Lecture AP Biology Chapter 23 The evolution of populations

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?
3. What is 1 thing you are grateful for today?


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): ___


The Evolution of
Populations
Chapter 23


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 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!
EVOLVING


Allele Frequencies:
• Gene with 2 alleles : p, q
p = frequency of dominant allele (A)
q = frequency of recessive allele
(a)

Note:
1–p=q
1–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 = 5
Allele 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.
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!
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:
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