Evolution as Genetic Change and Speciation

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Evolution as Genetic Change and Speciation
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A Populations Gene Pool

• A gene pool is all the alleles available in all
  of the individuals in a population

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How do we know a population is evolving?

• When there are changes in the frequency of
  alleles in the gene pool (aka Microevolution)
• How do the allele frequencies change?
• Natural Selection (directional, stabilizing,
  disruptive, or sexual selection)
• Random Change (mutations, genetic drift, or gene
  flow)
• Migration

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Genetic Drift

• The random change in the gene pool of a
  population
• All populations are subject to genetic drift, but
  small populations are impacted more
• Bottleneck Effect Occurs when a natural disaster
  greatly reduces the size of the population and
  the gene pool (population loses variation)
• Founder Effect Occurs when a few individuals
  from a population colonize an isolated island or
  new habitat (population loses genetic variation)

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Gene Flow

• Gene flow occurs when genes are exchanged with
  another population (ex migration)
• Gene flow can reduce genetic differences between
  populations, possibly uniting the gene pool in a
  single population

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Mutations

• The random change in the DNA of an organism
• The frequency of a mutation in the population
  will depend on the effects of natural selection-
  will it make the individual more likely to
  survive and reproduce?

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Hardy-Weinberg Equilibrium

• Evolution will NOT occur (allele frequencies
  remain constant) if the following conditions are
  met
• Random mating
• No emigration or immigration
• Population must be large
• No natural selection
• No mutations

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Evolution of a Species

• Biological Species Concept a species is a
  population that will interbreed and produce
  fertile offspring
• When a population evolves enough genetic change
  (either through natural selection, genetic drift,
  etc.) that it no longer interbreeds with the
  original population, it is considered a new
  species

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An Example of Speciation

1. A species contains one population or several
   interbreeding populations
2. A barrier comes between the populations
3. Natural selection occurs
4. Barrier is removed, reproductive isolation has
   occurred, two species now exist

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Other Reproductive Barriers

• Temporal (timing)- different mating seasons
• Behavioral- different courtship or mating
  behaviors
• Habitat- Fish living in streams verses open water
• Geographic- populations divided by barriers like
  glaciers, valleys, rivers, etc.