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CH. 22/23 WARM-UP
1. List 5 different pieces of evidence for evolution.
2. (Review) What are the 3 ways that sexual reproduction produces genetic diversity?
CH. 23 WARM-UP
1. In a population of 200 mice, 98 are homozygous dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous (bb).
a) The allele frequencies of this population are:B allele: ___ b allele: ___
b) The genotype frequencies are:BB: ___ Bb: ___ bb: ___
2. Use the above info to determine the genotype frequencies of the next generation:
B (p): ___ b (q): ___BB (p2): ___ Bb (2pq): ___bb (q2): ___
CHAPTER 23
THE EVOLUTION OF POPULATIONS
WHAT YOU MUST KNOW:
• How mutation and sexual reproduction each produce genetic variation.
• The conditions for Hardy-Weinberg equilibrium.
• How to use the Hardy-Weinburg equation to calculate allelic frequencies and to test whether a population is evolving.
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 population1.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. No mutations.2. Random mating.3. No natural selection.4. Extremely large population size.5. No gene flow.
If at least one of these conditions is NOTNOT met, then the population is EVOLVINGEVOLVING!
Allele Frequencies:• Gene with 2 alleles : p, q
p = frequency of dominant allele (A)q = frequency of recessive
allele (a)
Note:1 – p = q1 – q = p
Genotypic Frequencies:•3 genotypes (AA, Aa, aa)
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 = 5Allele Frequencies:
A = a =
Genotypic Frequencies:AA =Aa =aa =
HARDY-WEINBERG PRACTICE PROBLEM #2:PTC TASTERS
• Taster = AA or Aa Nontaster = aa
• Tasters = ____ Nontasters = ___q2 = q =
p + q = 1 p = 1 – q =
p2 + 2pq + q2 = 1
CAUSES OF EVOLUTION
CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM
1. No mutations.2. Random mating.3. No natural selection.4. Extremely large population size.5. No gene flow.
If at least one of these conditions is NOTNOT met, then the population is EVOLVINGEVOLVING!
Minor Causes of Evolution: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 EvolutionMajor Causes of Evolution:• Natural selection, genetic drift, gene flow
(#3-5)
MAJOR CAUSES OF EVOLUTION
#3 – Natural Selection#3 – Natural Selection• Individuals with variations better suited
to environment pass more alleles to next generation
MAJOR CAUSES OF EVOLUTION
#4 – Genetic Drift#4 – Genetic Drift• Small populations have greater chance of fluctuations in
allele frequencies from one generation to another• Examples:
• Founder Effect• Bottleneck Effect
Genetic DriftGenetic 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#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?
Natural selection can alter frequency distribution of heritable traits in 3 ways:
1.Directional selection2.Disruptive (diversifying) selection3.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
• IntrasexualIntrasexual – selection within same sex (eg. M compete with other M)
• IntersexualIntersexual – mate choice (eg. F choose showy M)
PRESERVING GENETIC VARIATION
• DiploidyDiploidy: hide recessive alleles that are less favorable
• Heterozygote advantageHeterozygote advantage: greater fitness than homozygotes• eg. Sickle cell disease
RUNNING TIME: 14:03 MIN
HHMI VIDEO:NATURAL SELECTION IN HUMANS
NATURAL SELECTION CANNOT FASHION PERFECT ORGANISMS.
1. Selection can act only on existing variations.
2. Evolution is limited by historical constraints.
3. Adaptations are often compromises.
4. Chance, natural selection, and the environment interact.
SAMPLE PROBLEM
Define the following examples as directional, disruptive, or stabilizing selection:
a) Tiger cubs usually weigh 2-3 lbs. at birthb) Butterflies in 2 different colors each
represent a species distasteful to birdsc) Brightly colored birds mate more
frequently than drab birds of same species
d) Fossil evidence of horse size increasing over time