Bottleneck Effect Vs. Founder Effect

Sewall Wright, R. A. Fisher, and J. B. S. Haldane are called the principal founders of theoretical population genetics due to their extensive research on inbreeding, mating systems, and genetic drift. Their work has led to the development of the concept of modern evolutionary synthesis (neo-Darwinian synthesis).

Genetic variation in a population is vital from an evolutionary standpoint as natural selection occurs as a function of this variation. Natural selection is a gradual process that allows for the selection of inheritable traits in organisms for better adaptability, thus, affecting evolution.

While this is a mechanism involved in macroevolution (at the level of species), a mechanism that occurs at the microevolutionary level is that of genetic drift. It involves the occurrence of a change in the allelic variations due to random sampling.

In other words, the genetic alleles observed in the progeny are a random sample of those possessed by the original/parent population.

The observable effects of genetic drift are relatively mild in a large population but are obvious and apparent in small populations, if there is an average distribution of allelic variation and frequency. But when the allelic frequency is too low, genetic drift can be observed even in a large population.

It is a random process and occurs as a result of two effects - bottleneck effect and founder effect.

It is also called a population bottleneck, and involves a drastic reduction in the size of a population due to environmental causes or human activities. Such a reduction in population results in the diminution of the genetic variation in the gene pool of that population.

The surviving population, hence, has a lower genetic variation and a correspondingly lower genetic diversity. This lost diversity is only gradually recovered over the course of time due to the chance occurrences of polymorphisms and mutations in the genetic material.

Also, such changes result in a decrease in survivability of the organisms as it affects their ability to adapt to changing environment.

The decreased variety in the gene pool and population size enhances the chances of inbreeding leading to genetic homogeneity, which in turn causes an accumulation of the unfavorable or dysfunctional genetic alleles. It changes the proportional distribution of allelic variants leading to allele fixation or loss of alleles.

This compounded effect is called a bottleneck effect, and the event that causes it is called a bottleneck event. It is called a bottleneck as the event resembles the functionality of a bottleneck, in a way that the selective survival of only few individuals occurs much like the restricted outflow of fluid from a bottle.

It can be applied in conjugation with the concept of minimum viable population size in the field of conservation biology. Here, the minimum population that has survived a bottleneck effect is identified and used to incorporate genetic diversity in the population by means of captive breeding programs.

This would not only protect the genetic diversity but would also be useful in preventing a species from becoming extinct. A reverse application could be administered in the field of agriculture by carrying out biological pest control. This type of pest control utilizes methods that eliminate the pests without causing any imbalance in the ecosystem.

Examples

❖ Wisent, also known as European bison (Bison bonasus), reached the brink of extinction in the early 20th century, however, was recovered later. The present day animals are all descendents of the last surviving 12 individuals and exhibit extremely low genetic variation, which has now started to manifest itself in the form of reproductive handicaps.

❖ The northern elephant seal had a total population of about 30 in the 1890s. Since then, this species has survived and proliferated due to inbreeding. The limited genetic diversity observed in this species has resulted in it being extremely vulnerable to diseases and genetic mutations.

❖ Bottleneck effects are induced artificially by the selective breeding of animals in case of dogs, cats, cattle, horses, etc.

❖ At present, any endangered species of plants or animals can be said to be going through a bottleneck event.

❖ The Cheetahs underwent a bottleneck event in the previous ice age which led to a highly reduced genetic variation. It has been reduced to such an extent that skin grafts from one cheetah can be transplanted onto others without any risk of rejection.

❖ The 1775 typhoon on the Micronesian island of Pingelap in the Western Pacific region eliminated 90% of the human population. Nahnmwarki Mwanenised, a man afflicted with achromatopsia, was one of the 20 survivors of this disaster. This condition is a rare genetic disorder that causes total color blindness and extreme sensitivity to light.

After six subsequent generations, 5% of the island's population had this disorder, and all of them were the descendents of Nahnmwarki Mwanenised.

It involves the loss of genetic variation due to the migration of a set of individuals from the parent population to a new habitat where this set establishes itself; thus, setting up a new habitat. This new population is called the founder population. The subsequent genetic changes that occur are said to be due to a founder effect.

In such a scenario, the set that separates from the parent population consists of genetic representatives of the original population with a disproportional distribution of allelic variants. The new environment in which they establish themselves brings about a selection between the diverse individuals allowing only a few to survive and reproduce. 

This surviving population has a marked reduction in genetic diversity and variation. Due to its limited size, inbreeding occurs prolifically, further reducing the diversity.

The adaptation of this set of individuals to a new environment causes them to be genotypically as well as phenotypically different from their parent population. 

In extreme cases, it may even cause speciation and eventual evolutionary development of a new species. This cumulative effect is called the founder effect. It may also be observed as a result of a population bottleneck, this is if the surviving population establishes itself in a new environment despite it not being a new population. 

It is often observed and applied in the case of microbiological cultivation of bacterial clones.

Examples

❖ The origin of mankind in Africa, and its subsequent migration to other parts of the planet establishing different races of people is an example of the founder effect.

❖ The French Canadians of Quebec are a result of the migration of the French to Canada between the years 1608 and 1760 as a result of French colonization. The genetic contribution of the original French founders is predominant and account for 90% of the gene pool.

❖ In humans, this effect can also arise due to cultural isolation and endogamy. One such population is that of the Amish people. Due to their inbreeding practices, they have a high incidence of phenomena such as polydactyly (a symptom of Ellis-van Creveld syndrome) and maple syrup urine disease.

❖ Similarly, fumarase deficiency is highly prevalent among the 10,000 members of the Fundamentalist Church of Jesus Christ of Latter Day Saints. This community engages in both endogamy and polygyny, and it is estimated that almost 75 to 80 percent of the community are blood relatives of just two men - John Y. Barlow and Joseph Smith Jessop.

❖ Ashkenazi Jews are highly susceptible to Tay-Sachs disease, a fatal condition in young children.

❖ The Parsi population of India that practices Zoroastrianism is commonly afflicted by developmental and psychological defects due to their policy of endogamy.

❖ The extraordinarily high population of deaf people in Martha's Vineyard has given rise to a form of sign language, unique to that locale.

While both these effects have a similar result on population genetics, they differ in regards to the underlying critical event that gives rise to them. In case of a bottleneck effect, the critical event in the form of an environmental catastrophe or a human activity causes population reduction,...

... whereas, in a founder effect, the critical event is an act of migration of a set belonging to the original population to a new habitat.

These events and their effects cause selection to occur but are not synonymous with natural selection.

The reason being, in case of natural selection, the genes and individuals are selected based on their possession of inheritable traits that would increase survivability (directed process), ...

...but in case of these effects of genetic drift, individuals survive and get selected based on pure chance not due to possession of any favorable attributes (random process).

In 1930, R.A. Fisher proposed that a continuous variation, measured by biometricians, could be produced by the combined action of many distinct genes, and that natural selection altered allelic frequencies in a population, thereby, resulting in evolution. At the same time, E.B. Ford demonstrated the power of selection due to ecological factors.

The combined work of Ford and Fischer caused a shift in emphasis on natural selection over genetic drift. However, recent research on eukaryotic transposable elements and their impact on speciation indicate mutations and genetic drift as major factors influencing the evolution of genomic complexity,...

...and as tools to help translate microevolutionary events into factors that affect macroevolution.