Random Genetic Drift

• Random genetic drift causes changes in allele frequencies through random gene sampling.
• Sewell Wright and others in the 20th century developed the principle of random genetic drift, which was unknown to Darwin.
• Allele frequencies are equally likely to increase or decrease, signifying that drift is unbiased.
• Genetic variation within a population is diminished by drift, causing one allele to become fixed.
• Simulations have demonstrated genetic drift.
• Genetically identical populations diverge over time.
• An experiment with Drosophila demonstrated that populations diverge.
• In the Drosophila drift experiment, brown-eyed mutation had no effect on fitness in 107 independent populations.
• Each population in the experiment started with 8 males and 8 females, with an initial mutant frequency of 0.5.
• After 19 generations, 30 populations in the experiment had lost the mutation, and 28 had fixed the mutation.
• All copies of a gene trace back to a single common ancestor.
• Gene trees illustrate the ancestry of gene copies.
• The Most Recent Common Ancestor (MRCA) of human mitochondrial DNA was a woman who lived about 150,000 years ago.
• The MRCA of the Y chromosome was a male who lived about 250,000 years ago.
• Many other individuals existed at the time of the MRCA.
• Different genes have different gene trees.
• Mutations accumulating on the gene tree produce differences in DNA sequences of a gene.
• Population size affects the strength of genetic drift, with smaller populations experiencing more drift.
• The intensity of drift is determined by the effective population size (Ne).
• A population's effective size represents the size of an idealized population with equivalent drift.
• "Idealized" refers to a constant population size where all individuals have an equal chance of producing offspring.
• The effective population size (Ne) is almost always smaller than the actual population size (N).
• The strength of drift is measured by 1/Ne. A small Ne indicates strong drift, while a large Ne indicates weak drift.
• Drift occurs in essentially all species because no population is infinitely large.
• Population size changes affect drift.
• Fluctuations occur in many populations.
• Ne is smaller than the population's average size when populations fluctuate.

Population Bottleneck

• A "population bottleneck" occurs when populations are reduced for a short period
• Intense drift occurs at this time
• Bottlenecks occur when an entire population is reduced in size.
• Bottlenecks occur when a new population is founded by a few individuals.
• Consequences include loss of genetic variation, seen in Northern Elephant seals who were hunted to near extinction.
• Northern Elephant seals are now one of the least genetically-variable mammals.
• Approximately 70,000 years ago some humans migrated out of Africa
• Each time populations are started anew, genetic variability is lost.
• Rare alleles become more common by chance, even if they are deleterious as seen in Polydactyly in Amish.
• Polydactyly is more common in Amish populations than other groups of European descent
• The US population was founded by 200 German immigrants in the early 1700s, now more than 250,000
• The polydactyly was caused by a single mutation in the founding community carried by Anna or Samuel Kings

Drift vs Selection

• Drift acts on all genes, all the time
• Selection acts on many, but not all, genes
• Selection and random genetic drift can occur at the same time.
• The relative strengths of drift and selection can be compared.
• Drift can be ignored when it is much weaker than selection.
• This happens when 1/Ne

Explore random genetic drift, a key evolutionary mechanism where allele frequencies change due to chance events. Simulations and experiments, like those with Drosophila, demonstrate how drift reduces genetic variation, leading to allele fixation and population divergence. Drift is unbiased, with allele frequencies equally likely to increase or decrease.