What is it?
This is the difference in DNA between members of the same species or a population.
It highlights the fact that members of the same species are similar but not identical.
What causes genetic diversity?
All members of the same population have the same genes but vary in alleles. The more genetically diverse a population is, the higher the number of alleles there
are. This essentially means that the gene pool is very great, increasing the heterozygosity between organisms.
Gene pool of a population of species can be made greater by:
 Mutation so that natural selection can occur.
 Gene flow occurs due to immigration to a population. This is because there is an addition of different alleles.
Meiosis
The way in which gametes divide to produce more sex cells could produce offspring with different characteristics.
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Crossing over occurs during prophase 1 where the chromatids of each chromosome in a bivalent structure ‘swap’ genes. This results in two new different
chromosomes.
Independent assortment during metaphase is where how the homologous chromosomes are aligned at the equator. This combination of chromosomes would
change the alleles present in each daughter cell when the cell divides.
Selective Breeding
Selective breeding is when two individuals of a species are chosen to reproduce to produce a new generation possessing a certain characteristic. This is also known as
producing high-yielding breeds, which is beneficial for human use. For example a farmer would selectively breed two cows which produce a better quality of milk so that it
could be sold for profit. Another example is if a gardener selects roses that are naturally great in height and have red-coloured petals to breed so that a new generation of
tall red roses could grow in the garden.
How it reduces genetic diversity?
If a selection of organisms with only certain traits are being bred then only certain alleles will continue to be passed on in a generation. This results in a low frequency of
alleles; the gene pool decreases and as a result the new population is less genetically diverse.
Arguments for and against the use of selective breeding
In reality there are a both positives and negatives of using this method.
Positives- the advantages are usually for humans where farmers or companies could gain profit or simply save money.
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It can increase tolerance of certain environmental conditions such as lack of water, low or extremely high temperatures. This means the survival of that species
would be guaranteed so more can be sold to obtain money. It also means the organisms can survive under conditions which are cheaper to supply.
Example - poor farmers in developing countries can grow and sell crops in unreliable weather and all year round, and this would mean they can increase their
profit.
There can be a high yielding of individuals with certain traits.
The organisms with higher resistance to diseases can breed so that the offspring will inherit the resistance gene and therefore increase their chance of survival. In
addition less medicinal care will be required, which also means that money can be saved.
Example - Sydney Rock Oyster
Selective breeding had allowed scientist to achieve success in causing populations of this oyster to develop resistance to diseases called Marteilia sydneyi and
also reduces winter mortality. The breeding time is reduced from 3 to 2 years, so that it is more suitable to sell for its demand.
Negatives- there are a number of ethical concerns to the organisms involved which are often unconsidered.
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Since there is a few alleles within a population, the results are the same as that of inbreeding; the organisms are more susceptible to genetic diseases due to
the accumulation of harmful recessive alleles.
The effect of certain characteristics in organisms could in fact result in discomfort.
Example - This is demonstrated in the intense selective breeding of pugs.
Their short legs mean
frequent dislocation of the
bone. This results in limping
and a low physical activity for
the pug.
Their short muzzles (noses)
cause laboured breathing.
The Founder Effect
A few members of the same species geographically isolate themselves from the population and colonize elsewhere. The reason for migrating in humans would be due to
religions or other beliefs.
How it reduces genetic diversity?
The founding population is usually very small and no allele would be favoured. Different alleles will be present to the original population; there may be a higher frequency
of one allele by chance and another allele might be completely eliminated -which results in a loss of genetic variety in the gene pool and thus genetic diversity.
Genetic diversity can also increase again overtime if mutations occur in an organism's DNA allowing the occurrence of natural selection. This introduces a new allele which
may be beneficial to the organism if it helps it to adapt to a certain habitat. Therefore the organism survives and reproduces, passing the new allele to the offspring. Over
time this allele will be more common within the population.
Example - The Fugates are a group of people who chose to live in the mountains in a single area of America. Two founders possess the recessive allele that results in
blue skin. This characteristic has become more common in the population due to generations of inbreeding and is now what characteristics them, since it is so distinct
from our own.
Genetic Bottleneck
This is a natural event, causing a mass loss of population. This dramatically reduces the gene pool and the number of different alleles present between the surviving
populations. No allele is favoured so the alleles which exist will be random and unselective. The new generation after a few years of reproduction will be large, but which
will also contain a few alleles.
This is very similar to The Founder Effect, only the cause of the small population is different.
Example - The Northern Elephant Seals
This species was hunted by humans 200 years ago and their original population was reduced to around 50 seals. This population have since reproduced but has very
low genetic diversity.