Founder effect: The original population (left) could give rise to different founder populations (right).
In population genetics, the founder effect
is the loss of genetic variation that occurs when a new population is
established by a very small number of individuals from a larger
population. It was first fully outlined by Ernst Mayr in 1942, using existing theoretical work by those such as Sewall Wright. As a result of the loss of genetic variation, the new population may be distinctively different, both genotypically and phenotypically, from the parent population from which it is derived. In extreme cases, the founder effect is thought to lead to the speciation and subsequent evolution of new species.
In the figure shown, the original population has nearly equal
numbers of blue and red individuals. The three smaller founder
populations show that one or the other color may predominate (founder
effect), due to random sampling of the original population. A population bottleneck may also cause a founder effect, though it is not strictly a new population.
The founder effect occurs when a small group of migrants that is
not genetically representative of the population from which they came
establish in a new area. In addition to founder effects, the new population is often a very small population, so shows increased sensitivity to genetic drift, an increase in inbreeding, and relatively low genetic variation.
Founder mutation
In genetics, a founder mutation is a mutation that appears in the DNA
of one or more individuals which are founders of a distinct population.
Founder mutations initiate with changes that occur in the DNA and can
be passed down to other generations.
Any organism—from a simple virus to something complex like a
mammal—whose progeny carry its mutation has the potential to express the
founder effect, for instance a goat or a human.
Founder mutations originate in long stretches of DNA on a single chromosome; indeed, the original haplotype
is the whole chromosome. As the generations progress, the proportion of
the haplotype that is common to all carriers of the mutation is
shortened (due to genetic recombination). This shortening allows scientists to roughly estimate the age of the mutation.
General
The founder effect is a special case of genetic drift,
occurring when a small group in a population splinters off from the
original population and forms a new one. The new colony may have less
genetic variation than the original population, and through the random
sampling of alleles during reproduction of subsequent generations, continue rapidly towards fixation. This consequence of inbreeding makes the colony more vulnerable to extinction.
When a newly formed colony is small, its founders can strongly
affect the population's genetic makeup far into the future. In humans,
who have a slow reproduction rate, the population will remain small for
many generations, effectively amplifying the drift effect generation
after generation until the population reaches a certain size. Alleles
which were present but relatively rare in the original population can
move to one of two extremes. The most common one is that the allele is
soon lost altogether, but the other possibility is that the allele
survives and within a few generations has become much more dispersed
throughout the population. The new colony can experience an increase in
the frequency of recessive alleles, as well, and as a result, an increased number who are homozygous for certain recessive traits.
The variation in gene frequency between the original population and colony may also trigger the two groups to diverge significantly over the course of many generations. As the variance, or genetic distance, increases, the two separated populations may become distinctively different, both genetically and phenotypically,
although not only genetic drift, but also natural selection, gene flow
and mutation all contribute to this divergence. This potential for
relatively rapid changes in the colony's gene frequency led most
scientists to consider the founder effect (and by extension, genetic
drift) a significant driving force in the evolution of new species. Sewall Wright was the first to attach this significance to random drift and small, newly isolated populations with his shifting balance theory of speciation. Following behind Wright, Ernst Mayr
created many persuasive models to show that the decline in genetic
variation and small population size accompanying the founder effect were
critically important for new species to develop.
However, much less support for this view is shown today, since the
hypothesis has been tested repeatedly through experimental research, and
the results have been equivocal at best. Speciation by genetic drift is a specific case of peripatric speciation which in itself occurs in rare instances.
It takes place when a random change in genetic frequency of population
favours the survival of a few organisms of the species with rare genes
which cause reproductive mutation. These surviving organisms then breed
among themselves over a long period of time to create a whole new
species whose reproductive systems or behaviors are no longer compatible
with the original population.
Serial founder effect
Serial
founder effects have occurred when populations migrate over long
distances. Such long-distance migrations typically involve relatively
rapid movements followed by periods of settlement. The populations in
each migration carry only a subset of the genetic diversity carried from
previous migrations. As a result, genetic differentiation tends to
increase with geographic distance as described by the "isolation by
distance" model. The migration of humans out of Africa is characterized by serial founder effects.
Africa has the highest degree of genetic diversity of any continent,
which is consistent with an African origin of modern humans. After the
initial migration from Africa, the Indian subcontinent was the first
major settling point for modern humans. Consequently, India has the
second-highest genetic diversity in the world. In general, the genetic
diversity of the Indian subcontinent is a subset of Africa, and the
genetic diversity outside Africa is a subset of India.
In island ecology
Founder populations are essential to the study of island biogeography and island ecology.
A natural "blank slate" is not easily found, but a classic series of
studies on founder population effects was done following the
catastrophic 1883 eruption of Krakatoa, which erased all life on the island. Another continuing study has been following the biocolonization of Surtsey, Iceland, a new volcanic island that erupted offshore between 1963 and 1967. An earlier event, the Toba eruption in Sumatra about 73,000 years ago, covered some parts of India with 3–6 m (10–20 ft) of ash, and must have coated the Nicobar Islands and Andaman Islands, much nearer in the ash fallout cone, with life-smothering layers, forcing the restart of their biodiversity.
However, not all founder effect studies are initiated after a
natural disaster; some scientists study the reinstatement of a species
that became locally extinct. Hajji and others, and Hundertmark & Van
Daele, studied the current population statuses of past founder effects
in Corsican red deer and Alaskan elk, respectively. Corsican red deer are still listed as an endangered
species, decades after a severe bottleneck. They inhabit the Tyrrhenian
islands and surrounding mainlands currently, and before the bottleneck,
but Hajji and others wanted to know how the deer originally got to the
islands, and from what parent population or species they were derived.
Through molecular analysis, they were able to determine a possible
lineage, with red deer from the islands of Corsica and Sardinia
being the most related to one another. These results are promising, as
the island of Corsica was repopulated with red deer from the Sardinian
island after the original Corsican red deer population became extinct,
and the deer now inhabiting the island of Corsica are diverging from
those inhabiting Sardinia.
Kolbe and others set up a pair of genetically sequenced and morphologically examined lizards
on seven small islands to watch each new population's growth and
adaptation to its new environment. Specifically, they were looking at
the effects on limb length and perch width, both widely varying
phenotypic ranges in the parent population. Unfortunately, immigration
did occur, but the founder effect and adaptive differentiation, which
could eventually lead to peripatric speciation, were statistically and
biologically significant between the island populations after a few
years. The authors also point out that although adaptive differentiation
is significant, the differences between island populations best reflect
the differences between founders and their genetic diversity that has
been passed down through the generations.
Founder effects can affect complex traits, such as song diversity. In the Common Myna (Acridotheres tristis),
the percentage of unique songs within a repertoire and within‐song
complexity were significantly lower in birds from founder populations.
Among human populations
Due
to various migrations throughout human history, founder effects are
somewhat common among humans in different times and places. The French Canadians of Quebec
are a classical example of founder population. Over 150 years of French
colonization, between 1608 and 1760, an estimated 8,500 pioneers
married and left at least one descendant on the territory.
Following the takeover of the colony by the British crown in 1760,
immigration from France effectively stopped, but descendants of French
settlers continued to grow in number mainly because of high fertility
rate. Intermarriage occurred mostly with the deported Acadians
and migrants coming from the British Isles. Since the 20th century,
immigration in Quebec and mixing of French Canadians involve people from
all over the world. While the French Canadians of Quebec today may be
partly of other ancestries, the genetic contribution of the original
French founders is predominant, explaining about 90% of regional gene
pools, while Acadians (descended from other French settlers in eastern
Canada) explain 4% and British 2%, with Native American and other groups contributing less.
In humans, founder effects can arise from cultural isolation, and inevitably, endogamy. For example, the Amish
populations in the United States exhibit founder effects because they
have grown from a very few founders, have not recruited newcomers, and
tend to marry within the community. Though still rare, phenomena such as
polydactyly (extra fingers and toes, a symptom of a condition such as Weyers acrodental dysostosis or Ellis-van Creveld syndrome) are more common in Amish communities than in the American population at large. Maple syrup urine disease affects about one out of 180,000 infants in the general population. Due in part to the founder effect, however, the disease has a much higher prevalence in children of Amish, Mennonite, and Jewish descent. Similarly, a high frequency of fumarase deficiency exists among the 10,000 members of the Fundamentalist Church of Jesus Christ of Latter Day Saints, a community which practices both endogamy and polygyny, where an estimated 75-80% of the community are blood relatives of just two men—founders John Y. Barlow and Joseph Smith Jessop.
The island of Pingelap
also suffered a population bottleneck in 1775 following a typhoon that
had reduced the population to only 20 people. As a result complete achromatopsia with a rate of occurrence of roughly 10% with an additional 30% being carriers of this recessive condition.
Around 1814, a small group of British colonists founded a settlement on Tristan da Cunha,
a group of small islands in the Atlantic Ocean, midway between Africa
and South America. One of the early colonists apparently carried a rare,
recessive allele for retinitis pigmentosa, a progressive form of blindness that afflicts homozygous
individuals. As late as 1961, the majority of the genes in the gene
pool on Tristan were still derived from 15 original ancestors; as a
consequence of the inbreeding, of 232 people tested in 1961, four were
suffering from retinitis pigmentosa. This represents a prevalence of 1
in 58, compared with a worldwide prevalence of around 1 in 4,000.
The abnormally high rate of twin births in Cândido Godói could be explained by the founder effect.
