Download 275 The founder effect

B io Factsheet
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Number 275
The Founder effect, genetic drift & bottlenecks
The colours of frog-hoppers are very variable and are determined
by several different alleles of a single gene. The range of colours of
the frog-hoppers and their frequencies vary between the different
islands (Fig.1) because the selection pressures on each island are
very different.
This Factsheet:
• Explains the concepts of genetic drift, genetic bottlenecks and
the founder effect
• Illustrates each concept with real examples
• Reviews recent exam questions on these concepts
Fig. 1 Frequencies of the two colour forms on 10 of the 12
islands
Key definitions:
Genetic drift: a reduction in the genetic diversity of a population
because, just by chance, some alleles were not passed on to the
next generation. Along with natural selection and mutation,
genetic drift is one of the basic mechanisms of evolution. Genetic
drift can result in a significant loss of genetic diversity especially
in a small population.
Genetic bottleneck: A change in allele frequency when a
population declines, loses alleles and then recovers.
St Martin's
Bryher
Founder effect: The establishment of a new population by just
a few individuals that are not genetically representative of the
population that they have left.
Tresco
Genetic variation is important because a population (of animals or
plants) with little genetic variability may not be able to survive
changes in their environment e.g. climate change. Genetic drift
reduces the genetic variation in a population and thus makes it
more vulnerable. The effect of genetic drift is most significant in
small populations e.g. of endangered species.
St Mary's
Genetic drift Case Study:
Populations of animals on islands are often small and geographically
isolated so genetic drift happens quite frequently.
Frog-hoppers (Philaenus sumarius) are found on many of the Isles
of Scilly.
St Agnes
1 A typical exam question would ask you to describe the
differences in the phenotypes of the frog-hoppers on the islands.
You need to get your eye in!
2. As you can see, on St Agnes and St Mary’s all the frog-hoppers
are striped; but on all the other islands there are some froghoppers of each type and, apart from one island, there are more
striped frog-hoppers than melanic ones.
So how is this an example of genetic drift?
• The populations of frog-hoppers on each island are small – the
founder effect
• By chance, one individual produces more offspring than others
• e.g. they may have found a place to avoid more predators
• Their allele becomes more common in the population
• The “chance” is different on each island
• So the populations on each island diverge
http://www.entomart.be/listetotale.html entomart
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Bio Factsheet
275 - The Founder effect, genetic drift & bottlenecks
www.curriculum-press.co.uk
Case Study
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Tilapia nilotica, is a species of fish found in Lake Turkana in
Kenya.
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The water level in Lake Turkana has fluctuated hugely over the last
10 000 years. When the water level was very high a crater on a
volcanic island in the lake filled up with water and fish. Over
thousands of years the water level fell and the crater became isolated
(Fig. 2).
the small population of fish in the crater were not a representative
sample of the fish in the main lake ( founder effect) and mutations
affecting one population were different from the mutations
affecting the second population
the two separated populations faced different selection
pressures thus they evolved differently (allopatric speciation)
the population of fish in the crater was small so genetic drift had
a significant effect
there was no gene flow between the two populations so their
gene pools became increasingly different
the two populations were reproductively isolated and eventually
they became unable to interbreed to produce fertile offspring
i.e. they had now become two separate species
Typical Exam Question
(a) What is meant by the term genetic diversity? (1)
(b) Low levels of genetic variation put a species at greater risk of
extinction. Explain why (3).
(c) In population genetics, what is meant by the term founder
effect? (2)
Fig. 2 Lake Turkana
Markscheme
(a) Difference in DNA/base sequence/alleles/genes;
(b) A selection pressure may change;
e.g. new predators/climate change/introduction of a
pollutant/disease;
If no variation exists all individuals within the population
will respond in the same way;
Many/all may die;
(c) A small number of individuals leave the main population/
found a new population;
the allele frequency in the founding group does not
represent the frequencies in the original population/
reference to reduced number of alleles;
Founders have different/fewer alleles/genes;
AVAILABLE
Genetic bottleneck
Northern elephant seals have reduced genetic
variation because of a population bottleneck
caused by humans hunting them. By the end of
the 19th century their population had been
reduced from about 150,000 to less than 100
individuals so their genetic variability had also
been reduced significantly. Since then, their
population has rebounded to over 150,000 but they are all
descendents of those 100 and they have much less genetic variation
than a population of southern elephant seals that humans didn’t
hunt as intensively.
Tilapia nilotica inhabits Lake Turkana itself but a different species,
Tilapia vulcani, inhabits the crater on the island. The two species
look very similar (Fig.3) and scientists believe there is common
ancestry.
Fig 3 Tilapia nilotica and Tilapia vulcani
Founder effect
Tilapia nilotica
A founder effect occurs when a new population is started (founded)
by a few members of the original population. This small population
size means that the new population may have a reduced and nonrepresentative genetic variation from the original population.
Tilapia vulcani
If the species that started off in Lake Turkana was Tilapia nilotica,
where did Tilapia vulcani come from?
Case Study
The Afrikaner population of Dutch settlers in South Africa is
descended from a small number of colonists. Worldwide, the
incidence of this disease is about 1 in 20,000, but the Afrikaner
population has a much greater frequency of the gene that is
responsible simply because an above average number of the
original Dutch colonists carried the gene.
This is a clear illustration of the founder effect and genetic drift:
• the flooded crater contained Tilapia nilotica from the lake
• when the water level fell, the crater population became isolated
from the lake population
• the fish in the crater could no longer breed with those in the
main lake and the two populations faced different selection
pressures
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Acknowledgements:
This Factsheet was researched and written by Kevin Byrne.
ISSN 1351-5136