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Outline 7:Evolution and the
Fossil Record
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The Study of Evolution
• Two domains: the present and the past• The present: Biology
– Genetics– Comparative Anatomy
• The past: Paleontology or Paleobiology– Comparative Anatomy of Fossilized
organisms
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Natural Selection
• Natural selection is one of the prime mechanisms of evolutionary change. The other mechanism is mass extinction.
• Charles Darwin, 1859: The Origin of Species by Natural Selection.
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One of the most famous books of
science.
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Artificial Selection vs. Natural Selection: Dog breeds produced by
artificial selection
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Natural Selection: Key Points
• 3 facts lead to a conclusion:• Fact 1. Overproduction of offspring• Fact 2. Variation within populations• Fact 3. Competition for resources• Conclusion: Survival of the fittest
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Voyage of HMS Beagle, 1831-1836
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Darwin’s finches: 13 different species evolved from a single ancestor from South America.
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The beaks of Darwin’s finches are adapted to the variety of food types available on the different
islands in the Galapagos Islands.
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Type specimens of finches collected by Darwin in the Galapagos Islands
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The Galapagos Islands. Darwin visited in October 1835
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Yours truly in the Galapagos Islands, 1996,
Fernandina Volcano
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Charles Darwin in 1837, age 28, after voyage of the HMS Beagle
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Age 40 Age 45
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Darwin in 1879, age 70.
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Darwin’s home, Down House, in England
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Darwin’s study at his home Down House
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Darwin’s grave in Westminster Abbey
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Near the Natural History Museum, London
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Darwin on the 10-pound note. Lincoln is on our $5 bill. Both men were born on the exact same date: Feb. 12, 1809.
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Sources of Variation
• Random mutations of DNA in the sex cells– >>99.9% are harmful or neutral– adaptive mutations are rare
• Sexual recombination– new variants created by mixing of genes– you have 4096 direct ancestors going
back 12 generations (212)
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Variations within Natural Populations are the Raw Materials for Natural Selection, including Sexual Selection.
Do you think I’m sexy?
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If genetic mutations are random, then is evolutionary change within organisms random?
Thus, is the history of evolution simply a series of random events?
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Chromosomes contain Genes composed of DNA
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The Anatomy of a Gene. Helical strands of DNA contain the blueprints for all forms of life.
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The DNA molecule contains nitrogenous bases (A,T,C,G) that code for different amino acids, which form proteins.
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Evidence of Evolution
• Homology, or homologous features• Analogous organs/features, or
Convergent Evolution• Vestigial organs• Adaptive radiation
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Homology
• The recognition of common ancestry of features. e.g. all primates have 5 fingers; apes and humans lack a tail; all tetrapods have similar limb bones.
• Studied by comparative anatomy of living and fossil organisms.
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Homologous features are used to recognize common evolutionary ancestry. Horses and humans share a common ancestor.
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Homologous features are used to recognize common evolutionary ancestry.
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Examples of homologous features in the forelimbs of tetrapods.
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Homologous development between different species indicates common ancestry.
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Analogous Organs or Convergent Evolution
• Produced by evolutionary convergence. Independent origin of similar features is called convergent evolution.
• Shows natural selection in operation.• e.g., insects, birds, bats, and pterosaurs
all evolved wings independently
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Analogous features, such as wings, indicate convergent evolution and notcommon ancestry.
Pterosaur
Bat
Bird
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Other examples of convergent evolution: armadillo and pangolin; or
monotreme and placental anteaters
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Vestigial Organs
• Organs no longer used show clear evidence of evolutionary change.
• e.g., pelvic bones of whales, vestiges of former legs
• e.g., human body hair, a vestige of former fur
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vestigialwhalehips
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Vestigial Organs in Humans. Mostly useless now.
e.g., appendix and tail bone
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Adaptive Radiation
• Natural selection can fill a variety of niches starting with a single species.
• e.g., Darwin’s finches in the Galapagos Islands, 13 species evolved from one ancestral species
• e.g., all birds evolved from Archaeopteryx
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The beaks of Darwin’s finches are adapted to the variety of food types available on the different
islands in the Galapagos Islands.
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Archaeopteryx, the first bird; Jurassic age.
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Archaeopteryx reconstructed
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Adaptive Radiation of Birds
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The Origin of New Species: Speciation
• Natural selection by itself will not increase the number of species on the planet.
• Species definition: Interbreeding populations reproductively isolated.
• New species must be reproductively isolated from their ancestral species.
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Lions and tigers share a common ancestor, but the hybrids are usually sterile. Thus, they are separate species.
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Liger
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Liger
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Liger
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Horses and donkeys share a common ancestor. Their hybrid, the mule, is sterile because of the odd number of chromosomes.
Embryo
Equus caballusEquus asinus
62 chromosomes
64 chromosomes
63 chromosomes
31 + 32 = 63
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Zonkey – Zebra and donkey hybrid
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Speciation, cont’d
• Allopatric speciation: produced by geographic isolation of populations.
• Natural selection causes an isolated population to adapt to its local environment.
• Given enough time, and no outside interbreeding, a new species will evolve.
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Rates of Evolution
• Phyletic gradualism - continuous and gradual change over time
• Punctuated Equilibrium - long periods of stasis punctuated by rapid change, probably associated with a bottleneckin population size.
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Geographic Separation and IsolationStasis Stasis
Punctuated Change
Populations with variations
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Rates of Evolution
• Large populations evolve very slowly or almost not at all. Advantageous mutations pass very slowly through a population.
• Small populations can evolve very rapidly. Advantageous mutations can be passed very quickly through the population.
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Evolution in Action
• Passing through a bottleneck:– Pesticide resistant insects– Antibiotic resistant bacteria
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An extreme selection event severely reduces population size and changes the composition of the gene pool of the species.
Bottleneck Effect
Small populations often go extinct, but some may survive with a changed gene pool
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Role of the Fossil Record• Provides the only actual record of
evolutionary change over geologic time.• Provides data on the timing of
evolutionary origins.• Shows mass extinctions to be a major
cause of evolutionary change.• Provides information on rates of
evolution.
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Evolutionary Origins
• Oldest life: 3.5 BY• Oldest nucleated cells: 2.1 BY• First animals: 600 MY• First tetrapods: 375 MY• First dinosaurs: 220 MY• First mammals: 220 MY• First hominids: 4 MY
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An example of the role of the Fossil Record:
The Evolution of Tetrapods
**
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Acanthostega and Ichthyostega
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Another Example:
The Evolution of Whales
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Mass Extinctions
• Survival of the luckiest?• Over geologic time, mass extinctions
may subvert the notion of survival of the fittest.
• Is evolutionary history predictable?
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The Record of Mass Extinctions
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Rates of Evolution
• Biologists cannot measure evolutionary rates. They have only the present. Darwin assumed gradual rates.
• Paleontologists can measure evolutionary rates. They have found punctuated equilibria to be the major pattern in fossil species rather than gradual change.