Download Ch. 23- Evolution of Populations

Ch. 23- Evolution of Populations- Guided Notes
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.
Ch. 23 Warm Up1. List 5 different pieces of evidence for evolution.
2. (Review) What are the 3 ways that sexual reproduction produces genetic diversity?
3. In a population of 200 mice, 98 are homozygous dominant for brown coat color (BB), 84 are heterozygous
(Bb), and 18 are homozygous (bb). SHOW YOUR WORK
a. The allele frequencies of this population are:
b. B allele: ___ b allele: ___
c. The genotype frequencies are:
d. BB: ___ Bb: ___
bb: ___
4. Use the above info to determine the genotype frequencies of the next generation:
a. B (p): ___
b. b (q): ___
c. BB (p2): ___
d. Bb (2pq): ___
e. bb (q2): ___
1. 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
2. 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
o Crossing Over (Meiosis – Prophase I)
o Independent Assortment of Chromosomes (during meiosis)
o Random Fertilization (sperm + egg)
3. Populations
 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
o Diploid species: 2 alleles for a gene (homozygous/heterozygous)
 Fixed allele: all members of a population only have 1 allele for a particular trait
o The more fixed alleles a population has, the LOWER the species’ diversity
4. Causes of evolution
 Conditions for Hardy-Weinberg equilibrium
o No mutations.
o Random mating.
o No natural selection.
o Extremely large population size.
o No gene flow.
 ***If at least one of these conditions is NOT met, then the population is EVOLVING!***
 Minor Causes of Evolution:
o #1 – Mutations
 Rare, very small changes in
allele frequencies
o #2 - Nonrandom mating
 Affect genotypes, but not
allele frequencies
 Major Causes of Evolution:
o #3 – Natural Selection
 Individuals with variations
better suited to environment
pass more alleles to next generation
o #4 – Genetic Drift
 Small populations have greater chance of fluctuations in allele frequencies from one
generation to another

Examples:
 Founder Effect: A few individuals isolated from larger population
o Certain alleles under/over represented
o EX: Polydactyly in Amish population

Bottleneck Effect: Sudden change in environment drastically reduces
population size
o EX: Northern elephant seals hunted nearly to extinction in California
o #5 – Gene Flow
 Movement of fertile individuals between populations
 Gain/lose alleles
 Reduce genetic differences between populations
5. How does natural selection bring about adaptive evolution?
o Fitness : the contribution an individual makes to the gene pool of the next generation
o Natural selection can alter frequency distribution of heritable traits in 3 ways:
 Directional selection: eg. larger black bears survive extreme cold better than small ones
 Disruptive (diversifying) selection: eg. small beaks for small seeds; large beaks for large seeds
 Stabilizing selection: eg. narrow range of human birth weight
6. Sexual selection
 Form of natural selection – certain
individuals more likely to obtain mates
 Sexual dimorphism: difference between 2
sexes
o Size, color, ornamentation, behavior
• Intrasexual – selection within same sex
(eg. M compete with other M)
• Intersexual – mate choice (eg. F choose
showy M)
7. Preserving genetic variation
• Diploidy: hide recessive alleles that are less
favorable
• Heterozygote advantage: greater fitness
than homozygotes
• eg. Sickle cell disease
8. 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.
9. Sample Problem
 Define the following examples as directional, disruptive, or stabilizing selection:
a) Tiger cubs usually weigh 2-3 lbs. at birth
b) Butterflies in 2 different colors each represent a species distasteful to birds
c) Brightly colored birds mate more frequently than drab birds of same species
d) Fossil evidence of horse size increasing over time
10. 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
11. 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 NOT met, then the population is
EVOLVING!
12. Hardy-Weinberg Principle
 Allele Frequencies:
o Gene with 2 alleles : p, q
 p = frequency of dominant allele (A)
 q = frequency of recessive allele (a)
o p+q=1
 Note:
 1–p=q
 1–q=p
13. Hardy-Weinberg Equation
 Genotypic Frequencies:
o 3 genotypes (AA, Aa, aa)
o p2 + 2pq + q2 = 1
 p2 = AA (homozygous dominant)
 2pq = Aa (heterozygous)
 q2 = aa (homozygous recessive)
14. Allele frequencies
15.
Genotypic frequencies
16. 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!
17. Hardy-weinberg practice problems:
1. The scarlet tiger moth has the following genotypes. Calculate the allele and genotype frequencies (%)
for a population of 1612 moths. SHOW YOUR WORK.
AA = 1469
Aa = 138
aa = 5
Allele Frequencies:
A=
a=
Genotypic Frequencies:
AA =
Aa =
aa =
2. Class survey Taster = AA or Aa

Tasters = ____

Nontasters = ___

q2 =

q=

p+q=1

p=1–q=

p2 + 2pq + q2 = 1
Nontaster = aa SHOW YOUR WORK