๐ฆ Evolution & Natural Selection: Allele Frequencies and the Engine of Change
Explore how natural selection shapes populations over generations, using allele frequencies and quantitative reasoning to understand why advantageous traits spread through populations.
Learning Goals
By the end of this lesson, you will be able to:
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Explain the mechanism of natural selection in terms of variation, heritability, and differential reproductive success
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Calculate and interpret allele and genotype frequencies in a population
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Predict how selection pressures alter allele frequencies across generations
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Distinguish natural selection from other evolutionary mechanisms such as genetic drift
Before You Begin
Before this lesson, you should know:
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Understanding of genes, alleles, and the difference between genotype and phenotype
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Familiarity with dominant and recessive inheritance (basic Punnett squares)
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Ability to work with fractions, decimals, and percentages
Notation & Symbols
Key notation and symbols used in this topic:
p
Frequency of the dominant allele in a population, expressed as a proportion between 0 and 1
q
Frequency of the recessive allele in a population, expressed as a proportion between 0 and 1
p+q=1
The sum of all allele frequencies for a single gene with two alleles equals 1
p2,2pq,q2
Expected genotype frequencies for homozygous dominant, heterozygous, and homozygous recessive individuals
ฮq
The change in recessive allele frequency across one or more generations
The Mechanism of Natural Selection
Natural selection is the process by which heritable traits that improve an organism's reproductive success become more common in a population over successive generations, driving evolutionary change.
Variation: Individuals within a population differ in heritable traits due to mutation and genetic recombination. Without variation, selection has nothing to act upon.
Heritability: Traits must be passed from parents to offspring through genes. Acquired characteristics (not encoded in DNA) are not inherited.
Differential Reproductive Success: Individuals with advantageous traits (higher fitness) produce more surviving offspring, so the alleles for those traits increase in frequency. Fitness is measured by reproductive output, not merely survival.
Allele Frequency Change: Evolution is quantified as a shift in allele frequencies. If a dominant allele has frequency p=0.6, selection may raise or lower this value across generations.
Example
In a beetle population, 60% of alleles for colour are dark (p=0.6) and 40% are light (q=0.4). On dark tree bark, predators eat more light beetles. If light beetles reproduce less, q decreases over generations โ say to 0.3 โ while p rises to 0.7, since p+q=1. This measurable shift is evolution in action.
100%
The diagram shows the recessive light-colour allele frequency declining from q=0.40 to q=0.17 over four generations as dark beetles are favoured by selection.
Worked Example
Basic
In a population of 400 peppered moths, 360 are light-coloured and 40 are dark-coloured. What fraction of the population is dark-coloured, expressed as a decimal?
Worked Example
Intermediate
During an industrial period, soot darkens tree bark. Over one generation the dark moth frequency rises from 0.10 to 0.28. Calculate the change and the percentage increase relative to the original frequency.
Worked Example
Advanced
A beetle population starts with dark allele frequency f0โ=0.20. Each generation the frequency is multiplied by a selection factor of 1.15. Estimate the frequency after 3 generations, and comment on why this model eventually breaks down.
Why Selection Changes Allele Frequency
Understanding the reasoning behind the rule:
Natural selection changes a population because individuals with advantageous traits survive and reproduce more. We can show mathematically why allele frequency shifts across generations.
Your Turn
Try this one yourself โ use the hint if you get stuck:
A bacterial population of 500 cells contains 25 that are antibiotic-resistant. After exposure to the antibiotic, only the resistant cells survive and reproduce until the population returns to 500. What is the resistant frequency before and after exposure, and what does the change illustrate?
Watch Out For...
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Thinking individual organisms evolve during their lifetime to meet needs (e.g., a giraffe stretches its neck and passes on a longer neck).
Evolution acts on populations across generations, not on individuals. Only heritable variation already present in the gene pool can change in frequency through natural selection.
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Believing natural selection has a goal or produces 'perfect' organisms.
Selection is not goal-directed. It only favours traits that improve reproductive success in the current environment; it cannot anticipate future conditions.
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Confusing 'fitness' with physical strength or health.
In evolutionary terms, fitness means relative reproductive success โ the number of offspring that survive to reproduce โ not athletic ability.
In the Real World
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Antibiotic resistance: resistant bacteria survive treatment and reproduce, so resistance allele frequency rises in the population โ a direct, measurable case of natural selection driving public-health policy.
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Artificial selection in agriculture: breeders select crops and livestock with desirable traits, compressing many generations of selection to boost yield, disease resistance, and drought tolerance.
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Pesticide resistance in insects: repeated spraying selects for resistant individuals, forcing engineers and farmers to rotate chemicals and model allele frequencies to manage resistance.
Extended Practice
Test yourself with a mix of multiple choice and short-answer questions:
Q1.
Which of the following is a requirement for natural selection to occur?
MCQ
Q2.
In a moth population, dark moths make up 10% of individuals. After pollution darkens tree bark, dark moths make up 70% a few generations later. What process best explains this?
MCQ
Q3.
Define 'fitness' in the evolutionary sense and give one example.
Fitness is an organism's relative reproductive success โ how many surviving, reproducing offspring it produces compared with others in the population. Example: a beetle whose colour camouflages it from predators survives longer and leaves more offspring, so it has higher fitness.
Q4.
A population of 500 beetles has 125 with allele frequency for a green-colour allele. If green beetles survive predation better and reach 60% of the population after several generations, explain what has happened to the allele frequency and why.
The frequency of the green allele has increased because green beetles have higher survival and reproductive success. As they leave more offspring carrying the green allele, that allele becomes more common in the gene pool over generations โ natural selection in action.
Extension: Quantifying Allele Frequency
Advanced
A harder challenge that connects to the next skill:
In a population of 1000 diploid beetles, 160 are homozygous recessive (bb) for body colour. Assuming Hardy-Weinberg equilibrium, estimate the frequency of the dominant allele B, then predict how selection against bb would change it.
Key Takeaways
1.Natural selection requires heritable variation, differential survival/reproduction, and more offspring than the environment can support.
2.Evolution is a change in allele frequencies within a population across generations โ not change in individuals.
3.Fitness means relative reproductive success, not strength or health.
4.Allele frequencies can be estimated and tracked quantitatively (e.g., using p+q=1 and p2+2pq+q2=1).
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Ready to Practice?
Now that you've learned Evolution & Natural Selection, put it to the test with a quick quiz!