Download IV. Evolution as Genetic Change

IV. Evolution as Genetic Change
*Populations can evolve over time in
different situations.
A. Natural Selection on Single-Gene Traits
-Natural Selection on single-gene traits
can lead to changes in allele frequencies
& thus to evolution.
Example : Brown vs. Black vs. Red
lizards. If brown is less visible, red &
black may be eaten by predators.
B. Natural Selection & Polygenic Traits
-When dealing with a range of phenotypes, a bell
curve is used for comparison.
-Natural selection can affect the distribution of
phenotypes in any of three ways :
1. Directional selection
2. Stabilizing selection
3. Disruptive selection
Directional Selection
Directional selection –
when individuals at one
end of the curve have
higher fitness than
individuals in the middle
or at the other end.
-The range of phenotypes
shifts as some individuals
die & others succeed, ex:
Darwin’s finches & increase
in large seed availability.
Stabilizing Selection
Stabilizing selection –
when individuals near
the center of the curve
have higher fitness
than individuals at
either end of the curve.
-The center of the curve
stays at its current
position, but narrows the
overall graph, ex: human
birth weight.
Disruptive Selection
Disruptive selection – when
individuals at the upper & lower
ends of the curve have higher
fitness than individuals near the
middle.
-Selection acts most strongly
against individuals of an
intermediate type.
-In disruptive selection, the
pressure of natural selection can
become strong enough to split a
single curve in two, creating two
distinct phenotypes, ex :
Darwin’s finches & availability of
large & small seeds only.
C. Genetic Drift
Genetic drift – random change in
allele frequencies that occurs in
small populations.
-In small populations, some
individuals with particular traits may
leave more descendants than others
by chance.
-Over time, a series of chance
occurrences of this type can cause an
allele to become common in a
population.
-Can occur when a small group
colonizes a new habitat.
Founder effect – change in allele
frequencies as a result of the
migration of a small subgroup of a
population.
D. Evolution vs. Genetic Equilibrium
*To understand how evolutionary change
works, we must consider if there are any
situations in which evolution doesn’t happen.
Hardy-Weinberg principle – principle
that states that allele frequencies in a
population will remain constant unless
one or more factors cause those
frequencies to change.
Genetic Equilibrium – situation in which
allele frequencies remain constant.
*No allele change = no evolution.
-In order to maintain genetic
equilibrium 5 conditions must be kept :
1. Random mating- no choosing mates.
2. Large population – no genetic drift.
3. No moving into/out of population.
4. No mutations.
5. No natural selection.