Evolutionary Biology: Phylogeny and Cladistics

History

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

species

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

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.

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.

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

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.

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

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

Understanding Cladograms Clade: a group comprising an ancestral

lineage and all its descendant lineages.

Characters Inheritable feature of an organism.

Physical characteristics (morphology) Genetic sequences Behavioral traits

Characters Homologous characters

A character possessed by two or more organisms due to their sharing of a common ancestor.

Characters Homoplasy

Similar characters in different taxa inferred to have arisen through convergent evolution, not a common ancestor.

No use in cladistics

euphorb spines

cactus spines

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.

Monophyletic group A group consisting of:

A common ancestor All descendants of that common ancestor

Paraphyletic group A monophyletic grouping that certain

descendent tax have been removed from due to their possession of additional synapomorphic characters.

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

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