mechanisms of evolution: drift & draft

Drift (+ forms)

Selectively neutral allele frequences drift by random change

• selective neutrality

  • no differential reproduction or natural selection when different alleles of a certain gene confer equal fitness benefits or do not affect fitness 

    • → random reproduction of alleles 

      • eg. human eye color

• Drift halts when an allele eventually becomes fixed, either by disappearing from the population or replacing the other alleles entirely

• 2 forms: founder effect & bottleneck

founder effect (form of genetic drift)

high frequencies of specific genetic traits inherited from the few common ancestors who first had them

• affects especially small populations with few founding members

• effects of recessive alleles on phenotype

  • eg. Ellis-Van Creveld Syndrome in Amish people

  • eg. Salla disease in Finland/Sweden

Ellis–van Creveld syndrome in Amish people

polydactyly: extra finger/toe

Amish people = religious group from Dutch ancestors who migratede to US

• religious beliefs: stay & marry in own group → little gene pool → If there are some carriers of a recessive allele in that limited group, the chance increases that children inherit the allele from both parents and thus become homozygous → disease then manifests

bottleneck (form of genetic drift)

Rapid dramatic changes cause most individuals to die without passing on their genes → The few survivors of these evolutionary "bottlenecks" then are reproductively very successful → large populations in subsequent generations

• extraordinary reduction in genetic diversity of a species since most variability is lost at the time of the bottleneck

• Low genetic variation among modern humans implies bottleneck(s) in our evolutionary past

  • we descend from a small number of humans who survived an evolutionary bottleneck

  • examples: drought in Africa & Toba supervolcano eruption

differences & similiarities between bottle neck effect & founder’s effect

They both lead to loss of genetic diversity but they have very different mechanisms

• bottleneck: original population is reduced bcs the majority of the individuals died in some sort of bottleneck event => surviving population has much lower degree in genetic diversity → all those alleles that went extinct with the indiviudals that died remain extinct

• founder: from the current population there is a smaller migrating population were the allele frequencies might not be representative for the original parent population => in successive generations those alleles that are carried by the original founders can become much higher in frequency

Draft (=hitch hiking)

• linkage: genes that are located close together in a chromosome tend to be inherited together

• hitch-hiking: a netral polymorphism is linked to alleles that are undergoing a selective sweep

  • fitness benefits → neutral allele increases (until it become fixed)

  • non-advantage → neutral allele decrease (to extinction)

drift & draft are both used for…

maintenance of genetic variation