Detailed, colourful, nicely displayed revision notes with images on AQA A-Level Biology on section 6, topic 14, Populations and evolution (Genetics, populations, evolution, and ecosystems ). According to the AQA A-Level specification, these notes are on section 3.7.2 and 3.7.3. My notes are collec...
A population is a group of organisms of the same species occupying a particular space at a
particular time that can potentially interbreed. The total number of alleles that are present
in a population is known as the gene pool whilst the allelic frequency expresses as a
decimal or percentage the proportion of a certain allele in a gene pool.
The Hardy-Weinberg Equation can be used to estimate the frequency of alleles in a
population and to see whether a change in allele frequency is occurring in a population over
time. In order to do this the following points are assumed:
no mutations occur to create new alleles.
there is no movement of alleles into or out of the population by migration.
the population is large.
there is no selection, so every allele has an equal chance of being passed to the next
generation.
mating is random.
The formulae for the Hardy-Weinburg principle are shown below:
p + q = 1.0 AND p2 + 2pq + q2 = 1
Where:
p = the frequency of the dominant allele (represented by A)
q = the frequency of the recessive allele (represented by a)
p2 = frequency of AA (homozygous dominant)
2pq = frequency of Aa (heterozygous)
q2 = frequency of aa (homozygous recessive)
Variation in phenotype
, There are variations in the alleles of genes for members of the same species due to a
number of genetic factors such as:
Random fertilisation - the gametes that are carrying different alleles will join
together randomly.
Meiosis - meiosis is the nuclear division that creates gametes and means that the
alleles will be assorted in the gametes at random.
Mutation - the mutation of an allele can go on to lead to the creation of another
new allele which can then be passed to the next generation
There may also be environmental influences, with the majority of phenotypic traits being
influences by environmental factors. An example is two plants that possess the same alleles
for the flower colour. However an environmental factor of one plant growing in a soil that
lacks a certain mineral may mean the pigment in one will not develop so will have a slightly
different flower colour. If a large enough sample is taken out of this trait then a normal
distribution will be shown as seen in the diagram showing this for height in adults.
Natural selection
The niche of a species is its role within the environment. Species which share the same
niche compete with each other and a better adapted species survive. The idea that better
adapted species survive is the basis of natural selection.
Many organisms have a unsustainably large number of offspring, for example fish may lay
thousands of eggs which cannot all survive due to limited resources. However Darwin
suggested that the reason may offspring are produced is so that there is greater
competition within the species (intraspecific competition) and therefore only those that
have the alleles best suited to the environment survive long enough to grow and reproduce
passing the alleles on to the next generation.
The variation in genotypes and phenotypes within a population increases the chance than a
species will survive in a habitat that is changing.
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