BIOL 105: Lecture 07 Genetic Drift

Defining Genetic Drift

• Genetic drift is defined as a random process that leads to changes in allele frequencies within a population over time.
• It is a mechanism of evolution alongside natural selection and gene flow.
• Evolution is fundamentally a change in allele frequencies in a population over time.
• Random processes have different possible outcomes, but the specific outcome is unpredictable.
• An example of a random process is flipping a coin, where the probability of heads or tails is usually 50%, but the actual result of any single flip is uncertain.

Genetic Drift Explained

• Genetic drift is a change in allele frequencies caused by “random sampling” in populations.
• Imagine a jar of marbles to understand genetic drift.
• It can be observed over time by tracking the frequency (proportion) of one allele.
• Drift is the random change of allele frequencies from one generation to the next due to “random sampling.”
• Genetic drift is always acting at some level in real populations and represents the constant “background noise” of evolution

Causes of Random Sampling

• “Random sampling” in real populations is caused by any process that randomly adds or subtracts fitness, irrespective of genotype.
• Oogenesis serves as an example; the allele that becomes the ovum is determined by effectively random “coin flips” during gamete formation.
• The “meiosis lottery” is a significant source of random sampling.
• Random environmental events, such as natural disasters, can kill individuals randomly.
• Resources and mates can be randomly encountered or lost.

Impact and Significance of Genetic Drift

• Drift is the sum of all these sources of randomness. It results in alleles changing randomly in frequency, similar to coin flips or sampling marbles.
• Because drift is always occurring:
  • It is the main driver of allele frequency change (evolution) at the genetic level.
  • It is the “null hypothesis” when testing for other evolutionary processes (e.g., selection).

Buri Drift Experiment

• The Buri drift experiment provides an example of genetic drift.
• Most populations in Buri’s experiment fixed (100% frequency) one of the two alleles.
• Hence, drift caused genetic diversity to be lost.
• Buri’s Experiment shows how drift causes the loss of genetic diversity.

Population Size and Strength of Genetic Drift

• The effects of genetic drift are more extreme in small populations.
• In smaller populations, there is a higher chance of fixing one of the alleles.
• Drift causes more variable and extreme changes in allele frequency in smaller populations.
• Alleles are fixed more rapidly in small populations, resulting in a loss of variation.
• All populations started at p=0.5, but have evolved genetic differences via drift. Smaller pops evolve differences faster.

Census Size vs. Effective Population Size

• Census Size (N_c):
  • The count of all individuals in the population.
  • This can be obtained by directly counting individuals or using a mark-recapture approach.
• Effective Population Size (N_e):
  • The number of breeding individuals in an idealized population that would show the same amount of genetic drift as seen in the population being studied.
  • Ne dictates the strength of drift, not Nc!

Factors Affecting Effective Population Size

• Not every individual contributes to the gene pool equally.
• For example:
  • In some animal populations, a single male may monopolize multiple females, preventing some males from breeding.
• Population size may also fluctuate through time.
• Reductions in effective population size can cause drift to become stronger.

Genetic Bottleneck

• The bottleneck effect magnifies the effect of genetic drift.
• A genetic bottleneck occurs when a population is greatly reduced in size.
• The bottleneck limits the genetic diversity of the species because only a small part of the original population survives.
• Cheetah populations have extremely low genetic diversity due to genetic bottlenecks.

Founder Effect

• Founder effects occur when some individuals become isolated from a larger population.
• The migrating population establishes a new founder population, which, after a few more generations, may exhibit different allele frequencies compared to the parent population.

Detecting Genetic Drift

• Drift causes populations to lose genetic variation.
• Genetic variation in a population can be quantified by heterozygosity.
• Populations that have experienced more drift are expected to have lower heterozygosity.
• Pacific wrens display a classic signature of the founder effect, with island populations showing much lower genetic diversity than mainland populations.
• Drift and founder events have shaped human evolution.

Discussion

• Reading a meta-analysis that investigates the relationship between population size and genetic variation in wild populations.
• Reinforcing ideas about how population size affects the strength of genetic drift

Key Points About Genetic Drift

• It is unbiased – the frequency of any allele is just as likely to go up as to go down (unlike selection).
• It is stronger in smaller populations – smaller “samples” = more drift.
• It causes genetic variability to be lost – Allele frequencies that fluctuate will eventually reach 0 or 1.
• It causes populations to become different – This can give populations the appearance of specialization to a site.
• Drift causes alleles to fix (reach 100%) – even in the absence of all selection.

Genetic Drift and Natural Selection

• Genetic drift is always occurring, even when selection is acting!

Interaction Between Selection and Drift

• Mutations with large fitness effects (s) can easily overcome drift.
• In contrast, mutations with small fitness effects (s) cannot overcome drift.
• In small populations, drift can cause deleterious alleles to fix!
• When drift is weaker (larger N_e), smaller effect mutations can overcome it.

Relative Strengths

• The ability of selection to overcome drift depends on their relative “strengths.”
  • Strength of drift = 1/N_e
  • Strength of selection = s
• If 1/N_e > s for an allele, drift will overwhelm selection.
• In small populations, natural selection needs to be very strong to overcome drift.
• In large populations, weakly selected alleles can overcome drift.

Fixation

• Fixation = an allele reaches 100%.
• Drift is random but will eventually cause the fixation (or loss) of all alleles.

Long-Term Effects of Drift

• In the long term, drift causes the eventual fixation of one allele.
• If we start with two alleles, A and a, at a locus:
  • In the long term, there are only two outcomes:
    • A = 100%, a = 0%
    • a = 100%, A = 0%

Fixation Probabilities

• Fixation probabilities are based on current allele frequencies.
• The probability that an allele will eventually fix is simply its current frequency (p).
• P_{fix} = p

New Mutations

• New mutations occur in a single copy in the population. Diploids have two copies of their genome, so the initial frequency of a mutation is 1/2N.
• P{fix} for a new mutation will fix is simply: P{fix} = 1/(2N)

Summary of Genetic Drift

• Genetic drift is the change in allele frequencies caused by chance events. It does not favor one allele or another.
• Drift tends to reduce genetic variation within a population and causes differences among populations to accumulate.
• The strength of drift is determined by the effective population size, where drift is stronger in smaller populations.
• An allele will evolve largely as if selection is not acting when s << 1/Ne, while it will evolve largely as if drift is not acting if s >> 1/Ne.