Small Population Size Effects

Genetic Drift

In small, reproductively isolated populations, special circumstances exist that can produce rapid changes in gene frequencies totally independent of mutation and natural selection.  These changes are due solely to chance factorsThe smaller the population, the more susceptible it is to such random changes.  This phenomenon is known as genetic drift.

In order to get a better understanding of the potential effect of population size on evolution, it is useful to carry out a simple coin flipping experiment.  The expectation is that heads will turn up 50% of the time because there are only two sides to a coin--heads and tails.  If you flip a coin 10 times, it may or may not result in 5 heads.

 Coin flipping experiment.  Take a coin out of your pocket.  Flip it 10 times and record the results.         Repeat the experiment twice.  Calculate the percentage of times the coin came up heads in each of         your three experiments and, finally, in all of your experiments combined.  What did you learn?

The more times that you flip your coin, the more likely it will approach the expected 50% heads.  If you do it an infinite number of times, it will be 50%.  In other words, when a sample is very small, the probable outcome may not occur.  As the sample increases in size, it will get progressively closer to it.

This kind of deviation from the expected outcome with small samples also occurs in genetic inheritance when breeding populations are very small.  For example, when women and their mates are both heterozygous (Aa) for a trait, we would expect that 25% of their children will be homozygous recessive (aa).  By chance, however, a particular couple might not have any children with this genotype (as shown below in the Punnett square on the right).

 genotypes of children 25% AA 50% Aa 25% aa genotypes of children 33.3% AA 66.7% Aa 0.0% aa

Unless other families have an unpredictably large number of homozygous recessive (aa) children for this trait to counter the random deviation, the population's gene pool frequencies will change in the direction of having fewer recessive alleles--genetic drift will occur.

The net effect of genetic drift on a small population's gene pool can be rapid evolution, as illustrated in the hypothetical inheritance patterns shown below.  Note that the red trait dramatically increases in frequency from generation to generation.  It is important to remember that this can occur independent of natural selection or any other evolutionary mechanism.

 Rapid genetic drift over three generations

Such distorting statistical anomalies occur regularly.  In small populations, they can have a rapid, significant effect on gene pool frequencies of subsequent generations.  In large populations, however, they are commonly neutralized by other families having children with countering genotypes.

Since genetic drift is measurably effective only in small populations, it must have played a major role in the early stages of human evolution when our populations were tiny.  However, even in large societies, such as the United States today, there are small, culturally isolated communities like the Amish and Dunkers of rural Pennsylvania and the Midwest that are mostly closed breeding groups.  In such sub-populations, genetic drift is still an important evolutionary mechanism.

Founder Principle

Another important small population effect is known as the founder principle or founder effect.  This occurs when a small amount of people have many descendants surviving after a number of generations.  The result for a population is often high frequencies of specific genetic traits inherited from the few common ancestors who first had them.

In the Lake Maracaibo region of northwest Venezuela, for instance, there is an extraordinarily high frequency of a severe genetically inherited degenerative nerve disorder known as Huntington's disease.  Approximately 150 people in the area during the 1990's had this rare fatal condition and many others were at high risk for developing it.  This disease usually does not strike until early middle age, after most people have had their children.  However, Huntington's can occur much earlier.  About 10% of its victims develop symptoms when they are younger than 20 years old.  There is no cure for this disease, but there has been a test for its genetic marker available since 1993.  All of the Lake Maracaibo region Huntington's disease victims trace their ancestry to a woman named Maria Concepción Soto who moved into the area in the 19th century.  She had an unusually large number of descendants and was therefore the "founder" of what is now a population of about 20,000 people with a high risk of having this unpleasant genetically inherited trait.

NOTE:  Huntington's disease is not unique to the Lake Maracaibo region.  It occurs throughout the world at relatively low rates.  About 200,000 people in the United States (.07% of the population) have it.  Perhaps, the most well-known victim of Huntington's was the American folk song writer and performer Woody Guthrie.

Another example of the founder effect has been discovered among the 16-18,000 Old Order Amish people of Lancaster County, Pennsylvania.  They are descended from a few dozen individuals belonging to an Anabaptist sect in Germany who migrated to Pennsylvania during the early 1700's.  Over the last 40 years of the 20th century, 61 babies with an extremely rare fatal genetic disorder known as microcephaly were born to 23 Amish families.  All of these families are descendants of a single Amish couple nine generations ago.  They were the founders of the population with the genes for microcephaly today.

It is also possible to find the results of a founder event even though the original ancestors are unknown.  For example, South and Central American Indians were nearly 100% type O for the ABO blood system and 100% positive for the Rh blood system.  Since nothing in nature seems to strongly select for or against blood types, it is likely that most of these people are descended from a small band of closely related "founders" who also shared these traits.  They migrated into the region from the north, probably by the end of the last Ice Age.

Bottleneck Effect

In many species, there have been catastrophic periods caused by rapid dramatic changes in natural selection, during which most individuals died without passing on their genes.  The few survivors of these evolutionary "bottlenecks" then were reproductively very successful, resulting in large populations in subsequent generations.  The consequence of this bottleneck effect is the extraordinary reduction in genetic diversity of a species since most variability is lost at the time of the bottleneck.

Bottlenecking also occurs at times in human populations as a result of major epidemics and catastrophic storms, earthquakes, and volcanic eruptions.  An example of this occurred on the small Micronesian island of Pingelap in the Western Pacific.  In 1775, a typhoon killed at least 90% of its people, thereby eliminating most of the genetic variation.  One of the 20 survivors was a man named Nahnmwarki Mwanenised.  He had achromatopsia, a very rare genetically inherited recessive eye condition that causes total color blindness and extreme sensitivity to light.  Six generations later, nearly 5% of the island's population had achromatopsia.  All of those who had it were descendents of Nahnmwarki MwanenisedBy comparison, only 1 in 33,000 people in the United States have it.  Not only did the Pinegleapese experience a dramatically reduced genetic diversity as a result of the 18th century storm, but unfortunately that surviving gene pool contained the genes for achromatopsia, making this an example of both the bottlenecking effect and the founder principle.