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POPULATION GENETICS
Chapter 23.1
Quick Review Natural Selection
Organisms better suited to the environment
SURVIVE & REPRODUCE
at a greater rate than those
less suited to the environment
and this is how…
SPECIES EVOLVE
Quick Review Natural Selection
Variation  Natural Selection  Speciation
Evolution Starts with Genetics
Mutation  Variation  Natural Selection  Speciation
Variation in Gene Pools

Collection of alleles within a population
Phenotypes: white, brown, gray
Genotypes: WW, Ww, ww
Hardy & Weinberg
We can study
changes in
phenotypes in a
population over time
We can calculate allele
frequencies based upon
the genotypes
We can convert
phenotypes into
genotypes
Thus, a math
equation will show
if evolution is
occurring
Ex: Allele Frequencies in Snapdragons

Collect data of phenotypes of a population
 320

red flowers, 160 pink flowers, & 20 white flowers
Convert phenotypes to genotypes
 320
RR
 160 RW
 20 WW

Calculate allele frequencies
R
alleles = 320 +320 + 160
 W alleles = 20 + 20 + 160
Ex: Allele Frequencies in Snapdragons

Hypothetically 500 flowers
 RR-red
 RW-pink
 WW-white

(320)
(160)
(20)
Allele Frequency
 800
/ 1000 R alleles exist (80% or 0.8)
 200 / 1000 W alleles exist (20% or 0.2)
Ex: Allele Frequencies in Snapdragons

Hardy-Weinberg:
 If
evolution is not occurring in this population
 Then allele frequency will remain constant over time

Therefore at any moment the population will have:
 80%
R alleles
 20% W alleles

If 10 years later:
 50%
R alleles
 50 % W alleles
 Then microevolution is occurring
Applying H.W.E.



This happens to nearly all populations for all traits
p represents the dominant allele (R)
q represents the recessive allele (W)
p+q=1
0.8 R alleles + 0.2 W alleles = 1
Solve this story problem

In certain Native American groups, albinism due to
a homozygous recessive condition. If the frequency
of the allele for this condition is 0.06, what is the
frequency of the dominant allele in this population?
p+q=1
P + .06 = 1
p = .94
Extrapolating H.W.E.

H.W.E. Equation 1:


H.W.E. Equation 2:




p + q = 1 (shows allele frequencies, but not genotype
frequencies)
(1) X (1) = 1
(p + q) X (p + q) = 1
p2 +2pq + q2 = 1
500 Snapdragon Ex:




p = .8 & q = .2
(.8)2 +2(.8*.2) + (.2)2 = 1
.64 + .32 + .04 = 1
320 + 160 + 20 = 500
Applying H.W.E.



p2 = homozygous dominant condition
2pq = Heterozygous condition
q2 = homozygous recessive condition
p2 +2pq + q2 = 1
RR + 2RW + WW = 1
Solve this story problem


In a certain flock of sheep, 4 percent of the
population has black wool
If black wool is a recessive trait, what percentage
of the population is heterozygous for this trait?
p +q = 1
p2 +2pq + q2 = 1
H.W.E. Conditions

When would allele frequencies not change over
time?
H.W.E. Conditions
 Very
 No
large population size
isolated disasters impacting allele frequencies
 Isolation
 No
 No
immigration/emigration (gene flow)
Mutations
 No
new genotypes/phenotypes
 Random
 No
 No
from other populations
Mating
picky females choosing one allele over another
natural selection
 No
environmental pressures selecting one allele over another
1. Mutations/Recombination

Rarely impact plants and animals
 ~1/100,000
genes
 98% of our DNA is junk (noncoding)
 Somatic cell mutations die with animal

Only Gamete mutations are inherited
1. Mutations/Recombination

Once a mutation occurs, nearly all phenotypic
variations will result from shuffling of existing alleles
2. Genetic Drift

Changes in the gene pool of a population
 Typical
Effect: decrease in genetic variation
 The smaller the population, the greater the effect
2. Genetic Drift: Bottleneck effect

Small population survives a disaster
2. Genetic Drift: Founder effect

Small population is responsible for colonization
3. Gene Flow

Individuals from different populations interbreed
 Result
of immigration/emigration
4. Natural Selection


Promotes favorable traits
Disposes unfavorable traits
4. Natural Selection: Environment

Changes in the biosphere, select better suited
alleles
4. Natural Selection: Sexual selection

Individuals choose mates based on particular traits
Summary: Microevolution

Changes that affect the gene pool of a population
Mutation  Variation  Natural Selection  Speciation