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Eastern Pennsylvania and the Founder Effect
Eastern Pennsylvania is home to beautiful farmlands and
countryside, but it's also a gold mine of information for
geneticists, who have studied the region's Amish culture for
decades. Because of their closed population stemming from a small
number of German immigrants -- about 200 individuals -- the Amish
carry unusual concentrations of gene mutations that cause a
number of otherwise rare inherited disorders, including forms of
dwarfism.
One form of dwarfism, Ellis-van Creveld syndrome, involves not only short stature but polydactyl (extra
fingers or toes), abnormalities of the nails and teeth, and, in about half of individuals, a hole between the two
upper chambers of the heart. The syndrome is common in the Amish because of the "founder effect."
When a small part of a population moves to a new locale, or when the population is reduced to a small size
because of some environmental change, the genes of the "founders" of the new society are disproportionately
frequent in the resulting population.
If individuals in the group tend to marry within it, there's a greater likelihood that the recessive genes of the
founders will come together in the cells that produce offspring. Thus diseases of recessive genes, which
require two copies of the gene to cause the disease, will show up more frequently than they would if the
population married outside the group.
In the Amish, in fact, Ellis-van Creveld syndrome has been traced back to one couple, Samuel King and his wife,
who came to the area in 1744. The mutated gene that causes the syndrome was passed along from the Kings
and their offspring, and today it is many times more common in the Amish population than in the American
population at large.
The founder effect is an extreme example of "genetic drift." Genes occurring at a certain frequency in the
larger population will occur at a different frequency -- more or less often -- in a smaller subset of that
population. As in the example of human diseases, genetically determined traits that would ordinarily be
uncommon in the overall gene pool might crop up with distressing frequency in a small subset of that pool.
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1. What mutations are common in this Amish population?
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2. What is the founder effect?
3. How did this mutation originally get into the gene pool?
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4. What is genetic drift?
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5. How does this population show genetic drift?
Bottlenecks and Founder Effects
Genetic drift can cause big losses of genetic variation for small populations.
Population bottlenecks occur when a population’s size is reduced for at least one generation. Because genetic
drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a
population’s genetic variation by a lot, even if the bottleneck doesn’t last for very many generations.
Reduced genetic variation means that the population may not be able to adapt to new selection pressures,
such as climatic change or a shift in available resources, because the genetic variation that selection would
act on may have already drifted out of the population.
An example of a bottleneck:
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.
Founder effects
A founder effect occurs when a new colony is started by a few members of the original population. This small
population size means that the colony may have:
- reduced genetic variation from the original population.
- a non-random sample of the genes in the original population.
For example, 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. This effect is easy to recognize in genetic diseases, but of course, the frequencies of all
sorts of genes are affected by founder events.
6. What is a population bottleneck or “the bottleneck effect”? ___________________________
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7. What does a bottleneck do to a population?
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8. What caused the bottleneck on the population of northern elephant seals?
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9. What is the founder effect?
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10. What is the result of the founder effect on the Afrikaner population?
Chance Events and the Bottleneck Effect
But random genetic drift is even more than this. It also refers to accidental random events that influence
allele frequency. For example,
"Chance events can cause the frequencies of alleles in a small population to drift randomly from generation to
generation. For example, consider what would happen if [a]... wildflower population ... consisted of only 25
plants. Assume that 16 of the plants have the genotype AA for flower color, 8 are Aa, and only 1 is aa. Now
imagine that three of the plants are accidently destroyed by a rock slide before they have a chance to
reproduce. By chance, all three plants lost from the population could be AA individuals. The event would alter
the relative frequency of the two alleles for flower color in subsequent generations. This is a case of
microevolution caused by genetic drift...
"Disasters such as earthquakes, floods, or fires may reduce the size of a population drastically, killing victims
unselectively. The result is that the small surviving population is unlikely to be representative of the original
population in its genetic makeup - a situation known as the bottleneck effect.... Genetic drift caused by
bottlenecking may have been important in the early evolution of human populations when calamities decimated
tribes. The gene pool of each surviving population may have been, just by chance, quite different from that of
the larger population that predated the catastrophe." (Campbell, N.A. in Biology 2nd ed. Benjamin/Cummings
1990 p.443)
Several examples of bottlenecks have been inferred from genetic data. For example, there is very little
genetic variation in the cheetah population. This is consistent with a reduction in the size of the population to
only a few individuals - an event that probably occurred several thousand years ago. An observed example is
the northern elephant seal which was hunted almost to extinction. By 1890 there were fewer than 20 animals
but the population now numbers more than 30,000. As predicted there is very little genetic variation in the
elephant seal population and it is likely that the twenty animals that survived the slaughter were more "lucky"
than "fit".
11. Give three examples of chance events that may influence the allele frequency of a population.
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12. What is a likely result of these chance events?
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13. Explain the bottleneck effect.
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14. Give an example of how a population was affected by the bottleneck effect.