definition of Terms•Systematics – study of the diversification of living forms, both past and present, and the relationships among living things through time.•Cladistics –   (from ancient Greek, klados, "branch"; originally called phylogenetic systematics) is a taxonomical technique for arranging organisms according to how they branch in the evolutionary tree of life.

IntroductionAll organisms:

Are composed of one or more cellsCarry out metabolismTransfer energy with ATPEncode hereditary information in DNA

Tremendous diversity of lifeBacteria-----whales----sequoia trees

Biologists group organisms based on shared characteristics and newer molecular sequence data

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SYSTEMATICSSince 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

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SYSTEMATICSDarwin 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

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SYSTEMATICSTwigs 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”

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SYSTEMATICS

Phylogenies depict evolutionary relationships

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SYSTEMATICS Evolution can occur rapidly at one time and slowly at

another (punctuated and gradual evolution)

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SYSTEMATICSOscillating selection: Traits can evolve in one direction, then back the other way

Evolution is not always divergent: convergent evolutionUse similar habitatsSimilar environmental pressures

Evolutionary reversal: process in which a species re-evolves the characteristics of an ancestral species

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CLADISTICSDerived 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

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CLADISTICSCharacters can be any aspect of the phenotypeMorphology - PhysiologyBehavior - DNA

Characters should exist in recognizable character statesExample: Teeth in amniote vertebrates has two states, present in most mammals and reptiles and absence in birds and turtles

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CLADISTICSExamples 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

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CLADISTICSDetermination of ancestral versus derivedFirst 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

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CLADISTICSOutgroup comparison is used to assign character polarityA 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

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CLADISTICSWhen 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

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CLADISTICSFollowing the character state-outgroup methodPresence of teeth in mammals and reptiles is ancestral

Absence of teeth in birds and turtles is derived

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CLADISTICSConstruction of a cladogramPolarize characteristicsClade: 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

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CLADISTICSA simple cladogram is a nested set of cladesPlesiomorphies: ancestral statesSymplesiomorphies: shared ancestral states, not informative about phylogenetics.

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CLADISTICS

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CLADISTICSHomoplasy: a shared character state that has not been inherited from a common ancestorResults from convergent evolutionResults from evolutionary reversal

If there are conflicts among characters, use the principle of parsimony which favors the hypothesis that requires the fewest assumptions

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CLADISTICS

Parsimony and Homoplasy21

CLADISTICS

A Cladogram; DNA22

CLADISTICS

23A Cladogram: DNA

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

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SYSTEMATICS AND CLASSIFICATIONClassification: how we place species and higher groups into the taxonomic hierarchyGenus, 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

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SYSTEMATICS AND CLASSIFICATIONPolyphyletic 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?

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SYSTEMATICS AND CLASSIFICATION

Monophyletic Group27

SYSTEMATICS AND CLASSIFICATION

Paraphyletic Group28

SYSTEMATICS AND CLASSIFICATION

Polyphyletic Group29

SYSTEMATICS AND CLASSIFICATIONPhylogenetic species concept (PSC)Focuses on shared derived characters

Biological species concept (BSC) Defines species as groups of interbreeding population that are reproductively isolated

Typological species concept (TSC) concept of a species as a group whose members share certain characteristics that distinguish them from other species

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