SBI 4U
Mechanisms of Evolution
Genetic Variation and Hardy-Weinberg Principle
Allele Frequencies and Genetic Equilibrium
 To understand how species change over time, think of a population simply
as a collection of genes.
 The collection of genes for all the traits in a population is called a GENE
POOL, and the gene pool of a population contains all the alleles for all the
genes
 A population in which allele frequencies do not change from generation to
generation is said to be GENETIC EQUILIBRIUM.
Hardy-Weinberg Principle
 For a population to undergo changes there must be genetic variation
 Demonstrated how the frequency of allele in the gene pool could be described
by mathematical formulas
 States that the frequency of dominant and recessive remains the same from
generation to generation as long as there is:
o A large population
o Random mating,
o Absence of forces (mutation, migration, and natural selection) that
change the proportion of the alleles at a given locus,
o **The original genotypes proportions will remain constant from
generation to generation
 Mathematical formula
p+q=1
frequency of dominant allele(A)
frequency of recessive allele (a)
(p + q)2 = 12
p2 + 2pq + q2 =1
frequency of AA genotype
frequency of aa genotype
frequency of Aa genotype
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SBI 4U
Mechanisms of Evolution
Note:
 In natural populations, the Hardy-Weinberg assumptions are rarely met.
 This principle is used to compare natural populations with an ideal situation.
E.g. #1
In Holstein cattle, one calf in 100 is spotted red, rather than black. Redness is due
to a recessive allele. What proportions of Holsteins are heterozygous?
Solution:
The frequency of red allele in the population is 1/100 or 0.01
q2=0.01  q=0.1
The frequency of the dominant allele is
p+q=1
p =1-0.1=0.9
The number of heterozygous in the population= 2pq
2(0.1)(0.9)=2(0.09)=18=18%
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SBI 4U
Mechanisms of Evolution
TRY THESE!
 E.g. #2
If 16% of individuals in a population have recessive trait, calculate the frequency of
the dominant allele in the population. Assume the population is in Hardy-Weinberg
equilibrium.
 E.g. #3
A biologist has found that 10% of a population of bats are hairless (recessive).
Assuming that the population is in Hardy-Weinberg equilibrium, determine the
genetic structure (genotype and allele frequencies) of the population
 E.g. #4
About 0.040% of the population of the US has cystic fibrosis (recessive disorder).
How can the proportion of people who do not carry even one such allele be
determined?
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SBI 4U
Mechanisms of Evolution
 E.g. #5
Inherited human disease phenylketonuria PKU; 1 every 1000 babies in US is born
with PKU which can result in mental retardation if untreated. The allele for
PKU=r babies homozygous =q2. What are the frequencies of dominant allele and
carriers?
 E.g. #6
In a population of 10 000 cats, you have identified two distinct phenotypes that can
result from two alleles at a single gene locus. One allele (B) produces dark brown
hair and the other (d) produces grey hair. If only 5 cats are grey-haired, estimate the
allele frequencies in the population. Assume the population is in the HardyWeinberg equilibrium.
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SBI 4U
Mechanisms of Evolution
Random Change-Patterns of Selection
 Evolution occurs when genetic equilibrium is disrupted.
Mutation




Produce totally new alleles for a trait
Can change the frequency of the alleles already present in the gene pool.
Review types of mutations (Also see p. 553 – gr. 12 text)
Recall, Polyploidy. This is a mutation whereby multiple sets of chromosomes
arise due to the improper reduction division during meiosis.
 Common in plants – Rare in animals.
o Ex. The Fern Ophioglossum reticulatum, contains 1260 chromosome
pairs (630n)
Migration
 Movement of individual organisms into (immigration) or out (emigrate) of a
population
 The movement of genes into or out of a population through migration is called
GENE FLOW.
Genetic Drift
 The phenomenon by which allele frequencies in a population change as a
result of random events or change.
Effect of Small Population
Effect of large Population
 Results may deviate from the
 Small effect on the change of
expected results
alleles in a population (not
significant
 E.g. flipping a coin 10X as
opposed to 10 000X
 Greater chance of 50% heads and
50% tails with the latter.
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SBI 4U
Mechanisms of Evolution
Bottleneck Effect
 A dramatic reduction in population size usually resulting in significant genetic
drift
 Often temporary
 E.g. northern elephant seal population b/c of overhunting.
Natural Selection
 The single most significant factor disrupting genetic equilibrium.
Stabilizing Selection
 Individuals with the average form of a trait have an advantage in terms of
survival and reproduction.
 Most common type of natural selection
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SBI 4U
Mechanisms of Evolution
 Most effective in a population that has become well adapted to its
environment.
 E.g. - birth weights.
- bill length of hummingbirds
Directional Selection
 Individuals with one of the extreme forms of a trait have an advantage in terms of
survival and reproduction
 E.g.
- anteaters with long tongues could more effectively prey on termites that
built very deep nests
- bill length of hummingbirds
Disruptive Selection
 Individuals with either extreme forms of a trait have an advantage in terms of survival
and reproduction
 E.g.
- light and dark colours of limpets blend with surroundings (intermediate colour
is preyed upon).
- bill length of hummingbirds
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SBI 4U
Mechanisms of Evolution
Sexual Selection
 Sexual dimorphism
o remarkable differences in the physical appearance of males and females
o Behavioural differences b/w sexes
 The preference choice of a mate is based on the presence of a specific trait.
 E.g. in the tropical beetle, females preferentially mate with males having an elongated
snout.
Species
Formation of Species
 A group of individuals that look similar and whose members are capable of
producing fertile offspring in the natural environment
Morphological Concept of vs
Species
Biological Concept of Species
 When differences in appearances are
used to classify an organism
 It has limitations b/c it does not account
for the reproductive compatibility of
morphologically different organisms
 Hybrid
o Offspring of two morphologically
dissimilar organisms
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 When organisms are classified
solely by whether or not they
naturally breed with one
another and produce fertile
offspring.
SBI 4U
Mechanisms of Evolution
Variation of Traits in a Population
Population
 Made up of all the member of the same species that live in a particular
location at the same time
 E.g. fish of a single species that live in a pond.
 In nature many traits in population show variation according to the bell curve
pattern.
 Variations results from mutation, recombination and crossing over.
Geographic Isolation
 The physical separation of members of a population by some barrier such as a
mountain or river.
 The gene pool of each group becomes isolated and the two can no longer
intermix.
 Over a period of time, each group will become more adapted to its particular
environment.
Reproductive Isolation
1. Ecological
 Occurs for example when one population may live on higher, drier mountains
than other, and may not seek out mates from a population living in deep moist
valleys.
2. Seasonal
 Occurs when one population mates in the spring while another mates in the
fall
3. Behavioural
 Related species of organisms such as birds often differs in their mating rituals,
which tends to keep the species distinct even if they occur in the same places.
4. Morphological
 May prevent mating b/w different variants of the same species.
 E.g. the Great Dane and Chihuahua are both members of the same species but
size differences may prevent copulation.
5. Gamete Incompatibility
 May arise to prevent the successful mating of two members of different
populations
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SBI 4U
Mechanisms of Evolution
 E.g. sperm from one population of frog may be unable to fertilize the eggs of
a different population.
Modes of Speciation
Allopatric Speciation
 The evolution of populations into
separate species as a result of
geographic isolation by a physical
barrier.
o a large river
o large bodies of water
o big mountains etc.
Sympatric Speciation
 The evolution of populations into
separate species within the same
geographic area.
 E.g. two species of grey tree frogs
Homework:
pg. 546 # 4, 5
pg. 549 # 1-3a-d
pg. 555 # 1,3,4,5,7,12
pg. 564#1-4,7
pg. 576#1,3,4,6,7
Chapter 12 Review
pg. 582-583#1,4,7,8,10,13,14,15,19,20,25a
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