Evolutionary biologists use phylogenetic trees and cladistics to study evolutionary relationships between organisms and construct classifications. Cladistics involves analyzing shared characteristics to hypothesize how groups of organisms evolved from common ancestors over time. A key assumption is that all organisms are related through descent from a shared ancestor. Cladograms graphically represent evolutionary relationships, with shared derived characteristics defining monophyletic clades. Parsimony is used to select the simplest phylogenetic tree that is best supported by evidence.

1. Evolutionary Biology: Phylogeny and
Cladistics

2. History
 Taxonomists have sought to construct a system that
would reflect evolutionary relationships.
 Use morphology to recognize and construct species

3. History
 Grande (2004)
 Proposes that we view morphology and morphological
variations in three ways
 Taxonomic
 Ontogenic
 Individual
 Ability to interbreed
 Biological species concept  Evolutionary species concept

4. Species Concept
 Morphological Species
 Generally used morphological criteria because this is how
most individuals have been compared.
 Biological Species
 Sexually interbreeding or potentially interbreeding group
of individuals normally separated from other species by
the absence of genetic exchange.
 Evolutionary Species
 species as an evolutionary entity.

5. Phylogeny
 Species evolve from existing species.
 Classification of species
 Homology
 2 species bearing the same phenotype caused by common
ancestry for the same genotype.
 Parallelism
 2 species with the same phenotype descended from a common
ancestor w/ different phenotype and genotype
 Convergence
 2 species with the same phenotype whose common ancestor is
very far in the distant past.

6. What is Cladistics?
 Method of hypothesizing evolutionary relationships
among organisms.
 Based upon the analysis of traits shared by
organisms.
 Cladogram: represents evolutionary relationships.
 Willi Hennig
 1950
 German zoologist

8. Assumptions of Cladistics
 The Characteristics of organisms change over time.
 All organisms are related by a descent from a
common ancestor
 There is a bifurcating or branching pattern of lineage
and splitting.

9. The Principle of Parsimony
 The principle of parsimony reasons that given
several solutions to a problem, the simplest one
should be accepted.
 The principle of parsimony does not:
 Provide absolute truth
 Necessarily reveal the true phylogeny
 The principle of parsimony does:
 Reduce conjecture
 Indicates those cladistic groupings best supported by the
available data

10. Understanding Cladogram
 The root of Cladogram represents the ancestral
lineage and the tips the descendants.
 Branching points in the cladograms are called nodes
and represents speciation events.
 Part of each lineage’s history is unique to it alone.
 Each lineage has ancestors that are unique and
ancestors that are shared with other lineages.
A B C D E F
TIME
speciation
Cladogram or Phylogenetic Tree
TAXA

11. Understanding Cladograms
 Clade: a group comprising an ancestral lineage and
all its descendant lineages.

12. Characters
 Inheritable feature of an organism.
 Physical characteristics (morphology)
 Genetic sequences
 Behavioral traits

13. Characters
 Homologous characters
 A character possessed by
two or more organisms due
to their sharing of a
common ancestor.

14. Characters
 Homoplasy
 Similar characters in different taxa inferred to have arisen
through convergent evolution, not a common ancestor.
 No use in cladistics

16. euphorb
spines
cactus
spines

17.  Plesiomorphy
 Considered to be ancestral or less derived
 Apomorphy
 Features considered to be derived from an ancestral
clade
 Synapomorphy
 An apomorphic feature that defines a clade.

18. Monophyletic group
 A group consisting of:
 A common ancestor
 All descendants of that common ancestor

24. Paraphyletic group
 A monophyletic grouping that certain descendent tax
have been removed from due to their possession of
additional synapomorphic characters.

25. Constructing a Cladogram
 Outgroup Method
 Works by identifying tow groups of taxa
 Ingroup: group being studies
 Outgroup: for the establishment of plesomorphic and
apomorphic character states in the ingroup
 The outgroup should comprise taxa closely related to the
ingroup so homologous characters can be established

26. Character states and Character Matrix
notochord vertebrae jaws skull lung enamel 4 true limbs
Outgroup 0 0 0 0 0 0 0
Tunicate 1 0 0 0 0 0 0
lamprey 1 1 0 1 0 0 0
Cladoselache 1 1 1 1 0 0 0
trout 1 1 1 1 1 0 0
lungfish 1 1 1 1 1 1 0
present 1
absent 0