Download 2.2 selection

Selection
• Variation occurs in all populations due to the
environment and genetics.
• Genetic variation results in characteristics
being passed from one generation to the next.
Meiosis:
crossing over & independent assortment of
chromosomes
Fertilisation:
Random fusion of ova and sperm
Mutation:
Spontaneous change in DNA code
• Variations allow organisms to be adapted to
their environment.
• How well an organism is adapted to its
environment is known as fitness. The best
adapted (fittest) organisms will be able to
compete best for resources (food, nest sites
etc.) and escape predators better than those
less well adapted.
• This is known as survival of the fittest and
explains the term Natural selection, in which
genetically inheritable features in a population
are edited to increase or decrease the
frequency of specific characteristics.
Variations in a population are due to different alleles for
characteristics which result from mutations.
Organisms with the best adaptations are more likely to
compete successfully or escape predators and
therefore live to reproduce. They pass on their
favourable alleles to the next generation.
Less adapted organisms will be less successful at
competing and die before reproducing, so their
alleles and characteristics are not passed on .
Over generations there is a change in the allele
frequency in a population.
This differential reproductive success maintains the
fitness of the population.
• Natural selection does not create
useful adaptations but rather edits
genetically inheritable features in a
population, increasing the
frequency of some while decreasing
the frequency of others over time
• There are two main types of selection
• Stabilising selection maintains the
constancy of features in a non-changing
environment
• Directional selection brings about a
change in frequency of a feature in a
changing environment, and accounts for
the diversity of organism
Stabilising (or Normalising)
Selection
•This occurs when the environment doesn't
change.
•Natural selection doesn't have to cause
change, and if an environment doesn't
change there is no pressure for a welladapted species to change.
•Fossils suggest that many species remain
unchanged for long periods of geological
time.
The Coelocanth
• This fish species was known only from ancient fossils
and was assumed to have been extinct for 70 million
years until a living specimen was found in a trawler net
off South Africa in 1938.
• So this species has not changed in all that time.
• Another example of stabilising selection can be seen
in the birth weight of humans.
• The heaviest and lightest babies have the highest
mortality and are less likely to survive to reproduce
and pass on their alleles.
Human Birth Weight
Birth weight in humans
is an example of
stabilising selection.
In stabilising selection the mean remains the
same but the range becomes smaller as the
extremes are removed.
Mean remains the same
Extremes selected against
range smaller
Directional Selection
•This occurs whenever the environment changes
in a particular way.
•The average organisms are no longer best
adapted to the environmental conditions, but the
best adapted individuals lie closer to one of the
extremes of the variation.
•There is selective pressure on the organisms.
•Pesticide resistance in insects is an example
of directional selection
• In insect populations there are mutations that confer
resistance to pesticides. These mutations make the
insects less fit than non-resistant insects in a pesticide
free environment and therefore their numbers are low.
• When pesticides are applied they act as an
environmental pressure, the resistant insects are
better adapted, survive and produce offspring as the
non-resistant insects without the resistance allele are
eliminated.
• Over time with continued pesticide application the
population can be replaced by resistant insects.
• In a population one extreme is selected against and
becomes less prevalent due to the change in the
environment.
• Bacterial resistance to antibiotics occurs in the
same way.
• In continuous features with a normally
distributed population this results in a change in
the mean as it shift to the right or left,
depending upon which extreme has been
selected against.
• In discontinuous features it causes one form to
be favoured over another.
Mean moves to the right
One extremes selected for
The peppered moth
• There are two forms of this moth.
• Light coloured moths are well camouflaged from bird
predators against the pale bark of birch trees
• dark moths are easily seen and eaten
• During the industrial revolution in the 19th century,
woods near industrial centres became black with
pollution.
• The black moths had a selective advantage,
camouflaged they were able to reproduce and pass on
their genes to their offspring, becoming the most
common colour
• Pale moths were easily predated and unable to pass
on their genes their numbers were reduced
Bacterial resistance to
antibiotics
• Antibiotics kill bacteria. Bacteria reproduce very
rapidly and have a naturally high rate of genetic
mutations. Occasionally a chance mutation
appears that makes the bacterium resistant to
that antibiotic.
• In an environment where the antibiotic is often
present, this mutant has an enormous selective
advantage since all the normal bacteria are
killed!
• The mutant cell is then free to reproduce and
colonise the whole environment without any
competition.
MRSA
methycillin resistant
staphylococcus aureus
KEY POINTS FROM PAGE 181