Ch. 22 Descent with Modification:

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Ch. 22 Descent with Modification

• A Darwinian View of Life

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View of life

• Before Darwin
• Darwin

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Whats the reason for diversity and numerous
similarities among species?

• On the Origin of Species by Means of Natural
  Selection. 1859. Charles Darwin.
• In his book, Charles Darwin made 2 major points
• 1. Species evolve from ancestral species and
  were not specially created.
• 2. Natural selection is a mechanism that could
  result in this evolutionary change.

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What was his reasoning?

• The voyage of the Beagle
• Galapagos islands
• Darwin focused on adaptation
• descent with modification
• observations

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Observation 1

• All species have such great potential fertility
  that their population size would increase
  exponentially if all individuals that were born
  reproduced successfully.

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Observation 2

• Populations tend to remain stable in size, except
  for seasonal fluctuations.

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Observation 3

• Environmental resources are limited.
• Production of more individuals that the
  environment can support leads to a struggle for
  existence, with only a fraction of offspring
  surviving.

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Observation 4

• Individuals of a population vary extensively in
  their characteristics/ no two individuals are
  exactly alike.

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Observation 5

• Much of this variation is heritable.
• Survival is not random, but depends in part on
  the hereditary constitution of the surviving
  individuals. Those individuals whose inherited
  characteristics fit them best to their
  environment are likely to leave more offspring
  than less fit individuals.
• This unequal ability to survive and reproduce
  will lead to a gradual change in a population.

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Summary of Darwins Ideas

• Natural selection
• Natural selection occurs from the interaction
  between the environment and the inherent
  variability in a population.
• Variations in a population arise by chance, but
  natural selection is not a chance phenomenon.

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Example of Natural Selection

• Ground Finches
• avg. beak depth (inherited trait) oscillates with
  rainfall.
• Wet years - feed on small seeds - avg. beak depth
  decreases.
• Dry years - small seeds less plentiful, survival
  depends on the finches being able to crack the
  less preferred larger seeds. Avg. beak depth
  increases.
• What does this study indicate?

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What does this study indicate?

• Natural selection is situational. What works in
  one environment may not work in another.
• The environment did not create beaks specialized
  for large or small seeds, but only acted on
  inherited variations already present in the
  population.

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Evidence for Evolution

• Biogeography
• Fossil record
• Comparative anatomy
• Comparative embryology
• Molecular biology