Genetic Drift and Population Genetics

Corn Pops (Corn Populations)

• $h^2 = 1.0$

Misapplication to Human Races

• IQ scores in the USA: comparing kids of African descent vs. others.
• Studies of post-WWII children raised in Europe, specifically children of African-American US soldiers, showed no significant difference in IQ scores.

Chapter 7: Genetic Drift

• Isolating drift by ignoring selection and mutation.

Genetic Drift

• Definition: Random change in allele frequencies in populations.
• Importance: Along with natural selection, it's a crucial process affecting allele frequencies.
• Randomness: Deterministic events are predictable, whereas random events are unpredictable (probabilistic).

Random Change in Allele Frequency

• Analogy: Like flipping a coin; the probability of an allele frequency going up is equal to the probability of it going down in any generation.
• Difference from Natural Selection: Natural selection causes frequency changes deterministically.

Allele Frequency

• Graphs depicting allele frequency (p) over generations, showing fluctuations.

Fixation and Loss

• Eventually, genetic drift leads to one allele becoming fixed and the other being lost.

Genetic Drift: Probabilistic Nature

• Random but Probabilistic: While we can't predict what will happen in any specific generation.
• Eventual Outcome: Eventually, one allele is fixed, and the other goes extinct.
• Prediction: We can make probabilistic predictions about fixation and loss.

Genetic Drift: Probabilistic Nature

• Graphs illustrating allele frequency changes with percentages.

Genetic Drift

• Random, but probabilistic; unpredictable in any one population.

Allele Frequency Starting at p = 0.5

• Graphs showing 20 populations all starting at $p = 0.5$.
• Example with 9 individuals and 18 gene copies.

Probability

• Graphs illustrating probability over allele frequency.
• Time scales: $t = 0.1N$, $t = 0.2N$, $t = 0.5N$, $t = N$, $t = 2N$ generations.

Probability of Allele Loss and Fixation

• Graph showing the probability of allele loss versus allele fixation.

Examples of Drift

• Observation of initial populations, noting bw homozygotes and bw75 homozygotes.
• Tracking allele numbers across generations (1, 5, 10, 15, 19).

Genetic Drift as Sampling Error

• Concept: Genetic drift is essentially a sampling error.
• Coin Flip Example: Probability of obtaining 7 heads and 3 tails in 10 coin flips is approximately 20%. However, getting 70 heads and 30 tails in 100 flips has a probability of less than 0.0063%.

Rate of Change and Population Size

• The rate at which an allele frequency changes due to drift depends on the population size.

Allele Frequency and Population Size

• Graphs illustrating allele frequency changes over generations for different population sizes:
  • $N = 500,000$
  • $N = 50,000$
  • $N = 5000$
  • $N = 500$
  • $N = 50$
  • $N = 5$

Drift Generalizations

• Allele frequencies fluctuate randomly until one allele becomes fixed (100%).
• Population size affects the rate of allele frequency change.
• The probability of allele A1 becoming fixed is equal to its initial frequency, p.
• Populations with the same initial p will diverge, with some becoming fixed for A1 and others for a different allele (1 – p).
• Heterozygosity (H) decreases proportionally to the rate of drift.
• In many isolated, initially identical populations, average p remains unchanged, but H declines.
• New mutations have a frequency of $1 ÷ 2N$.
• For new mutations that do become fixed, the average time to fixation is $4N$ generations.

Gene Flow

• Homogenizes populations, keeping them similar.
• Genetic drift differentiates populations.

Effective Population Size (Ne)

• Census Size vs. Effective Population Size: Census size isn't always the best measure; we need a term for population size as it affects evolution, which is the effective population size (Ne).

Factors Affecting Effective Population Size (Ne)

• Overlap of generations.
• Sex ratio.
• Small breeding groups (e.g., gorillas).
• Variable fertility.
• Population size fluctuation (e.g., bottlenecks reduce Ne).

Sex Ratio

• Unequal sex ratios can influence allele frequencies in the next generation.
• Example: If the original frequency of A1 is 0.4, an uneven sex ratio can cause the next generation's frequency of A1 to be greater than 0.4.

Small Breeding Groups: Gorillas

• Gorilla social structure impacts effective population size.

Gorilla Behavior

• Generally shy, unless threatened.
• Groups of 5–15, consisting of 1 dominant male, several adult females with young, and occasionally some less dominant males.
• The dominant male remains until displaced, then lives alone.
• Infanticide is common among newly dominant males.
• Food is usually abundant, with little travel and overlapping ranges.
• Nests are built on the ground or in trees.

Small Breeding Groups with Dominant Male

• Diagrams illustrating gene flow dynamics in smaller breeding groups focusing on the dominant male with varying representations of male and female distribution across generations.

Variable Fertility: Sexual Selection Among Males

• Diagram displays unequal likelihood of reproduction among males.

Bottlenecks

• Time and population size ($N$) diagram displays population bottlenecks.

Population Sizes

• Graph comparing effective population sizes (Ne) of various species (Gray whale, Gorilla, Human, Chimpanzee, D. melanogaster, C. remanei, C. elegans, E. coli).

World Population Growth Through History

• Graph showing world population growth through different ages (Stone Age, New Stone Age, Bronze Age, Iron Age, Middle Ages, Modern Age).
• Significant events marked, such as the Black Death (plague).
• Population estimates range from 2.5 million years B.C. to projected figures for 2025 A.D.

Breeding

• An example where a species went from 30,000 to 20 animals in the 1890s.

Drift and Inbreeding

• Number of Snakes graph displaying all snakes vs new juveniles, alongside interventions that reintroduced 20 males from outside populations into the inbred Swedish population.

Drift and Variation

• Drift applies to neutral variation.
• Drift reduces variation.
• Counteracting Process: Mutation can counteract the effects of drift by introducing new variation.