Contemporary stories of evolution! (a) antibiotic-resistant bacteria and (b)
a moth story
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Sub-index of biology notes on all aspects of EVOLUTION
Contemporary stories of evolution! (a) antibiotic-resistant bacteria and (b)
(10a) Antibiotic-resistant bacteria - a contemporary and worrying story of
and other examples of
evolution in practice including moths!
Example 1. Antibiotic resistant
The rapid growth and genetic development of
bacteria provide strong evidence for evolution.
This is a relatively new area of research and is
giving scientists for information as to how evolution works at the
Introduction - what are antibiotics?
An antibiotic is a type of antimicrobial
substance active against bacteria and is the most important type
of antibacterial agent for fighting bacterial infections.
Antibiotic medications are widely used in the
treatment and prevention of such infections.
They may either kill or inhibit the growth of
Antibiotics like penicillin (discovered in
1928) have proved an amazing medical development for treating infectious
(sometimes fatal) bacterial diseases.
The action of antibiotics in killing certain
bacteria has saved millions of lives over 90 years.
However, they have their limitations and a
particular antibiotic cannot provide a permanent long term
solution to particular infections - the reasons why are
discussed in detail further down.
How and why can bacteria become
The tale of
antimicrobial resistance by
Random mutations can occur in the DNA of any
living organism, and bacteria are no exception.
These mutations can lead to a change in
the bacteria's alleles, hence its characteristics (phenotypes), and, unfortunately, from our point of view,
can become less susceptible to the destroying action of an
This is evolution in action!
Strains of bacteria can develop with
genes-alleles that give the bacteria protection against a
The presence of these genetic variants
in the genome of the organism leads to the formation of
The strains of bacteria that offer
antibiotic resistance have an evolutionary advantage,
so are more likely to survive and multiply
bacterial population (in the environment of the antibiotic),
at the expense of less antibiotic resistant bacteria,
that will be killed/inhibited by the antibiotic,
so the gene (allele)
for antibiotic resistance becomes more common in the population.
This is illustrated in the diagram below.
The ability of a bacterial strain to
resist the effects of an administered antibiotic gives it an
genetic advantage and these new strains are better able to survive by
the process 'natural
selection' in the host (your body!).
The situation is made worse because bacteria
multiply rapidly, and this speed of reproduction allows the evolution of antibiotic-resistant strains of bacteria to take
place more quickly.
This increases the relative population of
antibiotic-resistant strains of bacteria.
cell division of
bacteria by binary fission.
This another case of 'natural selection',
the antibiotic-resistant bacteria become more common in the
i.e. a case of 'survival of the fittest',
because the development of antibiotic resistant genes/alleles in
the organism, means that these evolved strains of bacteria are
better able to survive in the environment of antibiotics!
This evolution is relatively rapid e.g.
taking place over a few years (or less?) rather than thousands
or millions of years - worrying!
Another contemporary example is the
development weeds resistant to a herbicide that once killed them -
the scientific explanation is exactly the same as for the
development antibiotic-resistant bacteria!
These days research scientist can monitor
the changes in the DNA of generations of bacteria and actually
genetically plot their evolution into different strains.
What is the problem for us?
Its quite simple! If we become infected with a
strain of bacteria that is resistant to the antibiotic treatment,
the treatment is ineffective!
The person is not immune to the
invasive bacteria AND the infection can spread more easily in
Drug companies are able to develop new
antibiotics BUT, unfortunately, they take a lot of time and
money to develop AND 'superbugs' are evolving which
are resistant to most common antibiotics - overuse is contributing
to the problem.
MRSA is one of the most common
superbugs that is difficult to treat and get rid of.
MRSA is often contracted by people in
hospitals and can be fatal if it gets into the bloodstream.
Superbugs have to be treated with
'cocktail' of the strongest acting antibiotics.
Why is antibiotic resistance by bacteria on the
Overuse and inappropriate use of
antibiotics has led to a great increase in the 'evolution' of
antibiotic-resistant strains of bacterial infections.
The more antibiotics are used, the greater
the chance of antibiotic-resistant strains of bacteria evolving,
the greater the problem becomes.
For many years antibiotics have been very
successful in treating bacterial infections (NOT viruses).
The death rate from infectious bacterial
diseases like pneumonia has fallen quite dramatically.
However, there are several reasons why
antibiotic resistance is on the increase:
The overuse of antibiotics
The more antibiotics are prescribed the
bigger the problem of antibiotic resistance - you are giving
more scope for different strains of rapidly reproducing bacteria
The use of antibiotics doesn't cause
resistant strains to develop, its just that by extensively using
them, you unfortunately create a situation where there is an
increased probability of an antibiotic-resistant strain
developing, and these strains have a genetic advantage over
the bacteria you are trying to treat.
The inappropriate prescribing of
Antibiotics are often prescribed by
doctors for viral conditions that antibiotics cannot treat
- perhaps under pressure from patients feeling unwell and
demanding a treatment? BUT, antibiotics are completely
ineffective against viruses e.g. flue or common cold viruses.
(Note: Antiviral drugs are
particularly costly to research and develop.)
Fully complete your prescription
Despite point (i), for a genuine bacterial
infection, it is really important you complete the full
course of your prescribed antibiotics.
This ensures all the bacteria are
destroyed, not only curing you of the infection, BUT preventing the bacteria form mutating into another
Use of antibiotics in
Farmers treat animals with antibiotics to
protect them against potential bacterial infections - obviously
the prevention of illness will increase the yield of milk or
meat from the herd.
Again, unfortunately, this mass treatment
of farm animals will lead to the development of
antibiotic-resistant strains of bacteria.
These could be passed on to humans in meat
and milk based products.
The overuse of antibiotics in farming has
caused sufficient concern for some countries to restrict the use
of antibiotics with farm animals.
Can we keep up with bacterial
Drug companies are always looking for a
new market and, encouraged by both government and the medical
profession, to develop new antibiotics that can combat these new
deadly strains of antibiotic-resistant bacteria.
However, it takes time and a lot of money
to develop new antibiotic products.
Because this process is so slow, it is
difficult to produce new antibiotics in time to keep up with the
rate of evolution of new strains of antibiotic-resistant
mutant bacteria - the stuff you are trying to treat!!!
I'm afraid this is a problem that is not
going away, however clever pharmaceutical research chemists are!
Culturing microorganisms like bacteria - testing
antibiotics and antiseptics gcse
biology revision notes
Keeping healthy: defence against
pathogens, infectious diseases, vaccination, drugs, monoclonal antibodies
(10b) Example 2 of evolution in
practice - a famous moth and its colours!
A tale of two
The white peppered moth with black spots (of melanin pigment)
The white peppered moth was, and is, common throughout
the UK, but not all of the time!
It is a relatively light coloured insect,
which, due to its wing markings, is relatively well camouflaged
against predatory birds, particularly in rural areas that are not
polluted from industry.
Its colour blended well light coloured
tree bark and especially lichen covered surfaces, and was less
susceptible to predation by birds.
In pre-industrialised Britain, no dark
coloured varities were reported until ~1848, when
moths were observed in grimy polluted areas such as the large and
highly industrialised city of Manchester.
The black peppered moth, in increasing
numbers, were found resting on any dark surface, whatever the
location and the numbers of lighter coloured peppered moths
decreased - because they stood out more against the 'dark'
background and were likely to caught by predatory birds - so natural
selection comes into play!
Due to this predation, the decrease in
the less well camouflaged light coloured peppered moth meant the
gene pool on average was decreasing in the alleles causing the
phenotype of the lighter colour.
The black peppered moth - with lots of the pigment melanin
The dark 'melanic' variety of this moth was
rarely observed in rural areas well away from industrialised areas.
It was also noted that the lighter coloured
white peppered moth was rarely observed in the darker landscape of
Victorian industrial towns - not well camouflaged.
In fact, the black peppered moth became
the dominant form in industrial Britain from the mid-19th
century to the 1950s.
The white peppered moths stood out more
against the 'darker' background of the industrial landscape were
much easier prey to catch by predators - mainly birds, less so
for the black variety of moth.
It has be shown by scientific research that
the dark coloured peppered moth is a mutant with the advantage of
blending into the industrial background and less easily seen by
The black peppered moth is much better
camouflaged than the white peppered moth in these areas.
Remember - the landscape did NOT change
the moth colour, the genes for 'white' and 'black' are already
in the gene pool - a single gene mutation is responsible for the
wings moths becoming black (melanic).
What environmental change in the industrial
revolution produced this natural selection situation?
In the late 18th, 19th and early 20th
centuries, many surfaces in towns and cities became dirty from
industrial smoke and dust, particularly soot from chimneys.
Also, air pollutants kill lichen on trees
and stone walls making them darker, so black peppered moths are
better camouflaged when resting on these surfaces as well as
industrial buildings and blackened houses.
Lichen are poisoned by air pollutants such
as sulfur dioxide, soot and heavy metal compounds.
Natural selection enables the less
vulnerable black peppered moth to survive in polluted industrial
areas much better than the white peppered moth.
Later in the 20th century, with the decrease
in 'dirty' industrial activity and lower air pollution levels, the
proportion of white peppered moths has increased considerably over
the past few decades.
Less polluted dark surfaces, less black
This tale of 'industrial melanism' has
also been observed other species like ladybirds.
This tale of the two moth colours is
considered a classic case of natural selection.
Survey statistics showed the emergence and
rise dominance of the black peppered moth (over the white
peppered moth) and its decline coinciding with the decline of
heavy polluting industry in the decades after the Second World
War (from the 1950s onwards).
It is now known from molecular genetics
that a single gene mutation is responsible for the wings moths
becoming black (melanic).
So, its actually the
tale of one mutation and two different alleles/genes!
Summary of learning objectives and key words or phrases
Be able to describe and explain the growth of antibiotic
resistant strains of bacteria because of the overuse of antibiotics.
Know that antimicrobial resistance happens when pathogens
like bacteria and fungi evolve and develop the ability to be unaffected by
drugs designed to kill them, so the pathogen organisms are not killed and
their population continue to grow.
Know that resistant pathogen infections can be very
difficult, and sometimes bacterial are impossible to treat, resulting in
death in extreme cases.
Know the story of the black peppered moth and white peppered
in terms of mutations favouring one environment (e.g. industrial - black)
and another (rural - white) and discuss this in terms of evolution of a
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