According to the Hardy-Weinberg principle, allele
frequencies stay constant from generation to generation
as long as certain conditions are met
Factors such as mutation, non-random mating and
selection can operate in natural populations
These factors may lead to changes in allele frequency
and bring about adaptation and evolution
The principal force in bringing about changes in
the genetic composition of populations is that
of Natural Selection
In 1835, Charles Darwin visited the
Galapagos Islands where he studied
many different species of plants
and animals and the variation
that existed between them
Following his observations of variation within
species, Darwin published his book The Origin of
Species (1859) in which he put forward his views on
the process of evolution by natural selection
Darwin wrote:
“As many more individuals of each
species are born than can possibly
survive; and as, consequently, there is a
frequently recurring struggle for
existence, it follows that any being, if it
vary however slightly in any manner
profitable to itself, will have a better
chance of surviving, and thus be
naturally selected”
Darwin published his work without any knowledge
of genetics or the nature of mutation, and the
re-examination of his theory in the light of modern
knowledge is known as Neo-Darwinism
• Organisms tend to produce a far greater number of
offspring than the the environment can support
• There is, therefore, a struggle for existence and a
high mortality rate as organisms compete for limited
resources
• Members of the same species display variation in all
characteristics; the main sources of variation are
mutation and the behaviour of chromosomes during
the process of meiosis
• Individuals who display variation that is better
suited to their immediate environment will compete
more successfully, have a better chance of survival
and be more likely to reach maturity and breed –
Survival of the fittest
• Organisms whose phenotypes are better suited to
their immediate environment are described as being
at a selective advantage
• Organisms whose phenotypes are less suited to their
immediate environment are described as being at a
selective disadvantage
• Over a period of time, the best suited variants will
predominate in the population and allele frequencies
will have changed
• Since environmental conditions are constantly
changing, then natural selection is forever favouring
the emergence of new forms which may culminate in
the origin of a new species
The brown rat is classed as a pest as it
forages for food intended for human
consumption and spreads disease
Rat populations have been
controlled by a rodenticide called
warfarin since the 1950’s
Warfarin is an anticoagulant which
acts by interfering with the way in
which Vitamin K is used in the
process of blood clotting
When food bait containing warfarin
is eaten by rats, their blood fails to
clot and they suffer from fatal
haemorrhages
Since the introduction of warfarin in the 1950’s,
populations of warfarin-resistant rats have been
identified in various parts of Britain
Large populations of rats compete for the available food
A random, spontaneous
mutation occurs within
the rat population
conferring resistance to
warfarin on one of
the members
Warfarin-resistant rat
The warfarin-resistant
rat is at a selective
advantage in areas
where warfarin
is used for pest control
The resistant rat is more likely to reach maturity
and breed and pass the resistance allele
onto some of its offspring
In this example, natural selection is exerting its
effect on a single major gene with two alleles
Some of the rats in
the first generation are
warfarin-resistant and
are more likely to breed
than their susceptible
brothers and sisters
several
generations
After many
generations, a high
proportion of the rat
population are
resistant to warfarin
as the mutant
allele spreads
throughout the
population
Natural selection may affect allele frequencies
within populations in several different ways
Three types of selection that operate
within populations are:
Stabilising Selection
Directional Selection
Disruptive Selection
For continuously varying characteristics, these
selection methods modify their frequency
distributions in different ways
Stabilising selection acts
against the extremes
within a range of
phenotypic variation
mode
unchanged
This type of selection
leads to a reduction in
the range of variation
within the population
without any change
in the mode
Stabilising selection
operates in an unchanging
environment to maintain
the best adapted genotypes
within the population
Stabilising selection occurs in the selection of
birth mass in humans
Infant mortality is greatest for babies of very high
or very low birth masses
Selection
pressure
against these
extreme
phenotypes
new
mode
Directional selection
operates in changing
environments and acts
for or against extremes
of phenotype within the
variable population; it
is the main type of
selection practised by
man when selecting
domesticated plants
and animals for the
improvement of stocks
This type of selection leads to
a reduction in the range of
variation within the
population, together with a
progressive shift in
the mode
The evolution of the long neck of the giraffe is thought to
have arisen by Directional Selection
Ancestral giraffes would have
displayed variation in neck
length with random, spontaneous
mutation giving rise to giraffes
with slightly longer necks than
the average of the population
During periods when food was
scarce, longer-necked giraffes
would survive as they would be
able to reach the available food
Natural, directional selection would favour the longnecked giraffes; these variants would survive, breed and
pass on their genes to the next generation
Longer-necked giraffes were at a selective advantage
during times of food shortage
Selection pressure
operating against
shorter-necked
giraffes when food
was scarce
Directional selection has
led to a reduction in the
range of neck length
within the population
together with a
progressive increase in
the mode
In time, longernecked giraffes
became more
common within
the population
Spontaneous
mutation in
growing
population
Bacterial
population
Bacterial population
exposed to antibiotic
Mutant bacterial cell is
resistant to a specific antibiotic
The resistant bacterial cell
survives and divides to
generate a population of
antibiotic-resistant
bacteria
The peppered moth (Biston betularia) exists in
two forms or morphs in Britain
This polymorphism involves a single gene locus with two
alleles; the recessive allele (c) determines the 'typical'
phenotype of light colour speckled with black; recessive
homozygotes display this phenotype
The mutant allele is dominant (C) and determines the
'carbonaria' form of the moth; the carbonaria form is a
dark-coloured melanic form
During the daytime, moths rest on tree
trunks and are preyed upon by birds
such as the blue tit
In unpolluted areas, trees are covered with
lichens and mosses and the light peppered
morph of the moth is well-camouflaged
against this background
The allele determining the carbonaria form of the moth
arose by spontaneous mutation and, prior to the second part
of the 19th century, the melanic moth was a rare specimen
In the largely unpolluted cities of Britain, the melanic morph
was probably at a selective disadvantage with
high mortality rates due to predation by birds
The less conspicuous peppered morph enjoyed a selective
advantage on the lichen covered trees and
fewer numbers were eaten by birds
By 1895, over 95% of moths sampled in the Manchester area
were of the melanic type
This marked increase in the frequency of
melanic moths was shown to be associated
with the Industrial Revolution when
large numbers of factories released
soot into the atmosphere
Sulphur dioxide from the atmospheric pollution killed the
mosses and lichens on the trees which then became
blackened by the soot particles in the smoke
Within these polluted areas, the melanic morph was less
conspicuous to predators and the light, peppered morph
proved to be at a selective disadvantage
Melanic moths suffered less predation by birds and the allele
for dark colour increased in frequency
The relationship between atmospheric pollution and the
increase in the frequency of melanic moths is called
Industrial Melanism
This gradual replacement of the ‘melanic’ allele for the ‘pale’
allele in areas of high pollution is an example
of transient polymorphism
The peppered morph is at a selective
advantage in unpolluted areas
The melanic morph is at a selective
advantage in polluted areas