Mechanisms Of Evolutionary Change: The Founder Effect

By definition, Founder effect^[1] is the loss of genetic variation that occurs when a small group of individuals is separated from a larger group and then establishes a new population. Originally, Founder effect was proposed in order to explain the origin of islands located in high elevations. Now, it serve as an event to justify the origins of species.

• The Founder effect is considered as a sub-type of Bottleneck effect^[2] , which is defined as the loss of the variation in the alleles after the founding of a new population from the main population. But unlike bottlenecks that last until the population is small, Founder effect is just an instant event.
• The alleles present in the new population may not necessarily reflect those of the larger population. And the more that the new population grows; it becomes more and more distinct from the original one.
• The chances of passing on the gene increase more when the individuals in the new population tend to mate within and interbreed.

A good illustration of this phenomenon can be seen below here:

During the 1940s, the discovery of the concept of the Founder effect can be attributed to scientist Ernst Mayr^[3] . Originally, Mayr called this phenomenon as the “peripatric speciation” or “founder-effect speciation“. In some New Guinea islands, Mayr observed that some populations of birds are very distinct from the main bird population. He hypothesized that this phenomenon could only occur by the isolation of a certain gene of the main population (resulting to bottleneck effect), genetic drift, and natural selection.

Throughout history, there have been several incidences where small groups of individuals move far from their home territories. By random chance alone, the new population develops different allele frequencies from that of the original population. Such isolation and evolution then lead to divergence between the two populations, with the founder population becoming a new species and being unable to interbreed with the former.

Huntington’s disease^[4] is a heritable progressive neurological disorder characterized by uncontrolled motor function, and loss of intellectual capacity. Individuals who inherit the defective dominant allele tend to lose their control over their brains, and not long after, will die.

• To date, it is present that this disease affects 0. 005 % (about 1 out of 20,000) people of European ancestry. Interestingly, fishing villages located near Lake Macaraibo (Venezuela) have the highest prevalence of this disease in the world.
• The origin of this disease^[5] can be traced back from a woman (in the 1700s) who carried the allele for Huntington’s disease and bore 10 children. These children and their children interbred and eventually created the lineage in that area.
• On the other hand, this disorder is least observed in some populations like the Chinese, Japanese, and African ancestry.

Another genetic condition governed by the principles of Founder effect is Tay-Sachs disease^[6]. This disorder is known to destroy the neurons of the central nervous system and is usually fatal during early childhood.

• The symptoms^[7] of the disease include intellectual disability, seizures, paralysis and a cherry red spot eye abnormality. Because of these symptoms, the Cajuns (from Louisiana) also call this disease “Lazy Baby” syndrome.
• Generally, about 0.4 % (or 1 out of 250 people) of the general population is a carrier of the allele for Tay-Sach’s disease. However, this disease has higher frequencies (about 3.7% or 1 out of 27 people) in the populations of the Ashkenazi, the French Canadians, and the Cajuns.
• In the 17th century, the Cajuns, who originated from the French Canadians, migrated to Louisiana and carried the allele for Tay-Sach’s disease in their gene pool. Since then, the Cajuns remained isolated to the point that people married even their close relatives.

Ellis-van-Creveld Syndrome is another heritable condition that affects bone growth, resulting to dwarfism (overall short stature) and polydactyly (presence of extra fingers or toes). Affected individuals may also have life-threatening heart ailments.

• This disorder is very rare in the general population^[8] (1 out of 200 000 people) but is very much common in the Old Order Amish descent of Pennsylvania and Western Australia.
• In the late 1930s, scientists Richard Ellis and Simon van Creveld first described the syndrome. Ten years later, the syndrome was formally described and was finally named after them.