1. 16–2 Evolution as Genetic Change
Section Outline
Section 16-2
16–2 Evolution as Genetic Change
A. Natural Selection on Single-Gene Traits
B. Natural Selection on Polygenic Traits
1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection
C. Genetic Drift
D. Evolution Versus Genetic Equilibrium
1. Random Mating
2. Large Population
3. No Movement Into or Out of the Population
4. No Mutations
5. No Natural Selection

2. Natural Selection on Single-Gene Traits
traits can lead to changes in _____
__________ and thus to ________.
In a hypothetical population of lizards,
individuals mostly demonstrate a
brown _________, which is ________,
so brown lizards are ________ ___ or
__, and make up 80% of the population.
Natural selection
single-gene
allele
frequencies
evolution
phenotype
dominant
genotype BB Bb

3. Natural Selection on Single-Gene Traits
20% of the lizard population show the
black _________, which arose as a
result of a non-lethal ____ ____
_______, and are ________ __.
According to _____-________, the
______ ________ of the recessive
allele q is _____ or _____, so…
phenotype
germ
cell
mutation
genotype bb
Hardy
Weinberg
relative
frequency
0.447
44.7%

4. Natural Selection on Single-Gene Traits
the _______ ________ of the
dominant p allele is _____. Because of
the black lizards’ darker color, they
absorb more heat from the sun, so they
warm up faster on cool days, allowing
them to move faster to avoid predation.
After 30 generations, the black
relative
frequency
55.3%

5. Natural Selection on Single-Gene Traits
_________ makes up ___ of the lizard
population, so the frequency of the
________ q allele is now 77.5%. The
change in the frequency of the alleles
indicates ________ caused by
______ ________.
phenotype
60%
recessive
evolution
natural
selection

6. Natural Selection on Polygenic Traits
In ________ _____, ______
_______ can affect a population in
three ways: _________ ________,
________ ________, and
________ ________.
polygenic
traits
natural
selection
directional
selection
stabilizing
selection
disruptive
selection

7. Figure 16–6 Graph of Directional Selection
Section 16-2
Key
Directional Selection
Low mortality, high fitness
High mortality, low fitness
Food becomes scarce.
Birds in the population with larger, stronger beaks find food in the large seeds
available in the population, while birds with smaller beaks have a hard time getting
enough food. Over time, birds with larger beaks make up most of the population.

8. Figure 16–7 Graph of Stabilizing Selection
Section 16-2
Male birds use their
plumage to attract
mates. Male birds in
the population with
less brilliant and
showy plumage are
less likely to attract
a mate, while male
birds with showy
plumage are more
likely to attract a
mate.
Stabilizing Selection
Male birds with
showier, brightly-
colored plumage
also attract
predators, and are
less likely to live
long enough to
find a mate.
The most
fit, then, is the male
bird in the middle--
showy, but not too
showy.
Key
Low mortality, high fitness
High mortality, low fitness
Selection against both extremes keep curve narrow and in same place.
Percentage of Population
Brightness of
Feather Color

9. Figure 16–8 Graph of Disruptive Selection
Section 16-2
Disruptive Selection
Largest and smallest seeds become more common.
Key
Low mortality, high fitness
Population splits into two subgroups specializing in different seeds.
Number of Birds in Population
Number of Birds in Population
High mortality, low fitness
Beak Size
Beak Size
Something like a change in the climate causes medium-sized seeds to become
scarce in the environment. Birds with smaller beaks are better at finding smaller
seeds, and birds with larger beaks are better at finding larger seeds, while birds
with medium-sized beaks (most of them) have trouble finding smaller and larger
seeds. Over time and many generations, the bird population may even become
two separate species.

10. If you flip a coin, there is a ___ chance
Genetic Drift
If you flip a coin, there is a ___ chance
of the coin coming up heads. If you flip
the coin 100 times, about __ should be
heads. If you flip the coin 4 times, you
are supposed to get _ heads, but you
probably won’t, because the sample size is small. _______ ____ is a
change in _____ ________ due to
random chance, especially in small
populations.
50% 50 2
Genetic
drift
allele
frequency

11. Genetic Drift: Founder Effect
Section 16-2
Sample of
Original Population
A population of beetles shows a wide variety
of colors and patterns. A storm randomly
blows a few individuals from the population
to two new isolated locations, like two
different islands. The few individuals become
the founders of new populations, which don’t
look much like the original.
Descendants
Founding Population A
Founding Population B

12. Genetic Drift: Founder Effect
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B

13. Genetic Drift: Founder Effect
Section 16-2
The two populations look very different from
each other and the original, but it was random
chance that made them that way.
Sample of
Original Population
Descendants
Founding Population A
Founding Population B

14. Evolution vs. Genetic Equilibrium
The _____-________ principle states
that allele frequencies will remain
________ unless one or more factors
cause those frequencies to change. If
those factors are not present, allele
frequencies remain constant, and a
_______ __________ exists.
Hardy
Weinberg
constant
genetic
equilibrium

15. Evolution vs. Genetic Equilibrium
Five conditions are required to maintain
_______________:
genetic equilibrium
1. random mating
2. large population
3. no movement in or out
4. no mutations
5. no natural selection

16. Evolution vs. Genetic Equilibrium
In order for ______ ______ to occur,
all members of the population must
have equal opportunity to produce
offspring. In natural populations, like
____, ______, ___, or _____________,
members compete or even fight for the
opportunity to mate.
random
mating
lions
wolves
elk
mountain sheep

17. Evolution vs. Genetic Equilibrium
For _______ __________ to occur, a
population must be large, so _______
___ doesn’t cause changes in allele
frequency by random chance. No
migration can occur, since movement in
and out of the population changes the
________ of ______.
genetic
equilibrium
genetic
drift
frequency
alleles

18. Evolution vs. Genetic Equilibrium
For _______ __________ to occur,
there must not be _________, which
introduce new ______, and no
______ ________ can take place
which gives any one _________ a
survival advantage over another. In
some populations, these conditions
may be met or nearly met over long
periods of time, and little or no
________ occurs.
genetic
equilibrium
mutations
alleles
natural
selection
phenotype
evolution

19. Should the Use of Antibiotics Be Restricted?
Read the Issues in Biology article on
p. 403 in your text. Then watch the
video Why Does Evolution Matter Now?
and discuss the question: Should the
use of antibiotics be restricted?