The Evolutionary History of Biodiversity

Phylogeny and the Tree of Life

Phylogenies show evolutionary relationships

Phylogeny›  the evolutionary history of a species or

group of related species› the cornerstone of a branch of biology

called systematic taxonomy› classifies organisms and their evolutionary

relationships using fossils morphology Genes molecular evidence

Phylogenies show evolutionary relationships

Taxonomy the ordered division of organisms into categories based on similarities and differences.

Binomial nomenclature two part naming system consisting of the genus /species. Developed by Carolus Linnaeus

Swedish biologist considered the father of modern taxonomy.

The binomial nomenclature Linnaeus used was developed in the late 16th and early 17th century by the Swiss botanists (brothers) Gaspard and Johann Bauhin, for some of the 6000 plants they described in their works, but it was Linnaeus who used it consistently and systematically.

Carl von Linné (Carolus Linnaeus)1707 – 1778

Hierarchical Classification

Hierarchical Classification Organisms are

classified into a hierarchies that group closely related organisms and progressively include more and more organisms. Each categorization at any level is called a taxon.

Phylogenetic trees The aim is to figure out

the evolutionary relationships among species.

Branching diagrams called phylogenetic trees hypothesize evolutionary relationships thought to exist among groups of organisms.

It does not show the ACTUAL evolutionary history of organisms.

Why a hypothesis?

Phylogenetic trees In a phylogenetic

tree the tips of the branches specify particular species and the branching points represent common ancestors.

Phylogenetic trees Phylogenetic trees

are constructed by studying features of organisms formally called characters.

Characters may be morphological or molecular.

Morphology Comparing physical structural

characteristics

Similarities due to shared ancestry

Homologous Structures

Convergent EvolutionThese animals have evolved similar adaptations for obtaining food because they occupy similar niches.

Analogous Structures

Convergent EvolutionSimilar solutions to similar problems

Analogous Structures

Molecular Systematics

Using DNA The more alike the DNA

sequences of two organisms, the more closely related they are evolutionarily.

Early phylogenetic tree of amniotes based on cytochrome c gene by Fitch and Margoliash (1967).

Note numbers on branches.

These represent estimated numbers of mutational changes in gene.

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Cladograms Cladograms are diagrams that display patterns

of shared characteristics. If shared characteristics are due to common

ancestry (are homologous) the cladogram forms the basis of a phylogenetic tree.

Within a tree a clade is defined as a group that includes an ancestral species and all of its descendants.

Cladistics is the science of how species may be grouped into clades.

Ancestral vs. Derived Characters Ancestral

› Character present in the common ancestor of both groups

Derived› Character that evolved in one group but not the other

What derived character is shared by all the animals on the cladogram on the next slide?

Cladograms and Phylogenetic Trees

A cladogram and a phylogenetic tree are similar, but not identical.

Traditional evolutionary taxonomy is subjective and therefore more prone to bias. A phylogenetic tree’s branches put more emphasis on certain characters rather than others.

Cladistics treats each character equally. It is

more objective by ignoring how some characters have a higher impact on evolution. It is thought that evolutionary success is dependent on high-impact events.

Rate of Evolution An organism’s evolutionary history is

documented in its genome The rate of evolution of DNA sequences varies

from one part of the genome to another Comparing the different sequences helps us to

investigate relationships between groups of organisms that diverged long ago

DNA that codes for ribosomal RNA and

mitochondrial DNA are both used› rRNA changes relatively slowly –used with taxa that

diverged hundreds of millions of years ago› Mitochondrial DNA evolves rapidly – used to explore

more recent events

The Molecular Clock Hypothesis

Used to measure the absolute time of evolutionary change.

Amount of genetic difference between sequences is a function of time since separation.

Assumes that the rate of molecular change is constant (enough) to predict times of divergence

Taxonomy in Flux When the authors of

your text book were in high school they were taught two kingdoms: plants and animals.

When your teacher was in high school she was taught five kingdoms: Monera, Protista, Plantae, Fungi and Animalia

Taxonomy in Flux Now biologists have

adopted a three-domain system › Discovery that there

are two distinct lineages of prokaryotes

Characteristic Bacteria

Archaea

Eukarya

Nuclear envelope No No YesMembrane-enclosed organelles

No No Yes

Introns No Yes YesHistone proteins associated with DNA

No Yes Yes

Circular chromosome

Yes Yes No