The outsiders ever find the island, but the

The founder effect is one way that nature can randomly create new species from existing populations.

In this lesson, learn about the founder effect and how it can be seen in all humans across the globe.

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The Logic Behind the Founder Effect

Think about the following scenario: A random group of ten men and ten women are suddenly stranded on a tropical island. Nineteen of the castaways have green eyes and one has blue eyes. The castaways decide that they have no chance of rescue, but they have plenty of supplies to start a new civilization. No outsiders ever find the island, but the civilization flourishes and many generations are born.Now, consider this question: What color of eyes will most people on the island have? Considering that all but one of the original castaways had green eyes, you would be correct if you guessed that most of the descendants would likely have green eyes.

You may not know the exact term for this phenomenon, but you have just demonstrated the logic behind what is known as the founder effect.

How the Founder Effect Works

A sequence of DNA that codes for a trait, such as eye color, is called a gene. Alleles are alternative forms of specific genes that are responsible for variations in a trait, such as green versus blue versus brown eyes.

By examining the number of people that have each of these different eye colors, you can determine the frequency of the alleles in the population.Occasionally, throughout history, small populations of a species have moved to an area that is sufficiently distant or physically isolated from the original population. This isolation prevents breeding between the two populations. By random chance alone, the allelic frequencies of one or more genes in the new population can be quite different than those of the original population. This shift in allelic frequency due to the creation of a new, isolated population is called the founder effect. Using the example of eye color from above, if a small group of people with only green eyes is isolated on an island, the allelic frequency of green eyes in the new (founder) population will be much higher than that of the original (source) population.The founder effect can occur during a migration if a small population moves sufficiently far from the home territory to prevent any interbreeding.

The founder effect is also evident on islands. Small populations isolated on islands, arriving either via flight or floating on debris, can have different allelic frequencies simply by chance. If the founder population has alleles that impact their survival, either positively or negatively, evolution can lead to greater divergence between the two populations. Eventually, the founder population can become a new species, related to the original but unable to interbreed.

Examples of the Founder Effect

There are several classic examples of the founder effect. We’ll start with the Pennsylvania Amish. In the 1700s, a small group (i.

e., a founder population) of Europeans settled in Eastern Pennsylvania. Among this small group was an individual who carried an allele for Ellis-van Creveld syndrome. Ellis-van Creveld syndrome is a very rare form of dwarfism, causing short stature, extra fingers (known as polydactyly), abnormal teeth and nails, and heart defects. The allele for Ellis-van Creveld syndrome is found at a frequency of 7% in the Pennsylvania Amish in comparison to only 0.1% in the general population. The low allelic frequency of 0.

1% was also the allelic frequency of the original European population from which the Amish migrated.The higher allelic frequency in the Amish community is most likely due to the founder effect. While the Amish live in close proximity to large, diverse human populations that would be capable of breeding, the culture of the Amish restricts marriage outside of the group. This results in genetic isolation and group interbreeding that allows the frequency of the allele for Ellis-van Creveld syndrome to not only persist but increase over time.

Another example is the blood types of Native Americans. The original colonizers of the Americas most likely arrived by crossing the Bering Strait land bridge around 20,000 years ago and gradually moved south through North America and into South America. This founder population probably had many allelic variations from the original population.While we have very little information on the allelic variation of the original population, today it is very rare to find a Native American with Type B blood. This suggests that in the founder population, the occurrence or frequency of the Type B blood allele was very low.

During much of the history of North America, until the arrival of the Europeans, the Native Americans, for the most part, would have been geographically isolated. This isolation, over thousands of years, resulted in the low frequency of Type B blood in Native Americans observed today.In a more contemporary example, in the 1980s, a pair of scientists were studying the large ground finches on the Galapagos Islands.

One island in particular, Daphne Major, attracted their attention. The island was a frequent foraging stopover for the finches, but they never stayed to reproduce on the island. In 1982, five finches stayed to breed, produced offspring, and established a permanent founder population.The scientists took physical measurements from both the original population of 238 birds and the founder population, which included 5 original birds and 22 direct offspring. The scientists discovered that the founder population had significantly larger beaks.

The change occurred over such a short time span that the size differences could not be explained by evolution via gradual change, the kind that could take many generations over many years. The five founder finches must have had larger beaks or a higher frequency of alleles for larger beaks. As a result of the founder effect, a population of finches with larger beaks has established itself on Daphne Major.

The Global Founder Effect

The founder effect can also be examined at the global scale.

The current theory of human evolution hypothesizes that humans evolved in Africa and migrated out in small bands to the rest of the globe. The founder effect should apply here as well. If you consider allelic frequency as the amount of possible variations of all genes, not just specific genes, you would suspect that people of native African descent should have greater allelic diversity across all genes in the genome. Populations farther removed from Africa, such as Europeans or Native Americans, should show less allelic variation.This is exactly what the genes illustrate.

Native Africans have a much greater allelic variation than non-Africans. Sequential migrations of groups of individuals, each travelling farther from the original population, created new founder populations with less allelic diversity than the African source population.If you are curious about your own founder effect, cheap genome sequencing is now allowing individuals to determine their allelic frequencies and trace their ancestors’ migration out of Africa!

Lesson Summary

Let’s review.

The founder effect essentially describes one cause of genetic variation, or lack thereof, in a population. Specifically, a sequence of DNA that codes for a trait, such as eye color, is called a gene. Alleles are alternative forms of specific genes that are responsible for variations in a trait, such as green versus blue versus brown eyes. By examining the number of people that have each of these different eye colors, you can determine the frequency of the alleles in the population.

Occasionally, throughout history, small populations of a species have moved to an area that is sufficiently distant or physically isolated from the original population. This isolation prevents breeding between the two populations. By random chance alone, the allelic frequencies of one or more genes in the new population can be quite different than those of the original population. This shift in allelic frequency due to the creation of a new, isolated population is called the founder effect.This effect can be found in any population, human or animal.

Classic examples include the high occurrence of a rare genetic disorder among the Pennsylvania Amish, the low frequency of Type B blood among Native Americans, and the large-beaked finches in the Galapagos.The global founder effect refers to our current theory of evolution, which hypothesizes that humans evolved in Africa and migrated out in small bands to the rest of the globe. Based on the founder effect, you would suspect that people of native African descent should have greater allelic diversity across all genes in the genome and populations farther removed from Africa, such as Europeans or Native Americans, should show less allelic variation.

This is exactly what the genes illustrate! Native Africans have a much greater allelic variation than non-Africans.

Learning Outcomes

Review the facts of this lesson the founder effect so that you can subsequently:

  • Compare genes and alleles
  • Discuss the founder effect and understand how it occurs
  • Reference classic examples of the founder effect
  • Detail the global founder effect and its relationship to the current theory of evolution
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