AS Biology_Unit 2: Genetic Diversity Fact File
Genetic diversity is the differences in the DNA of members of the same species within a
population.
Genetic diversity is increased through:





Mutation
Crossing over in Meiosis I
Random segregation
Random fertilisation of gametes
Random mating (out-breeding)
Genetic diversity is reduced through:





Selective breeding
Asexual Reproduction
The Founder Effect
Genetic Bottlenecks
In-breeding
Genetic diversity arises by mutations in the DNA forming new alleles. The combined
effects of when chromatids cross over and when random segregation occurs in
meiosis I along with random fertilisation produce new combinations of existing
alleles. Another way is when different alleles are introduced into a population when
individuals from another population migrate into them and reproduce, which is also
known as ‘gene flow’.
The Founder Effect
The Founder Effect occurs when a few individuals of
a population colonise a new, isolated region. Only a
small number of organisms have contributed their
alleles to the gene pool therefore their alleles may
not be representative of the original population. Due
to the narrower range of alleles, inbreeding may take
place and as a result over time, some alleles found in
the original population may be completely lost if
individuals fail to produce offspring, who can carry
the alleles.
Examples of the Founder Effect:
The Afrikaner population of Dutch settlers in South Africa is descended mainly from a few
colonists. Today, the Afrikaner population has an unusually high frequency of the gene that
causes Huntington’s disease, because those original Dutch colonists just happened to carry
that gene with unusually high frequency.
Genetic Bottlenecks
A genetic bottleneck is produced the population of a species reduces suddenly due to
unselective natural disasters or gradual environmental changes such as earthquakes, floods
and volcanic eruptions. The few individuals left will only have a small range of alleles
between them, so if they reproduce and the population increases again there will be reduced
genetic diversity because many of the original variety of alleles maybe lost in individuals who
didn’t survive or some alleles maybe overrepresented by the surviving individuals.
Examples of Genetic Bottlenecks:
Northern elephant seals have reduced genetic variation probably because of a
population bottleneck humans inflicted on them in the 1890s. Hunting reduced
their population size to as few as 20 individuals at the end of the 19th century.
Their population has since rebounded to over 30,000—but their genes still carry
the marks of this bottleneck: they have much less genetic variation than a
population of southern elephant seals that was not so intensely hunted.
How does the selection of high-yielding breeds of domesticated animals
and plants influence genetic diversity?
The selection of high-yielding breeds of domesticated animals and plant has an influence on
genetic diversity because it reduced it. When the desired alleles are selected to breed an
organism, the number of alleles present in the new organism is reduced.
Selective breeding
Selective breeding is also known as artificial selection, which involves humans choosing
desired characteristics for parent individuals to produce high-yielding breeds of domesticated
animals and strains of plants. Selective breeding reduces genetic diversity because the variety
of alleles in a population is limited to a small number of desired alleles. This means that when
an organism with the desired characteristic has been produced, only that one type of organism
will be continually be bred. The selection of high-yielding breeds of domesticated animals and
plants influence genetic diversity by reducing it because similar organisms with similar
characteristics will be reproduced again and again meaning that similar alleles will be bred
together over many generations.
Examples of selectively bred plants and animals:
1. Maize was bred from an unfamiliar tall grass-like plant called ‘Teosinte’. Larger and
fuller kernels were selected for in order to allow the crop to be a more effective food
source.
2. The banana plant is a hybrid and was produced as a result of the cross breeding of
two wild South Asian plants species called the Musa acuminata and Musa balbisiana.
The hybridized banana plant was unexpectedly tasty, seedless, was a bright yellow
colour and maintained an amusing shape.
3. Cattle have been selectively bred so that they have a high muscle to bone ratio, grow
and gain weight rapidly. Not only are cattle selectively bred for their meat but also for
their milk. Farmers will often breed together cattle that produce large amount of milk
with high fat and protein content as well as delivering milk rapidly.
4. Different breeds of chickens are bred with various characteristics, for example, the
egg-laying chickens are fed special high-calcium diets so they can make egg shells.
These chickens lay around 30 eggs a year. Chickens that are used for meat are bred
to grow quickly and have large leg and breast muscles.
Selective Breeding
Advantages



Produces high-yielding strains of crops and
animals which means they can be sold for a
lower price therefore food becomes more
affordable. This leads to a raised standard of
living.
Can be used to produce animals and plants
that have increased resistance to disease.
This means farmers have to use fewer drugs
and pesticides so their money is saved.
Animals and plants could be bred to have
increased tolerance of bad conditions where
other strain cannot survive on, such as
plants that grow in poor soil or on polluted
ground.
Disadvantages


Can cause health problems, for example,
dairy cows are often lame and have a short
life expectancy because of the extra strain
put on their bodies due to making and
carrying loads of milk. Another example is
the breathing problems experience by
selectively bred bulldogs.
Reduces genetic diversity, which results in
an increased spread of genetic disease and
increased susceptibility to new diseases
because of the lack of alleles in the
population.
There are several ethical implications that need to be considered when deciding to selectively
breed domesticated animals.





One may argue that selective breeding is the human interference with nature therefore
is wrong because humans do not have the right to develop new varieties of species
because variation will take place naturally as it has been done for the past millions of
years.
Many people may argue that the wellbeing of humans is more important than the
welfare of animals therefore as long as selectively breeding animals raises standard
of living and produces enough food to feed people, then it should be an acceptable
process. However, one may argue against this and suggest that it is hard to balance
between increased yield and animal welfare. Processes like selective breeding put
pressure on animal welfare because for example, the artificial insemination of cows to
double the milk yield per cow has put strain on a cow’s udder that it has led to mastitis
(inflammation of the udder), lameness and infertility.
Selective breeding leads to variety in animals with a narrower range of alleles therefore
genetic diversity is reduced. This may lead to some undesired alleles to be completely
lost forever, which may be useful to animals or mankind in the future.
This process may lead one to argue that because the desired alleles of animals are
selected, domesticated animals are valued for their usefulness to humans more than
anything else, for example, a farmer who selectively breeds a cow to produce more
meat may value the cow because it has the potential to raise his profits.
There is also the question as to what features should be selected when selectively
breeding and which person decides. One may argue that because selectively breeding
farm animals benefit humans, then the benefit of selectively breeding pets should make
the process of selectively breeding pets acceptable. It is questionable whether features
of cats, dogs, rabbits and other pets should have selected alleles only to make them
look more fashionable or desirable to their owner.