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Biology 11
Name: __________________
Unit 2: Evolution
M. Harvey 2014
Date: ____________
Modern Theories of Evolution notes
Chromosome, gene, allele
o Populations evolve, not individuals

Individual organisms do not evolve during their lifetime

Natural selection acts on individuals, but population evolve
CASE STUDY: Sickle Cell Anemia
o Sickle cell anemia is a genetic disease with severe symptoms,
including pain and anemia.
o Caused by a mutated version of the gene that helps make hemoglobin
•
A protein that carries oxygen in red blood cells.
o People with 2 copies of the sickle cell gene have the disease.
o People who carry only 1 copy of the sickle cell gene do not have the disease

May pass the gene on to their children.
 What happens at the DNA level?
 What happens at the cellular level?
o RBC with mutant hemoglobin can’t carry oxygen, and are “sickle-shaped”.
o This shape can sometimes clog blood vessels and interrupt blood flow.
Biology 11
Name: __________________
Unit 2: Evolution
M. Harvey 2014
Date: ____________
 What happens at the organism level?
o Carrier: at high elevation & when doing and intense exercise, may
occasionally show symptoms such as pain and fatigue.
o Person with sickle cell anaemia: in constant pain and always tired, rapid heart
rate, pale, have ‘crises’ – painful episodes.
 What does this have to do with evolution?
o Carriers of the sickle cell anemia are resistant to malaria, because the
parasites that cause this disease are killed inside sickle-shaped blood cells.
Modern Theories of Evolution
o A population

Is a localized group of individuals of the same species that are
capable of interbreeding and producing fertile offspring
o Gene pool

Is the total group of genes in a population at any one time

Consists of all genes in all individuals of the population
Biology 11
Name: __________________
Unit 2: Evolution
M. Harvey 2014
Date: ____________
o The Hardy-Weinberg Theorem

Punnet squares

This concept can be used to calculate the frequency of different
genotypes and to measure how fast allele frequencies are changing
within a population (how fast the population is evolving).

The principle also demonstrates that it is populations that evolve, not
individuals.

The Theorem is hypothetical

In real populations allele and genotype frequencies do change over
time
o The five conditions for non-evolving populations (rarely met in nature):

Extremely large population size

No gene flow (immigration/emigration)

No mutations

Completely Random mating

No natural selection
o Three major factors that can change allele frequencies and bring about most
evolutionary change

Natural selection

Genetic drift

Gene flow
o Natural selection

Some alleles get passed to the next generation in
greater numbers than others due to natural selection
o
Genetic Drift


Statistically, the smaller the population size

The less genes in the gene pool

The greater chance for change to have a big affect
Describes how allele frequencies can fluctuate unpredictably from one
generation to the next

Tends to reduce genetic variation
Biology 11
Name: __________________
Unit 2: Evolution
M. Harvey 2014
Date: ____________
o The bottleneck effect

A sudden change in the environment may drastically reduce the size of
a population

The gene pool may no longer be reflective of the original population’s
gene pool

Example:
Shaking just a few marbles through the narrow
neck of a bottle is analogous to a drastic
reduction in the size of a population after
some environmental disaster
By chance, blue marbles are
over-represented in the new
population and gold marbles
are absent.

Understanding the bottleneck effect can increase understanding of how
human activity affects other species

Organisms hunted until near extinction
o Gene Flow

Causes a population to gain or lose alleles

Results from the movement of fertile individuals or gametes through
immigration or emigration
o Speciation

Usually cause by part of a population becoming isolated in some way
that stops gene flow between two groups

Reproductive isolation

When part of a population becomes reproductively incompatible
with the other.

Can be caused by changes in behavior (nocturnal vs. not),
breeding season (fall vs. spring), or biology (gametes don’t
match up = miscarriage).

Geographic isolation

When part of a population becomes separated geographically
and the two groups do not interbreed.
Biology 11
Name: __________________
Unit 2: Evolution
M. Harvey 2014
Date: ____________

Mountain ranges, water bodies, etc.

Changing allele frequencies, continued mutation, and adaptation to
different habitat eventually leads to the two sub-populations
becoming separate species.
Theories of Evolution
o
Gradualism

Darwin’s theory of evolution is called gradualism

The idea that organisms are always evolving gradually over long
periods of time.
o
Punctuated Equilibrium

‘gaps’ in the fossil record led Niles Eldredge and Stephen Gould (1972) to
develop the Theory of Punctuated Equilibrium.

organisms do not evolve unless they have to because the environment
changes

During most of the Earth’s history the environment is stable and little
evolutionary change occurs.

Occasionally the environmental stability is
interrupted (punctuated) by major and
rapid change.
 Global warming
 New disease
 Ice age
 Geological shift
 Large Meteor
•
In any population there are relatively ‘unfit’ variants that manage to survive
on the ‘fringes’ in low numbers leaving little fossil evidence.
•
If the environment suddenly changes in a negative way for the ‘fit’ varieties,
their numbers drop and they can even become extinct.
•
If the environment change favors the variants, their population explodes and
they suddenly appear in the fossil record.
•
Major evolutionary changes correspond to major changes on Earth, and
entirely new species can replace former species.