Mechanisms of Microevolution

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Mechanisms of Microevolution
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There are 5 mechanisms that affect real
populations resulting in microevolution or
changes to allele frequencies within a
population.

• 1. Mutations
• are only important to evolution if the mutated
  DNA is in a gamete and passed on to offspring
• the new mutation may provide an advantage for
  natural selection
• Ex) Daphnia adapted to warmer water

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2. Genetic Drift

• Definition Chance changes in allele
  frequencies due to a small population size. The
  smaller the population the less likely all of the
  alleles in the parent gene pool will be reflected
  in the offspring gene pool
• a) Bottleneck Effect
• reduction of alleles in a population resulting
  from a disaster that drastically reduces
  population size.
• Example) earthquakes, volcanoes
• b) Founder effect
• occurs when a few individuals separate from a
  large population and establish a new one

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3. Gene Flow

• The gain or loss of alleles from a population
  by the movement of individuals or gametes. This
  could result from immigration or emigration.
• Ex) pollen blowing in the wind and fertilizing
  other plants

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4. Non-random Mating

• in-breeding or mating between closely related
  partners results in fewer heterozygotes and more
  homozygotes
• ex. self fertilization in plants
• assortative mating where a similar partner is
  chosen like toads choosing a same size partner,
  or dog breeding

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5. Natural Selection

• Individuals with favorable traits that make them
  better suited for their environment are more
  likely to survive to reproductive age and leave
  more offspring.
• http//www.pbs.org/wgbh/evolution/library/11/2/qui
  cktime/e_s_4.html

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Darwins 5 points About Natural Selection

• Population has variations.
• Some variations are favorable.
• More offspring are produced than survive
• Those that survive have favorable traits.
• A population will change over time.

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Three Ways Natural Selection Can Affect a
Population

• 1) Directional selection
• One extreme phenotype is favoured over all other
  forms.
• Ex. Increasing horse body size

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2) Stabilizing Selection

• Intermediate phenotypes are favoured.
• Ex) average human birth weight is around 7
  pounds (3-4 kg)

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3) Disruptive Selection

• Two extreme phenotypes are favoured.
• Ex) mimicry in butterflies
• Most African Swallowtails exhibit one of two
  forms. Each form looks like another species that
  tastes offensive to birds