Ch. 26 – Phylogeny and the Tree of LifeCh. 26 – Phylogeny and the Tree of Life

We can decide in which container to place a species by comparing its traits with those of potential close relatives

We will survey the diversity and describe hypotheses regarding how it evolved

Phylogeny

Systematics• Discipline focused on classifying

organisms and determining their evolutionary relationships (phylogeny)

Phylogeny and SystematicsPhylogeny and Systematics

Taxonomy• Science of classification• Sort and identify

closely related species• Specific group is a

taxonBinomial nomenclature• 2 part Latin name• Genus, species

Phylogeny and SystematicsPhylogeny and Systematics• Hierarchical classification• Linnaeus’ 7 level scheme:

– Kingdom– Phylum (Division for plants and

fungi)– Class– Order– Family– Genus – Species

• Domains are now above kingdoms– 3 exist

• Taxon– Toxonomic unit at any level of

the hierarchy

Linking Classification and PhylogenyLinking Classification and PhylogenyPhylogenetic Tree• Diagram that displays

evolutionary relationships• The sequence of branching

symbolizes historical chronology.– The last ancestor common to both the

cat and dog families lived longer ago than the last common ancestor shared by leopards and domestic cats.

PhyloCode• Resulted due to difficulties with

the Linnean system• Only names groups that include a

common ancestor of all its descendents

• Ranks would no longer exist

Linking Classification and PhylogenyLinking Classification and PhylogenyReading a phylogenetic tree• Branch points

– Divergence• Sister taxa

– Groups of organisms that share an immediate common ancestor

• Rooted– Represents most common

ancestor of all taxa in the tree

• Basal Taxon– Lineage that diverges early

in the history of a group• Polytomy

– Branch point from which more than two descendent groups emerge

Phylogenetic TreesPhylogenetic Trees1. Intended to show

patterns of descent

2. Sequence of branching in a tree does not necessarily indicate the actual (absolute) ages of the particular species

3. Don’t assume that a taxon evolved from the taxon next to it

• Analyzing the taxonomic distribution of homologies enables us to identify the sequence in which derived characters evolved during vertebrate phylogeny.

Fig. 25.11

Construct a cladogram based on the following traits (characters) in the character table

Fig. 25.11

Morphological & Molecular HomologiesMorphological & Molecular HomologiesHomology• shared ancestry; causes likeness• Can vary in appearance but still not

in the genesAnalogy• convergent evolution• acquisition of similar characteristics

due to sharing similar ecological roles

• External appearance very similar but internal anatomy, physiology, and reproductive systems are dissimilar

• Homoplasies– Analagous structures that arise

independently

Molecular Molecular HomologiesHomologies

Comparisons of DNA

Molecular Systematics

• Uses data from DNA and other molecules to determine evolutionary relationships

CladisticsCladisticsCommon ancestry is the primary criterion used to classify

organismsClades• Groups that include an ancestral species and all of its

descendentsMonophyletic groupParaphyletic groupPolyphyletic group

CladisticsCladisticsMonophyletic group• Allows a taxon to be equivalent to a clade• Group of taxa that consists of an ancestor and all of its

descendentsParaphyletic group• Ancestral species and SOME of its descendentsPolyphyletic group• Taxa with 2 or more different ancestors

Shared Ancestral & Derived CharactersShared Ancestral & Derived Characters

Shared Ancestral Character• Originated in an ancestor of the taxon• Backbone of mammalsShared Derived Character• Evolutionary novelty unique to a clade• Hair in mammals

Inferring Phylogenies using Derived CharactersInferring Phylogenies using Derived CharactersOutgroup• A species or group of species from an evolutionary lineage that is known

to have diverged before the lineage that includes the species we are studying (ingroup)

Evidence used for cladistics• Fossil record• Homology (morphology, embryology, reproduction)• DNA sequences• Biogeography• Direct observation

Gene Duplications & Gene FamiliesGene Duplications & Gene FamiliesTwo distinct types of homologous genesOrthologous genes• Found in different species and their

divergences traces back to the speciation events that produced the species

Paralogous genes• Multiple copies of genes have diverged from

one another in a species

Molecular ClocksMolecular ClocksMolecular Clock• A yardstick for measuring

the absolute time of evolutionary change based on the observation that some genes and other regions of genomes appear to evolve at constant rates

Neutral Theory• Most evolutionary change

has no effect on fitness

From Two Kingdoms to Three DomainsFrom Two Kingdoms to Three DomainsTwo Kingdoms• Plants and Animals Five Kingdoms• Monera, Protista, Plantae,

Fungi, Animalia• Prokaryotic v. Eukaryotic• Unicellular v. Multicellular• Autotrophic v.

Heterotrophic• How does each kingdom

classify?

From Two Kingdoms to Three DomainsFrom Two Kingdoms to Three DomainsThree Domains• Archaea• Bacteria• EukaryaSix-Kingdom system Eubacteria and

Archaebacteria Extreme

halophiles Thermoacidophil

es Methanogens

Alternatives to the 6 Kingdom SystemAlternatives to the 6 Kingdom SystemEight-Kingdom system -

proposed Protists split into 3

kingdoms Fungal like, plant

like, animal like

Still debate over splitting Archae and Eubacteria even further

Phylogeny SummarizedPhylogeny Summarized(potential FR…hint, hint)(potential FR…hint, hint)

Evolutionary history of organisms• Base on principle of parsimony (simplest explanation

is the best)Mechanisms of speciation• Geographic (allopatric speciation)• Sympatric speciation• Reproductive isolation by prezygotic barriers• Reproductive isolation by postzygotic barriersInvestigating phylogeny (evidence, strengths,

weaknesses)• Fossils• Anatomy/Morphology• Embryology• Molecular biology• Behavioral traits

Phylogeny SummarizedPhylogeny Summarized(potential FR…hint, hint)(potential FR…hint, hint)

Mechanisms of speciation• Geographic (allopatric speciation)

– Population of one species becomes physically separated by some geographic barrier such as a river, mountain range, etc.

– Long term isolation will lead to reproductive isolation

• Sympatric speciation– Occurs when new species arise as a result of reproductive isolation

within the population range• ex – result of polyploidy or switching mating behaviors

• Reproductive isolation by prezygotic barriers– Habitat, temporal, behavioral, mechanical or gamete incompatibility

• Reproductive isolation by postzygotic barriers– Reduced hybrid variability or fertility leads to speciation

Phylogeny SummarizedPhylogeny Summarized(potential FR…hint, hint)(potential FR…hint, hint)

Investigating phylogeny (evidence)• Fossils

– (strength) Determine time, extinct species– (weakness) not all species leave fossils; incomplete chronicle

• Anatomy/Morphology– (strength) homologous structures– (weakness) analogous structures, little diversity in taxa,

morphology may reflect environment or diet

• Embryology– (strength) similarities in patterns of development not present

in adults– (weakness) similarities lost in later development (chordates)

Phylogeny SummarizedPhylogeny Summarized(potential FR…hint, hint)(potential FR…hint, hint)

Investigating phylogeny (evidence)• Molecular Biology

– (strength) Lots of traits to study closely related species; most accurate

– (weakness) virtually no data for extinct species; variation within species blurs variation between species

• Behavioral traits– (strength) genetic based (frog calls)– (weakness) culturally transmitted or learned (bird calls)