explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Natural and Artificial Selection

Natural and Artificial Selection

Learning Objective

Explain how selection, the founder effect and genetic drift, including the bottleneck effect, may affect allele frequencies in populations.

Key Concepts

  • Allele frequency: proportion of a particular allele among all alleles at a locus in a population, denoted \$p\$ (for allele A) and \$q\$ (for allele a), where \$p+q=1\$.

  • Hardy–Weinberg principle: in a large, randomly mating population with no evolutionary forces, genotype frequencies are \$p^{2}\$ (AA), \$2pq\$ (Aa), \$q^{2}\$ (aa).

  • Evolutionary forces that can change \$p\$ and \$q\$: natural selection, artificial selection, genetic drift (including founder and bottleneck effects).

1. Natural Selection

Natural selection is a non‑random process where individuals with genotypes that confer higher fitness produce more offspring. This changes allele frequencies over generations.

  1. Variation: genetic differences (e.g., \$A\$ vs \$a\$) exist in the population.

  2. Differential survival/reproduction: fitness \$w\$ differs among genotypes (e.g., \$w{AA}>w{Aa}>w_{aa}\$).

  3. Inheritance: advantageous alleles are passed to the next generation.

The change in allele frequency due to selection can be expressed as:

\$\Delta p = \frac{p q (w{A} - w{a})}{\bar{w}}\$

where \$w{A}\$ and \$w{a}\$ are the average fitnesses of alleles \$A\$ and \$a\$, and \$\bar{w}\$ is the mean fitness of the population.

2. Artificial Selection

Artificial selection is a human‑directed form of selection. Breeders choose individuals with desirable traits (e.g., larger fruit, disease resistance) and use them as parents.

  • Often involves strong, directional selection.

  • Can rapidly increase the frequency of a target allele, sometimes to fixation (\$p=1\$).

  • May reduce genetic diversity if only a few individuals contribute to the next generation.

3. Genetic Drift

Genetic drift is a random change in allele frequencies due to sampling error in finite populations. It is most pronounced in small populations.

The probability that an allele will become fixed by drift alone equals its current frequency (\$p\$). The expected change per generation is zero, but variance increases with smaller \$N\$ (effective population size).

4. Founder Effect

The founder effect occurs when a new population is established by a small number of individuals from a larger source population.

  • Allele frequencies in the new population may differ markedly from the original due to the limited genetic makeup of the founders.

  • Rare alleles in the source can become common (or fixed) in the founder population, and vice‑versa.

Suggested diagram: A flowchart showing a large source population, a few founders, and the resulting allele frequency shift.

5. Bottleneck Effect

A bottleneck is a sharp, temporary reduction in population size caused by an event such as a natural disaster.

  • Surviving individuals constitute a random sample of the original gene pool.

  • Allele frequencies can change dramatically, and rare alleles may be lost.

  • After the population expands, the new gene pool reflects the post‑bottleneck frequencies.

Suggested diagram: Population size vs. time showing a sharp decline (bottleneck) and subsequent recovery, with allele frequency trajectories.

6. Comparison of Selection and Drift

FeatureNatural/Artificial SelectionGenetic Drift (Founder & Bottleneck)
DirectionalityNon‑random; favors alleles that increase fitness.Random; no preferential direction.
Population size dependenceEffective in any size, but stronger in large populations where selection pressure outweighs drift.Most pronounced in small populations (low \$N_e\$).
Speed of allele frequency changeCan be rapid if selection coefficient \$s\$ is large.Typically slower; depends on stochastic sampling.
Effect on genetic variationMay reduce variation at selected loci but maintain variation elsewhere.Generally reduces overall genetic variation, especially after bottlenecks.
PredictabilityPredictable based on fitness differences.Unpredictable; outcomes vary between replicates.

7. How These Forces Alter Allele Frequencies

  1. Start with Hardy–Weinberg equilibrium: \$p^{2}+2pq+q^{2}=1\$.

  2. Apply the evolutionary force:

    • Selection: use fitness values to calculate new \$p'\$ via the selection equation.

    • Drift: sample \$2N\$ alleles at random; new \$p'\$ follows a binomial distribution \$B(2N, p)\$.

    • Founder/Bottleneck: treat the reduced population as a new \$N\$ and apply drift.

  3. Repeat over generations to observe trajectories toward fixation, loss, or new equilibrium.

Summary

Both deterministic (selection) and stochastic (drift, founder, bottleneck) forces shape the genetic structure of populations. Natural and artificial selection drive adaptive change by favoring beneficial alleles, while genetic drift can randomly increase or eliminate alleles, especially in small or newly founded populations. Understanding these mechanisms is essential for interpreting evolutionary patterns observed in nature and in breeding programmes.