Download AP Chapter 23 Lecture - TJ

CHAPTER 23 THE EVOLUTION OF POPULATIONS
23.1 Mutation & Sexual Reproduction
I. Genetic variations
A. Variations within a population
1. Measured by average heterozygosity
B. Variations between populations
1. Geographic variations
a. Differences in genetic composition of separate populations
II. Mutations
A. Ultimate source of new alleles
1. A ∆ in nucleotide sequence
2. Needs to occur in cells producing gametes
3. Completely random & by chance
a. Impossible to predict
B. Point mutations
1. Usually harmful or no effect
2. But, every once in a while……
C. Mutations that alter gene # or sequence
1. Duplication (errors in meiosis), transposable elements,
translocation
2. Most harmful or of no benefit
a. If no severe effects can accumulate over time
b. Imagine if a beneficial gene was duplicated
1. Mammal olfactory
D. Mutation rates
1. In all organisms typically low
a. Plants & animals 1/100,000 genes/generation
1. Often lower in prokaryotes
2. Generation time can greatly influence
III. Sexual reproduction
A. Results in most of the genetic variations
1. Reshuffles existing alleles & deals them out randomly each
time
a. What causes reshuffling?
1. Cross-over
2. Independent assortment
3. Fertilization
23.2 The Hardy-Weinberg Equation
I. Basic Principle
A. Tests whether a population is evolving
II. Gene pools & allele frequencies
A. Terminology
1. Population
a. A group of individuals of the same species that live in the
same area & interbreed producing fertile offspring
2. Gene pool
a. All of the alleles within a population
b. Represented as a frequency
B. Allele frequencies
1. % of alleles within a given population compared to total # of
alleles in a population
a. Remember that each individual has
?
alleles/loci
b. p= Dominant allele
q= recessive allele
2. When dealing with 2 or more alleles/loci, sum of all
frequencies must equal 1 (100%)
a. p + q = 1
3. Example
RR = Red rr = White Rr = pink
340 individuals RR
40 individuals rr
180 individuals Rr
560 individuals
How many alleles in the population?
1120
What is the frequency of p & q?
p = .77
q = .23
III. The Hardy-Weinberg Principle
A. Describes a gene pool of a population that is not evolving
1. Hardy-Weinberg equilibrium
a. Allele & genotype frequencies do not ∆ from generation to
generation
2. A null hypothesis
a. There is no statistical difference between observed allele
or genotype frequency & expected frequency
B. The equation
1. p2 + 2pq + q2 = 1 (genotype frequencies)
a. p2  Expected frequency of homozygous dominant
b. q2  Expected frequency of homozygous recessive
c. 2pq  Expected frequency of heterozygous
C. Conditions for the Hardy-Weinberg Equilibrium
1. For a population to be in Hardy-Weinberg equilibrium (no
evolution) all of the below conditions must be met
a. No mutations
b. Random mating
c. No natural selection
d. Extremely large population size
1. Genetic drift
e. No gene flow
1. Migration
2. If allele frequencies are changing (evolution) then one or more
of the above conditions is not met
D. Applying the Hardy-Weinberg principle
1. Must assume 5 conditions are being met
In the town of Thomasville, 78% of Thomasens have extreme
intelligence, a dominant trait. What are the allele frequencies of this
p2 = .28
population?
p = .53 q = .47
q2 = .22
2pq = .49
What are the genotype frequencies of this population?
16% of the human population has a recessive trait. What are the
genotypic & allelic frequencies of the population?
7,000 AA & 3,000 aa individuals mate at random. In the first generation
of offspring, what would be the frequency of the 2 alleles? Frequencies
of the 3 genotypes? Assuming Hardy-Weinberg equilibrium , what
would be the values for the second generation?
Gene pool = 20,000 alleles (14,000 A & 6,000 a)
p = 14,000/20,000 = .7
q = 1–.7 or 6,000/20,000 = .3
p2 = .49
2pq = .42
q2 = .09
Second generation is the same (assuming equilibrium)
Actual 2nd generation
24,500 AA individuals
10,500 aa individuals
Has evolution occurred?
23.3 Altering Allele Frequencies in a Population
I. Intro
A. Deviations from H-W equilibrium is a potential cause of evolution
1. New mutations
a. Rare, so usually not significant enough to cause great
change from generation to generation
2. Nonrandom mating
a. Can affect homo & hetero genotypes but usually does not
affect allele frequency in a gene pool
3. Natural selection, genetic drift, gene flow
a. Alter allele frequency directly
b. Cause most evolutionary change
II. Natural selection
A. Lets review Key points
1. Individuals with certain heritable characteristics survive &
reproduce at a higher rate than other individuals
2. Individuals do not evolve
3. Only heritable traits are amplified or diminished
a. Organisms may be modified, & it may be a beneficial
modification, but it will not be inherited to the next
generation
4. Environmental factors vary from place to place & over time
B. Fruit fly & insecticide resistance
III. Genetic Drift
A. Concept
1. Chance events that cause an unexpected ∆ in allele frequency
a. More pronounced in smaller populations
B. Founder effect
1. A change in allele frequency due to the isolation of a small #
of individual from a larger population
a. A smaller gene pool to work from
b. Most pronounced in inherited disorders
1. See an increase in frequency in isolated human
populations
a. Amish of Lancaster County, Pennsylvania
1. 1 in 14 carries recessive allele for an unusual
form of dwarfism & polydactyly
C. Bottleneck effect
1. A type of genetic drift that occurs due to a natural disaster or
human actions
a. Population decreases
2. Prevents the majority of genotypes from participating in
reproduction
a. Inbreeding
b. Decrease genetic diversity
D. Sum up
1. May increase or decrease allelic frequency
a. Natural selection will decide
IV. Gene flow
A. The movement of alleles into & out of a population due to
movement of fertile individuals or their gametes
B. Tends to reduce genetic drift between populations
C. May increase or decrease
1. Natural selection will determine
23.4 Natural Selection Provides Consistency
I. Intro
A. Only natural selection consistently increases favorable allelic
frequencies
1. Increase reproductive advantage leads to adaptive evolution
II. A closer look at natural selection
A. Darwin’s Observations
1. All species populations have the potential to overproduce
2. Environmental resources are limited
3. Ind. in a population vary in their characteristics
4. Much of this variation in characteristics is heritable
5. A substantial amount of time is needed for changes in a
population to occur
B. Inferences based on his observations
1. Time needed for evolution to occur
2. There is overproduction of offspring
a. Leads limited resources (biotic & abiotic)
1. Leads to competition
3. Heritable variations exist within a population
4. These variations can result in differential reproductive
success
C. Relative fitness
1. The contribution an individual makes to the gene pool of the
next generation
a. Relative to the contributions of other individuals
2. Relative fitness is based on phenotype
a. Natural selection acts directly on phenotype
1. Indirectly on genotype
3. Degree of influenced by entire genetic and environmental
context
B. Directional, Disruptive, & stabilizing selection
1. Modes of natural selection
a. Example peppered moth
1. Prior to British industrial revolution 10% dark
2. 1950 ≈94% dark
3. 1994 ≈75% dark
III. The Key role of natural selection in adaptive evolution
A. Natural selection is the only consistent influence on favorable
allelic frequencies
1. Genetic drift
a. Can increase favorable allele frequencies but can also
decrease, usually the case
2. Gene flow
a. May introduce new advantageous alleles or
disadvantageous alleles
IV. Sexual selection
A. Intrasexual
B. Intersexual
V. The preservation of genetic variation
A. Deep Thoughts: Why does natural selection not remove
unfavorable traits, thus decreasing variability, in
a population?
B. Mechanisms
1. Diploidy
a. Heterozygotes hide recessive
2. Balancing selection
a. When natural selection maintains 2 or more forms in a
population
b. Heterozygote advantage
1. Sickle-cell anemia
c. Frequency-dependant selection
1. Fitness of a phenotype declines if it becomes too
common
d. Neutral variation
VI. Natural selection & the perfect organism
A. Deep Thoughts: If natural selection increases relative fitness,
than why are “harmful” or disadvantageous
alleles found in a population?
Why, after billions of years of natural
selection, is their no “perfect” organism?
1. Selection can only act on existing variations
a. Not all existing phenotypes are ideal
2. Evolution is limited by historical constraints
a. Ancestral phenotypes are not scraped & replaced by new
ones from scratch
3. Adaptations are often compromises
a. To gain 1 thing you must sacrifice another
4. Chance, natural selection, & the environment
a. Genetic drift
1. Best genes are not always selected
b. Environments can change unexpectedly