chapter 17 Flashcards
Describe the difference between
continuous and discontinuous variation.
● Continuous variation - shown by a characteristic that
can have any value over a range e.g. human height
● Discontinuous variation - shown by a characteristic
that can only take certain values e.g eye colour or blood
group
Explain the genetic basis of continuous variation.
Characteristics that show continuous variation
are often controlled by many genes. The
alleles at each gene locus have a small effect,
all of which are added together to control a
characteristic.
Explain the genetic basis of discontinuous variation.
Typically characteristics that show
discontinuous variation are controlled by one
or a few genes. The alleles present at these
gene loci have a large effect on the
characteristic.
State how the phenotype arises.
The phenotype results from the
interactions between the genotype and
the environment.
Why is the t-test used?
The t-test is used to determine whether
there is a statistically significant
difference between the means of two
data sets that show normal distribution.
Why is genetic variation important for selection?
Variation in a population increases the likelihood that
some individuals will have a phenotype which is
better suited to the environment, particularly as
environmental changes occur. These individuals will
have more reproductive success.
Explain why natural selection occurs.
Natural selection occurs because populations have the
capacity to produce many offspring and increase their
numbers exponentially. The individuals of the population must compete for resources; those who are better adapted (due to genetic variation) will survive, reproduce and pass on their alleles to the next generation.
Describe stabilising selection.
In stabilising selection, the extremes of a
characteristic are selected against. This does
eliminate some genetic variation, and so tends
to occur when environmental conditions are
constant for a long time.
Give an example of stabilising selection.
Human birth weights - babies born too far
below or above the optimum weight may not
survive infancy. They therefore cannot
reproduce and pass on their alleles. The
extremes are selected against.
Describe and give an example of directional selection.
When environmental conditions change, individuals
with a characteristic away from the mean will be
better suited to the new environment, and this
extreme is selected for. An example of this is
antibiotic resistance in bacteria.
What is disruptive selection?
The extremes of a characteristic are
favoured and the mean is selected
against. This can result in two
subpopulations with different
phenotypes, and is important for
evolutionary change.
How does selection affect allele
frequency in a population?
Environmental factors exerting the forces of selection
change the allele frequency of a population by increasing advantageous alleles. Selection does not affect the probability of new mutant alleles arising, it affects the frequency of the alleles already in the population.
What is genetic drift?
A change in allele frequency in a small
population due to chance.
Describe the founder effect.
The founder effect is a type of genetic drift. It occurs
when a small subpopulation is isolated from the larger
parent population. This small population has less
genetic variation and may not the same proportion of
alleles as there was in the original population.
What is the bottleneck effect?
Population numbers are significantly reduced in
one generation, which reduces genetic variation
in the next generations. This means the next
generations are less likely to survive
environmental changes.
