Natural Selection on
Polygenic Traits
Natural Selection on Polygenic
Traits
• Effects of natural selection on polygenic
traits are very complex
• Can affect the distributions of phenotypes
in any of three ways:
– Directional selection
– Stabilizing selection
– Disruptive selection
Directional Selection
• Individuals at one end of the curve have a
higher fitness than individuals in the
middle or at the other end
• Entire curve moves as the trait changes
– Ex. Finches and beak size – food became
scarce; finches compete for food; bigger
beaks are able to find food, survive and
reproduce = beak size increases
Directional Selection
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Stabilizing Selection
• Individuals near the center of the curve
have higher fitness than individuals at
either end of the curve
• Keeps the center of the curve at its current
position, but narrows the graph
– Ex. Weight of human infants – small babies
less likely to survive and large babies have
difficulty being born = average-sized babies
are favored
Stabilizing Selection
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Disruptive Selection
• Individuals at upper and lower ends of the
curve have higher fitness than individuals
near the middle
• Can cause graph to split into two, creating
two distinct phenotypes
– Ex. Bird – 2 different seed sizes (small and
large); birds with large beaks and birds with
small beaks survive = two distinct beak sizes
Disruptive Selection
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Genetic Drift
• Allele can become more or less common BY
CHANCE
• Random change in allele frequency is genetic
drift (caused BY CHANCE!)
• In small populations, individuals that carry a
particular allele may leave more descendants
than other individuals, just BY CHANCE. Over
time, a series of chance occurrences of this type
can cause an allele to become common in a
population
Genetic Drift
(continued)
• May occur when a small group of
individuals colonies a new habitat. These
individuals may carry alleles in different
relative frequencies than the larger
population that they came from. If so, the
population that they create will be
genetically different from the parent
population
Genetic Drift: The Founder Effect
• Founder Effect – allele frequencies change as a
result of the migration of a small subgroup of a
population:
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift: The Founder Effect
• Founder Effect – allele frequencies change as a
result of the migration of a small subgroup of a
population:
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Drift: The Founder Effect
• Founder Effect – allele frequencies change as a
result of the migration of a small subgroup of a
population:
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Genetic Equilibrium
• Hardy and Weinberg – 2 scientists that
asked: “Are there any conditions under
which evolution will NOT occur?”
• If allele frequencies stay the same – the
population does NOT evolve
Genetic Equilibrium
(continued)
• 5 conditions are required to maintain genetic
equilibrium from generation to generation:
– Random mating – equal chance of passing genes
– Large population – genetic drift has less effect
– No movement into or out of the population – must
maintain gene pool
– No mutation – can’t have new alleles
– No natural selection – no “survival of the fittest”
Genetic Equilibrium
(continued)
• Meeting these five conditions is difficult to
do and often cannot be met, so evolution
will occur
– Many organisms mate with chosen mates
based on things like strength, color, etc
– Not all populations are large
– Migration occurs all the time
– Mutations are bound to happen
– Natural selection/survival of the fittest often
cannot be avoided!