Mechanisms of Evolution

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Mechanisms of Evolution
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Populations, not individuals evolve

• Within its lifetime, an organism cannot evolve a
  new phenotype in response to its environment.
• Evolution occurs as a populations genes and
  their frequencies change over time.
• All the alleles of a populations genes make up
  the gene pool.
• The percentage of any specific allele in the gene
  pool is called the allelic frequency.
• If the frequency of alleles remains the same over
  generations it is called genetic equilibrium.

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Changes in genetic equilibrium

• A population that is in genetic equilibrium is
  NOT evolving.
• Mechanisms for genetic change
• Mutation change in the DNA
• Some mutations are harmful
• Others are useful
• Genetic drift the change of allelic frequencies
  by chance events.
• Example Amish in Pennsylvania tend to
  intermarry. Caused a recessive allele for short
  arms legs to go from 1 in 1000 to 1 in 14.

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Changes in genetic equilibrium continued

• Gene flow when individuals LEAVE a population
  they take their genes with them. If others ENTER,
  they bring in NEW genes to the gene pool.

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The evolution of Species

• Speciation is the evolution of a new species. It
  occurs when members of similar populations no
  longer interbreed to produce fertile offspring
  within their environment.

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Physical barriers can prevent interbreeding

• Geographic isolation occurs whenever a physical
  barrier divides a population.
• Ex lava flows, changes along continental shelves
  in the oceans can create islands.

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Reproductive isolation can result in speciation

• Reproductive isolation occurs when formerly
  interbreeding organisms can no longer mate and
  produce fertile offspring.
• Some occurs because the genetic material of the
  populations is so different that fertilization
  cannot occur.
• Another form is when one population mates during
  fall and another mates in the summer (then they
  cant mate with each other). This is aka temporal
  isolation.

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A change in chromosome Numbers and speciation

• Many new species of plants and animals have
  evolved in the same geographic area as a result
  of polyploidy( multiple sets of chromosomes) .
• Examples include wheat, cotton, apples, and
  bananas.

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Speciation can occur quickly or slowly

• Gradualism species originate through a gradual
  change of adaptations. (EX camels)

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• Punctuated equilibrium speciation occurs
  relatively quickly, in rapid bursts, with long
  periods of genetic equilibrium in between. (EX
  Elephants)

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Patterns of Evolution

• Divergent evolution evolution in which species
  that once were similar diverge (change in
  different directions)
• Occurs when population adapting to different
  environmental conditions change, becoming less
  alike as they adapt, resulting in a NEW species.

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• Convergent evolution distantly related
  organisms evolve similar traits.

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• Coevolution process by which 2 species evolve
  in response to changes in each other over time.
• EX many flowering plants can reproduce ONLY if
  the shape, color, and odor of their flowers
  attract a specific type of pollinator. An
  evolutionary change in one, can be followed by a
  corresponding change in the other.