Chapter 20
Genes Within Populations
AP Biology 2012
20.1 Genetic Variation and Evolution
Evolution – changes over time
Darwin – descent with modification
Darwin’s mechanism is natural selection
organisms with a desirable characteristic
produce more offspring that live than
those that do not
Opposing theory – inheritance of acquired
characteristics – giraffe example
Based on genetics – which mechanism makes
the most sense?
Enzyme polymorphism
polymorphism – the presence in a population
of more than one allele of a gene at a
frequency greater than that of newly arising
mutations
Common in insects and plants
20.2 Changes in Allele Frequency
Was once believed that genetic variation was
blended from one generation to the next
- blending inheritance
Thought that variation was lost due to
blending
Question – Wouldn’t all members of a
population eventually only exhibit dominant
traits?
Hardy-Weinberg Principle – proportions of a
genotype remain constant as long as:
1. No mutation takes place
2. No immigration or emigration takes
place
3. Random mating is occurring
4. The population size is very large
5. No selection occurs
Hardy-Weinberg Equation
(p + q)2 = p2 + 2pq + q2
Where did we see this before?
Cat Example:
If 16% of cats are white and white is recessive
Then q2 = 0.16
q = 0.4
p = 0.6 b/c p + q = 1
Genotypic Frequencies would be:
BB  p2 = (0.6)2 = .36 or 36%
Bb  2pq = 2(0.6)(0.4) = .48 or 48%
HW can be used to find evidence for evolution
Populations change
Natural Selection occurs
Mating is not random
Immigration and emigration
20.3 Five Agents of Evolutionary
Change
1. Mutation changes alleles
mutation rates are very low
1/100,000 cell divisions
however – ultimate source of change
*mutation is not a result of natural
selection
2. Gene flow occurs when alleles move between
populations
a. New organism with different alleles
comes to the area
b. gametes spread – seeds or pollen
c. mating between adjacent populations
3. Nonrandom mating shifts genotype
frequencies
a. assortative mating – phenotypically similar
individuals mate – increases homozygous
b. disassortative mating – phenotypically
different individuals mate – increases
heterozygous
4. Genetic drift
change in genetic frequencies
A. Founder Effect – few individuals “found” a
new population, alleles they do not have are
lost
B. Bottleneck Effect – few individuals survive
and then produce the new population
5. Selection favors some genotypes over others
A. Artificial selection
B. Avoiding predators
C. Matching climatic conditions
D. Pesticide resistance
20.4 Fitness and Its Measurement
A phenotype with greater fitness usually
increases in frequency
Why?
Toad Example
2 phenotypes of toads – green and brown
Green 4.0 to next generation
Brown 2.5 to the next generation
G – 4.0/4.0 = 1
B – 2.5/4.0 = 0.625
What do you think should eventually happen?
20.5 Interactions Among
Evolutionary Forces
Sometimes drift doesn’t favor the allele favored
by selection?
How would you explain this statement?
Gene flow may promote or constrain
evolutionary change
How would you explain this?
20.6 Maintenance of Variation
Frequency-dependent selection – fitness of a
phenotype depends on its frequency within
the population
Negative frequency-dependent selection –
rare phenotypes are selected
Positive frequency-dependent selection –
common phenotypes are selected
Oscillating selection – favored phenotype
changes as the environment changes
Example – Ground Finch
A. large bills favored during drought
B. small bills favored during wet seasons
In some cases heterozygous may exhibit greater
fitness than homozygous
Example – Sickle Cell Anemia
Heterozygous individuals are resistant to
malaria
20.7 Selection Acting on Traits
Affected by Multiple Genes
Disruptive Selection removes intermediates
Example
African black-bellied seedcracker finch
Some individuals have large beaks –
for large seeds
Some have small beaks – for small
seeds
No intermediates – no medium seeds
Directional Selection – selection that eliminates
an extreme, causing those genes to become
less frequent
Example – Fruit flies
Some have genes that cause them to move
toward light – often leading to death
Over time less flies are moving toward
light
Stabilizing Selection – selection that eliminates
both extremes, increase of the common
phenotype
Example – Infant birth weights
Highest survival is found between 6 & 7 lbs
20.8 Experimental Studies of
Natural Selection
In groups
• Identify the biology of the guppy
• Identify why the guppy is a good organism to
study
• Describe the laboratory experiment
• Describe the field experiment
• What were the results
20.9 The Limits of Selection
What we cannot do:
Chickens cannot lay larger or more eggs
b/c shells would become thinner
Racehorses cannot get any faster
b/c we bred them faster than mutations
occur – no faster horses in 50 yrs
Some selection on one gene can be affected by
another -- seen in epistasis
Epistasis – when one gene modifies the effect
of another – example – fur color in cats