Chapter 25: Tracing Phylogeny

Phylogeny Phylon = tribe, geny = genesis or origin The evolutionary history of a species or a group of related species.

Phylogeny Found in fossils and the fossil record.

Fossils Any preserved remnant or impression of a past organism.

Types of Fossils 1. Mineralized 2. Organic Matter 3. Trace 4. Amber

Mineralized Fossils Found in sedimentary rock. Minerals replace cell contents. Ex: bone, teeth, shells

Organic Matter Fossils Retain the original organic matter. Ex: plant leaves trapped in shale. Comment – can sometimes extract DNA from these fossils.

Trace Fossils Footprints and other impressions. No organic matter present.

Amber Fossil tree resin. Preserve whole specimen. Usually small insects etc.

Fossils - Limitations Rare event. Hard to find. Fragmentary. Dating.

Fossil Dating Methods 1. Relative - by a fossil's position in the strata relative to index fossils. 2. Absolute - approximate age on a scale of absolute time.

Absolute - Methods 1. Radioactive 2. Isomer Ratios

Radioactive Estimated from half-life products in the fossil. Ex: Carbon - 14 Potassium - 40

Isomer Ratios Ratio of L- and D- amino acid isomers. L- used by living things. D- not used by living things.

Death L- form  D- form Age can be calculated from the ratio of L-/D- as long as the temperature of the area is taken into account.

What do fossils tell us? That the geographical distribution of organisms has changed over time. Reason? – The land formations of the earth have changed.

Continental Drift The movement of the earth's crustal plates over time. Drift is correlated with events of mass extinctions and adaptive radiations of life.

Pangaea 250 million years ago. One super continent. Many life forms brought into contact with each other.

Mesozoic era Pangaea began to break up. 180 million years ago.

Result Geographical Isolation. New environments formed. Old environments lost. As the environments changed, so did Life.

Example Australian fauna and flora are unique. Separated early and remained isolated for 50 million years.

Mass Extinctions The sudden loss of many species in geologic time. May be caused by asteroid hits or other disasters.

Examples Permian Extinction Cretaceous Extinction

Permian Extinction 250 million years ago. 90% of species lost.

Cretaceous Extinction 65 million years ago. Loss of the dinosaurs. Good evidence that this event was caused by an asteroid that hit in the Yucatan, causing a “nuclear winter”.

The crater

Result of Mass Extinctions Areas are open for the surviving species to exploit. Rapid period of speciation (adaptive radiation). Many new species are formed in a very short period of time.

Systematics The study of biological diversity. Uses evidence from the fossil record and other sources to reconstruct phylogeny.

Systematics - concerns 1. Phylogeny- tracing of evolutionary relationships. 2. Taxonomy- the identification and classification of species.

Taxonomy Natural to humans. Modern system developed by Linnaeus in the 18 th century.

Linnaeus Taxonomy 1. Binomial Nomenclature – two names for each organism. Ex - Homo sapiens 2. Hierarchical System – arranges life into groups. Ex - Kingdom  Species

Goal of Systematics To have Taxonomy reflect the evolutionary affinities or phylogeny of the organisms.

Problem How to group taxa so that the phylogenetic relationships are correct ?

Ideal Situation Monophyletic Grouping - a single ancestor gave rise to all species in the taxon.

Other Possibilities Polyphyletic - grouping where members are derived from two or more ancestral forms. Paraphyletic - grouping that does not include all members from an ancestral form.

Problem Not all “likeness” is inherited from a common ancestor. Problem is of homology vs analogy.

Homology and Analogy Homology – likeness attributed to shared ancestry. Ex: forelimbs of vertebrates Analogy – likeness due to convergent evolution. Ex: wings of insects and birds

Convergent Evolution When unrelated species have similar adaptations to a common environment. Ex: Sharks and dolphins

Only one is a cactus

Need Methods to group organisms by similarities and phylogenies. One possible method is Molecular Systematics.

Molecular Systematics Compares similarities at the molecular level. Ex: DNA, Proteins

DNA Comparisons A direct measure of common inheritance. The more DNA in common, the more closely related.

DNA Comparison Methods 1. DNA-DNA Hybridization 2. Restriction Mapping 3. DNA Sequencing All three methods have been used, but DNA Sequencing is becoming the most common.

Protein Comparisons Examines the Amino Acid sequence of homologous proteins. Ex: Cytochrome C Study

Schools of Taxonomy 1. Phenetics 2. Cladistics

Phenetics Makes no phylogenetic assumptions. Taxonomic affinities based on measurable similarities. Ex: Numerical Taxonomy

Cladistics Branch points defined by novel characteristics. Branch pattern may not reflect evolutionary history.

Cladistics

Problem ?

Classical Evolutionary Taxonomy Balances Phenetics and Cladistics with overall homology.

Result Taxonomy will become Genealogies, reflecting the organism’s "Descent with Modification“.

Summary Recognize the use and limits of fossils. What happens to evolution in mass extinctions. What is phylogeny?

Summary What is a phyletic tree? How is molecular systematics used in phylogeny?