POPULATION GENETICS
In-build mechanisms for
adaptation
(or evolutionary mechanisms)
Biology Stage 3
Text: Chapter 23
Human Biology Stage 3
Text: Chapter 15
Key ideas
Population genetics
Observable facts
 Natural selection
 Mutations
 Biological fitness
 Genetic variation
 Genetic drift
 Speciation
 Bacterial resistance
We will be looking at
population genetics.
These concepts are
not related to
evolution, although in
your textbook they are
automatically linked.
The processes
involved in population
genetics are directly
observable and can
be studied
scientifically.
The theory of evolution
Remember …

The theory of evolution proposes that life
arose from non-life and that this process has
taken billions of years. It is closely linked to the
Big Bang theory, being a continuum in the
spectrum of the ‘evolving’ universe. This
theory is based on an atheistic worldview.
Definitions
Species
A group of individuals who are able to interbreed,
producing fertile offspring
Population
A group of organisms of the same species living
together in a particular place at a particular time
Gene pool
The sum of all alleles in a given population
Allele frequencies
How often a particular allele occurs in a
population
Natural selection
Definition
 Natural selection
occurs when
environmental
conditions favour one
allele at the expense
of other alleles
 The result is that the
process of natural
selection enhances
the chances of
survival in particular
environments
Despite this definition,
natural selection is
mostly not a strong
enough process to favour
single genes. Rather,
environmental conditions
act to favour multiple
characteristics that
enhance survival.
Example
 The difference between
the Inuit and Masai (pg.
251)

Population size and allele frequency
In large populations:
Individuals make only a small contribution to the gene
pool.
 Allele frequencies are relatively stable and change slowly
over time
 These patterns depend on:
◦ Dominance
◦ Co-dominance
◦ Monogenic inheritance
◦ Polygenic inheritance

In small populations:
Individuals make a relatively large contribution to the gene
pool.
 Change in allele frequency can occur rapidly.
 Many changes are due to chance events.

Population bottlenecks
A bottleneck is caused
by events that create
a sudden drop in
population size (e.g.
wars, natural
disasters or migration)
or prevent individuals
from reproducing.
 This reduces mating
possibilities and can
cause a genetic
bottleneck.

Example
 Noah’s flood
In-breeding
Reduced mating
opportunities often
lead to an increase in
in-breeding or
consanguinity.
 In-breeding results in
reduced genetic
diversity in the
population’s gene pool.

The Pharaoh and Inca nobility
practised consanguinity to
preserve the 'purity' of their
gene line.
Consanguinity
the Old Order Amish community of Lancaster
County in Pennsylvania, USA is a fundamentalist
religious sect who do not marry out or use modern
technology.
 The Amish community numbers around 18 000
people and shares only eight family names.
 Within the community there is an above average
incidence of inherited illnesses including a rare
form of microcephally (small brain), albinism,
dwarfism, cretinism, webbed fingers and limb
girdle muscular dystrophy.

Genetic drift
Genetic drift is the random fluctuation of allele
frequencies in a population from one
generation to the next.
Example
 The frequency of a particular trait could, for no
obvious reason, drift from 2% in generation A,
to 11% in generation B, to 5% in generation C



Genetic drift is often a consequence of a
genetic bottleneck
It results from in-breeding brought about by the
limited mating possibilities in a small
community.
The effects of genetic drift


The effects of genetic drift
can be amplified by
differences in the number
of children raised by
couples or individuals dying
prematurely.
Genetic drift can result in:
traits being lost from
small populations.
unusual traits, not
commonly found in the
parent population, and
that are often nonadaptive, becoming
established.
Genetic drift
The founder effect



Founder effect is an example of a genetic bottleneck.
Founder effect occurs when a small number of people
migrate and settle in a new area.
The founding population carry only a small fraction of
the original population's genetic variation. As a result,
they may differ both genetically and in appearance,
compared with the parent population.
The founder effect
Pitcairn Island, in
the Pacific, was
colonised by the
mutineers from
HMS Bounty in
1790.
Today the
population of the
island is around
44 people.
Gene flow
Migration is describes as gene flow from one
population to another
 This brings in new alleles into the population
 The allele frequencies for that gene will be
altered over time
Example
 Chinese only had Rh+ allele until Europeans
came and brought in Rh- allele

Barriers to gene flow
Populations are
often kept apart by
physical or social
barriers
 These may result
in populations
becoming less
alike over time.
 Isolation due to
barriers results in
separate gene
pools

Barriers include:
 Geographical
barriers
 Sociocultural
barriers
Heterozygous advantage



Genetic diseases
result in changes to
the allele frequency in
a gene pool.
Some alleles that
cause fatal diseases
can persist in the
population because, in
some cases, it can be
advantages to be the
carrier of a fatal allele
This is called
heterozygous
advantage.
Examples
 Sickle cell anaemia
 Tay-Sachs disease
Sickle-cell anaemia
Occurrence
 African continent
Advantage
 Individuals have some immunity to malaria
Tay-Sachs disease
Tay-Sachs is
lipid metabolism
disorder caused
by a missing
enzyme.
 This results in
the accumulation
of a fatty
substance in the
nervous system
 Death usually
occurs at age 4
or 5

Occurrence
 Individuals of Jewish descent
from Eastern Europe (Ashkenazi
Jews)
Advantage
 Individuals have some immunity
to tuberculosis (TB)
Speciation
The development of
different species
over time
Steps to speciation:
Variation
 A single population
exists with a
common gene pool
Isolation
 A barrier forms and
divides the
population into 2
Selection
 Different selection
pressures exist for
these 2 populations
Speciation
 Over time, the allele
frequencies in each
of the populations
change
 When the two
populations no
longer interbreed, a
new species is said
to have ‘evolved’
Mutations
Mutations
 A change in the DNA sequence.
 Can result from DNA copying mistakes,
exposure to radiation, exposure to
chemicals or infection by viruses.
 Mostly have deleterious effects
Gene mutations
 Changes in the DNA of a single gene
◦ Example: Phenylketonuria
Chromosome mutations
 All or part of the chromosome is affected
◦ Example: Down’s syndrome
Mutations
Somatic mutations
 A mutation in the body cells
Germinal mutations
 A mutations in the gametes
Three types of mutations
National Human Genome Research Institute - NIH
Evolutionary mechanisms
Alfred Wallace
Charles Darwin
Charles Darwin and Alfred Wallace proposed that
natural selection and mutations are the two main
forces that drive evolutionary mechanisms. The
theory requires that new genetic material is created
over time by random natural forces.
In-built mechanisms for
adaptation

Creation scientists
propose that God
created living
organisms with an
immensely complex
yet flexible genetic
code that enables
them to adapt and
change to survive in
different
environments.

This theory proposes
that existing DNA is
modified according
to natural selection,
genetic drift and
population
bottlenecks, rather
than new genetic
material being
created.