Factors that affect allele frequencies

Genetic Variation Most traits in a population vary from one extreme to another (eg. Height, weight)

Allele Frequency How often a particular allele appears in a population. 30% of us have the brown phenotype Homozygous Individuals carry two copies of the allele, while heterozygous individuals carry one.

Gene Flow Movement of individuals between populations moves the alleles carried by the individuals Can change the allele frequency in both populations Can also spread new alleles that arise in one population

Gene Flow Population: 15 blue (75%) 5 Green (25%) Population: 5 blue (25%) 15 Green (75%) Population: 13 blue (65%) 7 Green (35%) Population: 7 blue (35%) 13 Green (65%) Migration between habitats

Genetic Drift Genetic Drift – The gradual change in allele frequencies due to chance. Especially strong in small populations

Genetic Drift Population 17 blue (85%) 3 green (15%) Population 15 blue (93.75%) 1 green (6.25%) Random events cause the death of some individuals

Bottleneck Bottleneck - occurs when a population’s size is reduced. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population’s genetic variation by a great deal

Cheetah wildlifephotos/cheetah.jpg Cheetahs are so closely related to each other that they do not reject skin grafts

Northern Elephant Seal Bottleneck Effect - Reduced to 20 individuals in Now 30,000 individuals, with no detectable genetic diversity

Founder Effect Founder Effect - If a population began with a few individuals, one or more of whom carried a particular allele, that allele may come to be represented in many of the descendants.

Founder Effect Population 15 Blue (75%) 5 Green (25%) Population 2 Blue (50%) 2 Green (50%) Population 13 Blue (82%) 3 Green (18%) Population 8 Blue (50%) 8 Green (50%) Access to new habitat Some individuals migrate Genetic makeup of that group populate new habitat

Founder Effect In the 1680s Ariaantje and Gerrit Jansz emigrated from Holland to South Africa, one of them bringing along an allele for the mild metabolic disease porphyria. Today more than South Africans carry this allele and, in every case examined, can trace it back to this couple — a remarkable example of the founder effect

Tristan de Cunha Islands An 1814, 15 British colonists founded a settlement on Tristan da Cunha, a group of small islands in the Atlantic Ocean, midway between Africa and South America One of the early colonists apparently carried a recessive allele for retinitis pigmentosa, a progressive form of blindness that afflicts homozygous individuals. Of the founding colonists' 240 descendants on the island in the late 1960s, 4 had retinitis pigmentosa. The frequency of the allele that causes this disease is ten times higher on Tristan da Cunha than in the populations from which the founders came.