Causes

• The founder effect occurs whenever a group with a non-random sample of genetic variations is somehow cut off from its original population. The founder effect also happens when a large population is suddenly reduced to only a few remaining members, generally referred to as a bottleneck effect. In human populations, the founder effect frequently occurs as the result of migration, colonization, cultural splits, the imposition of national or geographic boundaries, or endogamy--the practice of marrying exclusively within your ethnicity, class, religion or other social group.
  
  

Results

• As a result of the founder effect, groups exhibit less genetic diversity over time, making them more and more homogenous, in part because of inbreeding. As the genetic lines intermingle, certain genetic variations will disappear from the population, while others will become more and more prominent, as a result of either random selection or natural selection.
  
  

Complications

• Because there is less genetic diversity in a small population, it is more susceptible to extinction. This is because members of that population are more vulnerable to changes in their environment. If a sudden change in the environment occurs, and the population cannot adapt because there are fewer genetic variations for natural selection to utilize, the population may not survive the change. Additionally, the founder effect may cause an increase in the frequency with which recessive alleles are passed on. In particular, this can cause some genetic diseases (which often occur when a member inherits two of the same recessive alleles for the disease) to become more and more common in the population.
  
  

In Non-Human Populations

• The founder effect can be observed perhaps most readily in the study of island ecology. Charles Darwin himself noticed this phenomenon while traveling through the Galapagos Islands, when he noticed that what had originally started out as a single species of finches originating from the coast of South America had over time evolved into 13 different species of finches, each exhibiting different traits depending upon which island it was on.
  
  

In Human Populations

• In human populations, examples of the effects of the founder effect are relatively common because of various historical migrations, isolation and endogamy. One common example is the Amish. Since they started out with only a very few founders, very rarely recruit newcomers and marry only within the community, the Amish exhibit a high level of genetic similarity and a low level of genetic diversity. Another example is the Afrikaner population of South Africa, which kept itself relatively isolated from the larger population throughout the country's history. As one result of the founder effect, they now carry the gene that causes Huntington's disease more often than other populations.