Transcript f215 variation and population genetics student version

F215 Variation and Population
Genetics
By Ms Cullen
Examples of Variation
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Sex
Blood group
Shoe size
Tongue rolling
Height
Plant mass
Finger length
Continuous Variation
Discontinuous Variation
Polygenic Inheritance
• Not all variation caused by genes is discontinuous.
• Sometimes variation can be caused by many genes at different loci,
or many different alleles of the same gene.
• This means there are many different possibilities, so the variation is
continuous.
• When characteristics are influenced by the combined effect of
many genes it is known as polygenic inheritance.
• Polygenic characteristics tend to show continuous variation.
• Inherited characteristics that show continuous variation are usually
influenced by many genes. For example, human skin colour comes
in lots of different shades of colour.
• Inherited characteristics that show discontinuous variation tend to
be influenced by only one gene. For example, violet flower colour
(either coloured or white) is controlled by only one gene. We call
this monogenic inheritance.
Environment
• Variation is also affected by the environment.
• Examples can be climate, food, lifestyle etc
• Environmental factors can change over an organisms
lifetime and therefore so can their characteristics.
• Can you think of any examples of environmental
variation?
Both?
• Some variation is caused as a result of both genes
and environment.
• Can you think of any examples?
VP = VG + VE
Both genotype and environment can
contribute to phenotypic variation.
Gene pools and populations
• A population is the number of individuals of the same
species within a particular area.
• A species is a group of similar individuals which can
breed with each other and produce fertile offspring.
• The gene pool for that population will be the complete
range of alleles available within that population.
• New alleles can be created as a result of mutations.
• The allele frequency is how often an allele occurs within
a population (usually expressed as a percentage).
• The frequency of an allele will change over time this is
evolution.
The Hardy-Weinberg Principle
• This predicts the frequency of alleles in a population
won’t change from generation to generation.
• But this will only occur in certain conditions; a large
population, with no immigration, emigration,
mutations or natural selection.
• If allele frequencies do change then it will be as a
result of one or more of the above.
The Hardy-Weinberg Principle – allele
frequency
p+q=1
• p = the frequency of the dominant allele.
• q = the frequency of the recessive allele.
• The total frequency for all possible alleles within a
population is 1.0. So the frequency of dominant and
recessive alleles must add up to 1.0.
• Q: In a population of plants the allele R (red) is
dominant and has a frequency of 0.4. Allele r (white)
is recessive. What is its frequency?
The Hardy-Weinberg Principle –
genotype frequency
p2 + 2pq + q2 = 1
• p2 = the frequency of the homozygous dominant
genotype.
• 2pq = the frequency of the heterozygous dominant
genotype.
• q2 = the frequency of the homozygous recessive
genotype.
• The total frequency for all possible genotypes within a
population is 1.0. So the frequency of all the genotypes
must add up to 1.0.
The Hardy-Weinberg Principle –
genotype frequency
Example:
• If there are two alleles for flower colour (R and r)
then there are three possible genotypes. RR, Rr and
rr.
• Q: If the frequency of RR (p2) is 0.34 and the
frequency of Rr is 0.27. What is the frequency of rr
(q2)?
Remember Evolution occurs via
Natural selection:
• Variation with individuals.
• Selection Pressures eg predation, competition, disease.
• Best adapted individuals survive, breed and pass on
beneficial alleles.
• This results in a greater proportion of the next
generation having the beneficial allele.
• They in turn survive, breed and pass on the beneficial
allele.
• The frequency of the beneficial allele within the gene
pool will increase.
• This process is called natural selection.
Stabilising Selection
• Most of the time organisms
are well adapted to their
surroundings.
• The alleles present in the
species gene pool are
advantageous for survival.
• If the environment remains
stable the same alleles will
be selected in successive
generations.
• As a result, nothing
changes.
http://wps.prenhall.com/esm_freeman_biosci_1/0,6452,499573-,00.html
Directional Selection
• This occurs when there is a
change in the environment.
• This can cause a change in the
selection pressures on a
population.
• A variation that may not have
been previously advantageous,
may become so.
• Or a new variation may arise
due to a mutation.
• These will result in directional
(or evolutionary) selection.
Genetic Drift
• Evolution is not always a direct result of natural selection,
sometimes it happens purely by chance!
• It usually happens in smaller populations, with few selection
pressures. Particularly in island populations.
• If one or two individuals have a better success at breeding
then their alleles will become more popular within the
population.
• While other alleles may be lost if some individuals do not
have offspring.
• This causes a change in the gene pool and the population’s
characteristics. This has occurred by chance rather than
natural selection and is known as Genetic Drift.
• Genetic drift is also common when a population becomes
suddenly smaller, for example after a natural disaster.
Example of Genetic Drift
• Native American tribes show different blood group
frequencies.
• Blackfoot Indians are mostly blood group A.
• Navajos are mainly blood group O.
• Blood group does not affect survival or reproduction,
therefore it is not evolution by natural selection.
• In the past human populations were much smaller and
often isolated
• The blood group differences are due to genetic drift.
• By chance the allele for blood group A was passed on more
frequently by Blackfoot Indians. Over time the allele and
blood group A became more common.
Speciation
• This is the formation of a new species.
• It usually occurs as a result of either:
• individuals becoming physiologically or
morphologically different to members of the original
species.
• individuals no longer being able to breed and
produce fertile offspring with members of the
original species.
• When a new group splits from the original species it
is known as isolation.
Isolation
• Geographical isolation – species can be
separated by a physical barrier, a river,
mountain etc.
• When the two groups become so different
that they can no longer breed and produce
fertile offspring it is called reproductive
isolation.
• They have now become different species and
there is no gene flow between them.
Geographical Isolation
White-tailed antelope squirrel
Harris’ antelope squirrel
Reproductive Isolation can be caused
by one or more of these:
• Temporal (seasonal) isolation – different
breeding seasons.
• Gamete isolation – sex cells from different
species are incompatible.
• Behavioural isolation – different courtship rituals
are not attractive to other species.
• Mechanical isolation – structural differences in
the anatomy of reproductive organs prevents
sperm transfer between individuals of different
species.
The Species Concept
• The classic definition of a species can cause problems
if you can’t see an organism reproduce!
• For example a species may reproduce asexually, or be
extinct, or just might not be practical to watch them
reproduce in the wild.
• Scientists sometimes use the phylogenetic species
concept to help classify organisms.
• Phylogenetics is the study of evolutionary history of
groups of organisms (it is also known as the cladistic
or evolutionary species concept)
• All species have evolved from common ancestors.
A cladogram showing the evolutionary relationship between
seven insect groups
Artificial and Natural Selection
Task:
• Explain how artificial selection is used to
produce varieties of bread wheat (triticum
aestivum).
• Explain how artificial selection has been used
to produce the modern dairy cattle.
• Compare the similarities and differences
between natural selection and artificial
selection.