Evolution Notes

Learning Goals

• Understand the 5 forces of evolutionary change.

• Know a brief version of the story describing how Darwin and Wallace developed the Theory of Evolution via Natural Selection (TENS).

• Know the 4 preconditions necessary for evolution via natural selection to occur.

• Be able to describe a number of examples that illustrate that evolution either has occurred or is currently occurring.

What is Evolution?

• Biological Evolution:- Change in allele frequencies within a population across generations.

  • Change in allele or genotype frequencies within a population over time.

• Important Notes:- Evolution does not occur within individuals.

  • Evolution does not occur in communities, ecosystems, biomes, etc. For example, a biome cannot evolve over time.

  • Evolution is not necessarily directional.

5 Forces of Evolutionary Change

• Selection:- Natural selection.

  • Sexual Selection.

• Mutation.

• Random Genetic Drift.

• Migration.

• Non-random (assortative) mating.

• Question:- Which of these forces increase genetic diversity?

  • Which of these forces decrease genetic diversity?

Selection

• Changes in allele frequency due to differential survival and or reproduction caused by:- The abiotic and biotic environment.- Abiotic: Temperature, drought, flood, etc.
  - Biotic: Food availability, competition with other species and members of your own species, predation, etc.

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Selection (Continued)

• Changes in allele frequency due to differential survival and or reproduction caused by:- Members of your own species.- Males competing with one another for access to mates (Male:Male Competition).
  - Females being choosy about which males they will mate with (Female Choice).

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Mutation

• Random changes in the nitrogenous base sequence of DNA of either the nuclear, mitochondrial, or chloroplast genome.

• Mutation can occur anywhere in the genome.

• If it occurs in those portions of the genome that are functional, mutation can have either no effects, minor effects, or major effects.

Random Genetic Drift

• Random changes in allele frequency due to random occurrence of death.

• More pronounced in small populations.

• A special case of this is population bottlenecks.

Random Genetic Drift (Example)

• Example using characters from Gilligan's Island to illustrate how allele frequencies can change randomly over time due to chance events.

• The example shows different genotypes (AA, Aa, aa) and how their frequencies can fluctuate.

Random Genetic Drift (Monte Carlo Simulations)

• Graphs illustrating genetic drift with different population sizes (N = 8, 16, 32, 64, 128, 500) over generations.

• The graphs show how allelic frequency (p) changes over time due to random genetic drift.

• Smaller populations exhibit more pronounced fluctuations in allele frequencies.

Migration

• Migration and Founder Effect illustrated using the Hawaiian Islands.

• Founder effect happens when a small group of individuals colonize a new area, the new population only contains the alleles present in the founders of the group. This will prevent some alleles from being present while increasing the presence of others reducing the genetic diversity of the new population.

Non-Random (Assortative) Mating

• When individuals preferentially mate with other individuals who DO have the same genotype or phenotype.

• When individuals preferentially mate with other individuals who DO NOT have the same genotype or phenotype.

• This tends to alter genotype frequency in a population but does not necessarily alter allele frequency within a population.

Discovery of the Theory of Evolution via Natural Selection

• Key Figures:- Charles Darwin (born 12 February 1809).

  • Alfred Russel Wallace (born 8 January 1823).

• Current State of Evolutionary Thought at the beginning of the 19th century:- Earth is young.- James Ussher – Ussher Chronology (1650) set the date of creation as 22 October 4004 B.C.E.
  - Worked backward from known events and then used the length of time between biblical events (using genealogies and lifespans of biblical figures) to arrive at the date.

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  • Earth is now as it was when created (stasis).

  • Species are immutable.

Discovery of the Theory of Evolution via Natural Selection (Continued)

• Current State of Evolutionary Thought at the beginning of the 19th century:- George Cuvier (1796) compared fossil mastodons and mammoths to modern elephants and concluded that they were different species. This was the first definitive evidence that organisms can go extinct.

  • James Hutton (1788) proposed that current geology was due to gradual geological processes operating over long periods of time.

  • Charles Lyell later elaborated these ideas, that came to be known as Uniformitarianism, in his three-volume work, Principles of Geology (1830-1833).

  • Jean-Baptiste Lamarck (Philosophie zoologique, 1809) proposed that organisms arose via spontaneous generation but were then modified by the use or disuse of characteristics that were then passed on to the next generation.- The inheritance of acquired characteristics.

Lamarck's Theory of Evolution

• Explanation of Lamarck's theory with the example of giraffes.

• The giraffe ancestor lengthened its neck by stretching to reach tree leaves, then passed the change to offspring.

• Acquired variation is passed on to descendants.

Charles Darwin and Alfred Russel Wallace - Influences

• Thomas Robert Malthus (1798). An Essay on the Principle of Population.- Laid out the idea that populations grow exponentially leading to overpopulation, starvation, competition for resources, disease, war, etc.

Charles Darwin and Alfred Russel Wallace - Influences (Continued)

• William Paley (1802). Natural Theology.- Used the clockwork-like precision of designs in nature to support the idea that creation was designed by a divine creator.

Preconditions for Natural Selection

1. More offspring are produced than can possibly survive.

2. Interindividual phenotypic variation exists.

3. Interindividual phenotypic variation is heritable.

4. Interindividual phenotypic variation results in interindividual variation in either reproductive success or survival.