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THE EVOLUTION OF
POPULATIONS
CHAPTER 23
SMALLEST UNIT OF EVOLUTION
Microevolution: change in the allele
frequencies of a population over generations
• Darwin did not know how
organisms passed traits to
offspring
• 1866 - Mendel published his
paper on genetics
• Mendelian genetics supports
Darwin’s theory  Evolution is
based on genetic variation
SOURCES OF GENETIC VARIATION
• Point mutations: changes in one base (eg.
sickle cell)
• Chromosomal mutations: delete, duplicate,
disrupt, rearrange  usually harmful
• Sexual recombination: contributes to most
of genetic variation in a population
1. Crossing Over (Meiosis – Prophase I)
2. Independent Assortment of
Chromosomes (during meiosis)
3. Random Fertilization (sperm + egg)
Population genetics: study of how
populations change genetically over
time
Population: group of individuals that
live in the same area and interbreed,
producing fertile offspring
• Gene pool: all of the alleles for all genes in
all the members of the population
• Diploid species: 2 alleles for a gene
(homozygous/heterozygous)
• Fixed allele: all members of a population
only have 1 allele for a particular trait
• The more fixed alleles a population has,
the LOWER the species’ diversity
HARDY-WEINBERG PRINCIPLE
Hardy-Weinberg Principle: The allele and
genotype frequencies of a population will
remain constant from generation to
generation
…UNLESS they are acted upon by forces
other than Mendelian segregation and
recombination of alleles
Equilibrium = allele and genotype
frequencies remain constant
CONDITIONS FOR HARDY-WEINBERG
EQUILIBRIUM
1.
2.
3.
4.
5.
No mutations.
Random mating.
No natural selection.
Extremely large population size.
No gene flow.
If at least one of these conditions is NOT met,
then the population is EVOLVING!
Hardy-Weinberg Principle
Allele Frequencies:
• Gene with 2 alleles : p, q
p = frequency of dominant allele (A)
q = frequency of recessive allele (a)
p+q=1
Note:
1–p=q
1–q=p
Hardy-Weinberg Equation
Genotypic Frequencies:
• 3 genotypes (AA, Aa, aa)
2
p
+ 2pq +
2
q
=1
p2 = AA (homozygous dominant)
2pq = Aa (heterozygous)
q2 = aa (homozygous recessive)
ALLELE
FREQUENCIES
GENOTYPIC
FREQUENCIES
STRATEGIES FOR SOLVING H-W PROBLEMS:
1. If you are given the genotypes (AA, Aa, aa),
calculate p and q by adding up the total # of
A and a alleles.
2. If you know phenotypes, then use “aa” to find
q2, and then q. (p = 1-q)
3. Use p2 + 2pq + q2 to find genotype
frequencies.
4. If p and q are not constant from generation to
generation, then the POPULATION IS
EVOLVING!
HARDY-WEINBERG PRACTICE PROBLEM #1
The scarlet tiger moth has the following genotypes.
Calculate the allele and genotype frequencies (%)
for a population of 1612 moths.
AA = 1469
Aa = 138
aa = 5
Allele Frequencies:
A=
a=
Genotypic Frequencies:
AA =
Aa =
aa =
HARDY-WEINBERG PRACTICE PROBLEM #1
The scarlet tiger moth has the following genotypes.
Calculate the allele and genotype frequencies (%)
for a population of 1612 moths.
AA = 1469
Aa = 138
aa = 5
Allele Frequencies:
A =(2*1469)+(1*138)= 3076
a =(1*138)+ (2*5)= 148
p = 3076/(3076+148)= .954 q=148/(3076+148)=.046
Genotypic Frequencies: Total alleles = 2*1612=3224
AA = p2 = 0.910
Aa = 2pq= 0.087
aa = q2= 0.002
HARDY WEINBERG PRACTICE #2
• A hypothetical population of 10,000 humans has 6840
individuals with the blood type AA, 2860 individuals with blood
type AB and 300 individuals with the blood type BB.
• What is the frequency of each genotype in this population?
AA =
AB =
BB =
• What is the frequency of the A allele?
• What is the frequency of the B allele?
• If the next generation contained 25,000 individuals, how many
individuals would have blood type BB, assuming the
population is in Hardy-Weinberg equilibrium?
HARDY WEINBERG PRACTICE #2
What is the frequency of each genotype in this population?
AA = 6840/1000=0.684 AB = 0.286 BB = 0.03
What is the frequency of the A allele?
((2*6840) + (1*2860))/20000= .827 (p)
What is the frequency of the B allele?
((1*2860)+(2*300))/20000= 0.173 (q)
If the next generation contained 25,000 individuals, how many
individuals would have blood type BB, assuming the
population is in Hardy-Weinberg equilibrium?
• q2=0.030, 0.030*25000=750
•
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CAUSES OF EVOLUTION
CONDITIONS FOR HARDY-WEINBERG
EQUILIBRIUM
1.
2.
3.
4.
5.
No mutations.
Random mating.
No natural selection.
Extremely large population size.
No gene flow.
If at least one of these conditions is NOT met,
then the population is EVOLVING!
Minor Causes of Evolution:
#1 - Mutations
• Rare, very small changes in allele
frequencies
#2 - Nonrandom mating
• Affect genotypes, but not allele
frequencies
Major Causes of Evolution:
• Natural selection, genetic drift, gene flow
(#3-5)
MAJOR CAUSES OF EVOLUTION
#3 – Natural Selection
• Individuals with variations better suited to
environment pass more alleles to next
generation
MAJOR CAUSES OF EVOLUTION
#4 – Genetic Drift
• Small populations have greater chance of
fluctuations in allele frequencies from one
generation to another
• Examples:
• Founder Effect
• Bottleneck Effect
Genetic Drift
FOUNDER EFFECT
• A few individuals isolated from larger
population
• Certain alleles under/over represented
Polydactyly in Amish population
BOTTLENECK EFFECT
• Sudden change in environment drastically
reduces population size
Northern elephant seals hunted
nearly to extinction in California
MAJOR CAUSES OF EVOLUTION
#5 – Gene Flow
• Movement of fertile
individuals between
populations
• Gain/lose alleles
• Reduce genetic
differences between
populations
HOW DOES NATURAL SELECTION BRING
ABOUT ADAPTIVE EVOLUTION?
Fitness : the contribution an individual makes to
the gene pool of the next generation
Natural selection can alter frequency
distribution of heritable traits in 3 ways:
1. Directional selection
2. Disruptive (diversifying) selection
3. Stabilizing selection
Directional Selection:
eg. larger black bears
survive extreme cold
better than small ones
Disruptive Selection:
eg. small beaks for
small seeds; large
beaks for large seeds
Stabilizing Selection:
eg. narrow range of
human birth weight
SEXUAL SELECTION
• Form of natural selection – certain individuals
more likely to obtain mates
• Sexual dimorphism: difference between 2 sexes
• Size, color, ornamentation, behavior
SEXUAL SELECTION
• Intrasexual – selection within same sex (eg. M
compete with other M)
• Intersexual – mate choice (eg. F choose showy
M)
PRESERVING GENETIC VARIATION
• Diploidy: hide recessive alleles that are less
favorable
• Heterozygote advantage: greater fitness than
homozygotes
• eg. Sickle cell disease
HHMI VIDEO:
NATURAL SELECTION IN HUMANS
RUNNING TIME: 14:03 MIN
NATURAL SELECTION CANNOT FASHION
PERFECT ORGANISMS.
1.
2.
3.
4.
Selection can act only on existing variations.
Evolution is limited by historical constraints.
Adaptations are often compromises.
Chance, natural selection, and the
environment interact.