Genetic Drift Random change in allele frequencies from 1 generation to the next. Individuals who survive & pass on their genes are not necessarily the “fittest” Leads to a loss of genetic variation 2 types: Bottleneck & Founder Effect

Genetic Drift: Bottleneck Effect A bottleneck effect is a sudden reduction in the number of alleles in a population

Genetic Drift: Founder Effect Occurs when a small part of a population moves to a new locale (or when the population is reduced to a small size because of some environmental change) The genes of the "founders" of the new society are disproportionately frequent in the resulting population. The smaller the new population, the more likely the allelic frequencies will differ from the original population.

The Founder Effect The Amish community in eastern Pennsylvania was formed by ~200 German immigrants. It is a closed community, and members marry other Amish people. This creates a small gene pool (lack of genetic variability) for offspring, and a founder effect occurred because of this

The Founder Effect Ellis–van Creveld syndrome is a relatively rare autosomal recessive disorder for the US population This disorder occurs more frequently in the Amish community because of the limited gene pool, and is an example of the founder effect. The mutated genes have been traced back to one couple who came to the area in 1744, and it has now been passed on to their kids, grandkids, great grandchildren, and so on.

Founder Effect Some people from the mainland colonize an island Island gene pool is not as variable as the mainland’s Original mainland population

Genetic Drift Examples Cheetahs are so closely related to each other that skin grafts from one cheetah to another do not provoke immune responses. This suggests an extreme population bottleneck in the past. Elephant Seals were reduced to 20 individuals in 1896. Now there are 30,000 individuals with no detectable genetic diversity. Overhunting almost caused extinction, now has begun to recover.

Gene Flow Movement of alleles from one population to another. Occurs when individuals leave one population, join another, and breed. Immigration - alleles entering the gene pool Emigration - alleles leaving the gene pool

Gene Flow Population B Population A Offspring of immigrated bird has a genotype of Hh Offspring of immigrated bird has a genotype of Hh Population B Selection pressure against dominant phenotype has created a homozygous recessive population (hh) Population A Selection pressure against recessive phenotype has created a homozygous dominant population (HH)