17.2 Evolution as Genetic
Change in Populations
MRS. MACWILLIAMS
ACADEMIC BIOLOGY
I. How Natural Selection Works
**Evolutionary fitness- success in passing genes to the
next generation
A. Natural Selection on Single-Gene Traits
1. Natural selection for a single-gene trait can lead to changes in
allele frequencies and then to evolution.
Ex. Mutation in one gene that determines body color in lizards can
affect their lifespan. So if the normal color for lizards is brown, a
mutation may produce red and black forms.
•
•
If red lizards are more visible
to predators, they might be
less likely to survive and
reproduce. Therefore the
allele for red coloring might
not become common.
Black lizards might be able to
absorb sunlight. Higher body
temperatures may allow the
lizards to move faster, escape
predators, and reproduce.
B. Natural Selection on Polygenic Traits
**Remember, polygenic traits have a wide range of
traits
**The fitness of individuals may vary from one end of
the curve to the other.
**Natural selection can affect the range of phenotypes
and hence the shape of the bell curve.
1. Stabilizing Selection- individuals near the center of
the curve have higher fitness than individuals at either
end
2. Directional Selection- individuals at one end of the
curve have higher fitness than individuals in the middle
or at the other end
3. Disruptive Selection- individuals at the upper and
lower ends of the curve have higher fitness than
individuals near the middle
Natural Selection and Adaptation (Rock Pocket Mouse)
Watch Video at:
http://www.hhmi.org/biointeractive/making-fittest-naturalselection-and-adaptation
II. Genetic Drift
**In small populations, individuals that carry a particular allele
may leave more descendants than other individuals, by chance.
Over time, a series of chance occurrences can cause an allele to
become more or less common in a population.
A. Genetic Bottleneckschange in allele frequency
following a dramatic
reduction in the size of a
population
Ex. Disaster kills many
individuals in a population,
and surviving population’s
gene pool may contain
different gene frequencies
from the original gene pool.
B. Founder Affect- allele frequencies change as a result
of the migration of a small subgroup of a population
III. Evolution Versus Genetic Equilibrium
**Genetic Equilibrium- allele frequencies in the population
remain the same. If allele frequencies don’t change, the
population will not evolve.
A. Sexual Reproduction and Allele Frequency
1. Gene shuffling during sexual reproduction produces
many different gene combinations
2. Meiosis and fertilization do not change allele
frequencies by themselves, therefore a population of
organisms could remain in genetic equilibrium
B. Hardy Weinberg Principle- Describes
conditions under which evolution does not
occur
**Allele frequencies remain constant unless
one or more of the following 5 factors
cause those frequencies to change
1. Large Population
 Genetic drift can cause changes in allele
frequencies in small populations; Large
population size helps maintain genetic
equilibrium
2. No Mutations
 If mutations occur, new alleles may
be introduced into the gene pool and
allele frequencies will change
3. Random mating
 All members of the population must have an equal
opportunity to produce offspring and must mate with
other members of the population at random
 In natural populations, mating is NOT random.
• Ex. Female peacocks choose mates on the basis of
physical characteristics such as brightly patterned
tail feathers  alleles for those traits are under
selection pressure
4. No Gene Flow/Migration
a. Individuals who join a population may introduce new
alleles into the gene pool.
b. Individuals who leave may remove alleles from the
gene pool.
c. Thus, for no alleles to flow into or out of the gene pool,
there must be no movement of individuals into or out of
a population.
5. No Natural selection
a. All genotypes in the population must have equal
probabilities of surviving and reproducing
b. Phenotypes have NO selective advantage over
another
Should Antibiotic Use Be Restricted?
• Many disease-causing bacteria are evolving resistance to
antibiotics.
• Bacterial populations have ALWAYS contained a FEW individuals
with mutations that enable them to destroy, inactivate, or eliminate
antibiotics. But those individuals didn’t have a higher fitness, so
mutant alleles didn’t become common.
• Doctors began prescribing antibiotics widely and farmers started
feeding them to their animals to prevent infection.
• Antibiotics have become a REGULAR PART OF THE
ENVIRONMENT for the bacteria.
• In this environment, resistant bacteria have a higher fitness, so the
resistance alleles increase in frequency.
• Resistant alleles can be transferred from one bacteria to another on
plasmids.
• Many bacteria are evolving resistance not to just one antibiotic,
but to almost ALL medicines!
• SHOULD ANTIBIOTIC USE BE RESTRICTED?