Chapter Overview

1
Chapter Overview

• Introduction to geological eras
• Episodes of mass extinction
• History of plate tectonics
• Methods of dating fossils
• Classification/ phylogeny
• Cladistics

2
Geological eras

• Precambrian
• Paleozoic
• Mesozoic
• Cenozoic

3
Geological eras

• Precambrian
• 4.6 Billion to 543 million years ago
• One-celled fossils date back to 3.5 bya
• Oxygen forms large component of atmosphere
• Eukaryotic cells seen from 2.2 bya
• 600 mya multi-cellular life began

4
Geological eras

• Paleozoic
• 543 million to 245 million years ago
• Began with "Cambrian explosion"
• Diversification of plants animals
• Colonization of terrestrial habitats
• Ends with Permian mass extinction (245 mya)

5
Geological eras

• Mesozoic
• 245 million to 65 million years ago
• Reptiles radiate dominate land (dinosaurs)
• Gymnosperms form vast forests
• Ends with Cretaceous mass extinction (65 mya)

6
Geological eras

• Cenozoic
• 65 million years ago to present
• Radiation of mammals, birds, angiosperms

7
Geological record - mass extinctions

• Geological record is marked by mass extinctions
• E.g. Permian Cretaceous
• Cretaceous may have been caused by impact
• Caused devastating global fires
• Debris clouded out sunlight
• Dinosaurs went extinct

8
Geological record - Plate tectonics

• Earth's crust is not static
• Crust is in plates that move relative to one
  another
• Continental drift explains fossil record
  current distributions
• E.g. mammals in Australia South America

9
How do we age fossils rocks

• Relative dating
• Sedimentary layers are deposited on top of one
  another
• Lower layers are older
• Geological epochs usually identified by presence
  of typical fossils

10
How do we age fossils rocks

• Absolute dating
• Use radioactive isotopes
• Decay with predictable half-life
• E.g. 14C (most common C is 12C) decays to 14N
  with half-life of 5,730 years

11
How do we age fossils rocks

• If an animal dies with a 14C to 12C ratio of
  1/500, what will the ratio be after 5,730 years?
• After 11,460 years?
• Older rocks fossils require radioisotopes with
  longer half-life (e.g. Uranium-238 4.5 billion
  yrs)

12
Organizing diversity

• Classification
• Grouping species by similarity of form
• Into genera, families, orders, etc.
• Systematics
• Organization represents evolutionary history
• Each taxon should be monophyletic (clade)
• Derived from a common ancestor

13
Measuring similarity

• Recently diverged taxa should be more similar in
• Morphology
• Behavior
• Biochemistry
• Amino acid sequences of homologous protein
• Nucleotide sequences in DNA
• Example new world vultures

14
New world vulture example

• Traditionally classified in Family Falconidae
• with hawks, eagles, falcons
• DNA hybridization
• Measuring the binding affinity between DNA from
  different species
• More closely related species bind more strongly
• Found vultures more similar to storks!

15
New world vulture example

• Similarities between new world vultures raptors
  is probably due to convergent evolution
• Hooked bill then analogous not homologous

16
Cladistics

• Formalized method of drawing a phylogenetic tree
  by assigning the order of branching events
• Cladistics requires knowledge of the ancestral
  states of characters used in analysis
• Then groups clades by "shared derived characters"
• Uses the principle of parsimony to decide between
  competing trees

17
Cladistics example

• Consider the three animals below
• Characters
• Feet webbed or not (not webbed ancestral)
• Have crest or no crest (no crest ancestral)
• Bill hooked or not (not hooked ancestral)

18
Molecular clocks

• Perhaps differences in DNA sequence can be used
  to estimate the time two species diverged in past
• If mutations accumulate steadily through time,
  longer time since divergence means more changes
• Calibrate with fossil evidence
• Different parts of genome accumulate mutations at
  different rates
• Perhaps not so reliable?