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Chapter 15 Section 3
SHAPING EVOLUTIONARY THEORY
Mechanisms of Evolution
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First, although not a “mechanism” it’s the basis for the defined
mechanisms, the Hardy-Weinberg principle.
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When allelic frequencies remain constant, a population is in genetic
equilibrium.
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Ex: although the number of owls doubled, the ratio of gray to red owls is
the same.
Slide 2 continued
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To be in equilibrium, the population must meet 5 conditions (in other words
the population must not have any of the 5 mechanisms of evolutionary
theory going on):
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1. no genetic drift
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2. no gene flow
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3. no mutation
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4. mating must be random
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5. no natural selection
*populations in nature might meat some of these requirements, but hardly
any population meets all 5 conditions for long periods of time.
Mechanisms of Evolution: genetic drift
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What’s an allele again? It’s an alternative form that a single gene may have
for a particular trait.
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Any change in the allelic frequencies (# of each kind of allele in the
population) that results from chance.
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These effects are more pronounced in smaller populations.
Mechanisms of Evolution: genetic drift continued
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Founder Effect- small sample of a population settles in a location
separated from the rest of the population.
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Alleles that were uncommon in the original population might become common
in the new population
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Bottleneck- population declines to a very low # then rebounds.
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https://www.youtube.com/watch?v=Q6JEA2olNts
Mechanisms Video: 5 Fingers of Evolution
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http://ed.ted.com/lessons/five-fingers-of-evolution#watch
Pinky- Shrinking Population (aka genetic drift- which is more pronounced in a smaller populationfounder effect & bottleneck
Ring- Nonrandom Mating
Middle- Mutation
Pointer- Gene Flow
Thumb (up/down)- natural selection
Mechanisms of
Evolution:
Gene Flow
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Mechanism of evolution
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Can occur during migration of individuals from one population to another
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When the migrating individuals breed with the new population, they
contribute their genes to the gene pool of the local population
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Makes gene pools of the same species more similar to one another
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Ex: wind carrying seeds from parent population to another population,
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Ex: animals driven off of herd join a new population
Mechanisms of Evolution:
Nonrandom Mating
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Mechanism of Evolution
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What would completely random mating look like? Individuals regardless of
environment, heredity, or social interaction. Potential mates have an
equal chance of being selected. As long as mating was random and no
other mechanisms of evolution were happening, no evolution would occur
in this population.

The result of nonrandom mating is that some individuals have more
opportunity to mate than others and thus produce more offspring (and
more copies of their genes) than others. It is simply easier to mate with a
nearby individual, as opposed to one that is farther away. Also, especially
in animals, individuals compete for mates and active selection of mating
partners occurs. This goes directly against the concept of randomness.
Mechanisms of Evolution: Mutation
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Mechanism of evolution
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Mutation, a driving force of evolution, is a random change in an
organism’s genetic makeup, which influences the population’s gene pool.
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Mutations give rise to new alleles; therefore, they are a source of genetic
variation in a population.
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Mutations may be harmful or benign, but they may also be beneficial.
Mechanisms of Evolution:
Natural Selection
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Mechanism of evolution
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Individuals in a population are not equally adapted to the environment
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Best traits survive
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Types of natural selection
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Directional
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Stabilizing
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Disruptive
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Sexual