Descent with Modification

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Descent with Modification
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Pre-evolutionary Thought

• scala naturae
• Aristotle
• essentialism
• spontaneous generation

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Road to Evolution

• discovery of fossils
• certain organisms had become extinct
• certain organisms that currently exist are not
  represented in the fossil record (did not always
  exist)
• organisms that currently exist are more similar
  to recent fossils (those closer to the surface)
  than to older fossils (those more deeply buried)

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Road to Evolution

• geology
• Hutton
• gradualism
• Lyell
• Principle of Uniformitarianism
• taxonomy
• revealed gaps in scala naturae

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Road to Evolution

• Lamarck
• ladder of life
• new species arise from spontaneous generation
• each species represents a stage in its evolution
  from an earlier simpler ancestor to some more
  perfect and complicated form
• species change over time to become more complex

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Darwinian Evolution

• Darwin and Wallace proposed evolution by natural
  selection
• On the Origin of Species by Means of Natural
  Selection (1859)
• descent with modification
• predicated on natural selection

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Natural Selection

• variability
• overproduction of offspring
• Malthus
• competition
• survival of different types

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What is evolution?

• change over time
• explains the diversity of organisms and the
  similarity between groups of organisms

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What is evolution?

• all living things are derived from a common
  ancestor
• apparent differences between organisms are the
  result of slow, gradual changes from an original
  form
• evolution occurs at the population level

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Evidence of Evolution

• comparative anatomy
• homology vs. analogy
• molecular biology
• comparative embryology

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Population Genetics and Evolution

• evolution can be explain in genetic terms
• change in allele frequencies in a population over
  time
• gene pool
• huge genetic variation

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Gene Pool

• How do changes in the gene pool arise?
• mutations in DNA are the ultimate source of much
  variation, but these are rare
• independent assortment of chromosomes at meiosis
  
• crossing over at meiosis
• chromosomal mistakes may contribute (e.g.,
  chromosomes fusing together or splitting apart
  nondisjunctions)
• fertilization between gametes from strains
  (races) with significant variation ("hybrid
  vigor")

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Hardy-Weinberg Populations

• very large population size
• isolation from other populations
• no mutations
• random mating
• no natural selection

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Gene Pool

• How do changes in the gene pool occur?
• genetic drift
• gene flow
• mutation
• nonrandom mating
• natural selection

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Genetic Drift

• change in gene frequency in a gene pool due to
  chance
• occurs in small populations
• reduces the populations genetic variability
• increases the genetic variability between
  populations

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Genetic Drift

• bottleneck effect

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Genetic Drift

• founder effect
• occurs when a small number of pioneer individuals
  of an original population colonize a new area
• the new population could have a much different
  genetic ratio than the original one

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Other Variations in the Gene Pool

• gene flow
• occurs when a population gains or loses alleles
  as a result of migration
• mutations
• change in DNA nucleotide sequence
• source of genetic variation, although rare

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Nonrandom Mating

• limited mobility and mating based on behavior may
  alter mating choice
• assortative mating

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Natural Selection

• acts on individuals
• affects the frequency of a heritable trait in one
  of 3 ways
• stabilizing selection
• directional selection
• disruptive selection

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Speciation

• cumulative effect of evolution is evolution of a
  new species
• What is a species?

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Biological Species Concept

• proposed by Ernst Mayer (1942)
• population or group of populations whose members
  have the potential to interbreed and produce
  fertile offspring