Core idea
Natural selection is the mechanism driving evolutionary change.
It acts on existing variation within a population, not on individuals trying to change.
Over time, it causes heritable advantageous traits to become more common, helping explain the biodiversity of life on Earth.
Exam link: evolution = change in allele frequencies over generations.
Darwin’s theory replaced Lamarckism by providing a convincing mechanism for evolution.
Paradigm shift = a major change in scientific thinking when a better explanation replaces an older one.
Sources of variation
Mutation creates new alleles.
Sexual reproduction creates new combinations of alleles.
Natural selection can only act if there is variation between individuals.
Key exam wording: selection acts on phenotypes, but evolution is measured as change in allele frequencies.
Conditions needed for natural selection
There is variation between individuals in a population.
More offspring are produced than can survive: overproduction of offspring.
Individuals compete for limited resources such as food, space, territory, light, water, or mates.
Some individuals survive and reproduce more successfully because of their heritable traits.
These successful alleles become more common in the next generation.
Selection pressures
Selection pressure = any factor that affects survival or reproductive success.
Abiotic factors can act as selection pressures, for example high temperature, low temperature, drought, or other density-independent factors.
Biotic pressures also matter in practice, especially competition, predation, and mate choice.
Always link pressure → differential survival/reproduction → change in allele frequency.
Fitness and survival
Fitness means the survival value and reproductive potential of a genotype.
The “fittest” organism is not necessarily the strongest; it is the one that leaves more surviving offspring.
Natural selection commonly occurs through intraspecific competition.
Better-adapted individuals are more likely to survive, reproduce, and pass on alleles.
Exam trap: fitness is about reproductive success, not just lifespan.
This image is useful for discussing sexual selection and the classic guppy system, where colour patterns can be shaped by both predation and mate choice. Source
Heritability
For evolutionary change to occur, the trait must be heritable.
Acquired characteristics caused by the environment during an individual’s lifetime are not heritable if they do not change the base sequence of genes.
Therefore, natural selection does not act by passing on traits acquired by use/disuse.
Good exam phrase: only genetically based variation can be inherited.
Sexual selection
Sexual selection is a type of selection pressure involving success in attracting mates.
Traits that improve mating success may spread even if they have survival costs.
It can involve physical traits or behavioural traits.
These traits can act as signals of overall fitness.
Syllabus example: plumage of birds of paradise.
This image illustrates sexual selection, where elaborate male plumage can increase mate attraction and therefore reproductive success. Source
Interpreting natural selection in data
In exam questions, describe the pattern as: variation exists → selection pressure acts → individuals with the advantageous phenotype have higher fitness → their alleles become more frequent.
Do not say organisms “needed” to change.
Do not say individuals evolved; populations evolve.
For graph/data questions, always state whether there is a change in phenotype frequency, genotype frequency, or allele frequency.
John Endler’s guppy experiments
Students must be able to interpret data from John Endler’s experiments with guppies.
Core idea: guppy colour patterns are influenced by a balance between natural selection (for example, avoiding predators) and sexual selection (female mate choice).
In environments with stronger predation, conspicuous males are more likely to be removed.
In environments with lower predation, brighter male colour may increase due to mate preference.
In data questions, identify the selection pressure, then link it to differential reproductive success.
This page explains guppies as a classic example of the interaction between natural selection and sexual selection, helping with interpretation of Endler-style questions. Source
Artificial selection vs natural selection
Artificial selection = humans deliberately choose which individuals breed based on desired traits.
Used in crop plants and domesticated animals.
Natural selection is not goal-directed; it results from environmental selection pressures.
Unintended outcomes such as antibiotic resistance in bacteria are due to natural selection, not artificial selection.
Best comparison:
Artificial selection: selector = humans
Natural selection: selector = environment / selection pressures
HL only: Gene pool and allele frequencies
Gene pool = all the genes and their different alleles present in a population.
Natural selection changes the allele frequencies of the gene pool.
Geographically isolated populations may show different allele frequencies because different selection pressures act on them.
When answering HL questions, connect trait advantage to increase in frequency of the allele(s) underlying that trait.
Neo-Darwinism = Darwin’s natural selection combined with genetics.
This image helps visualize how allele frequencies relate to genotype frequencies and provides a baseline for showing when a population is or is not evolving. Source
HL only: Types of selection
Directional selection: one extreme phenotype is favoured; the population mean shifts in one direction.
Stabilizing selection: the intermediate phenotype is favoured; extremes are selected against.
Disruptive selection: both extremes are favoured; intermediates are selected against.
All three types can lead to changes in allele frequency.
In graph questions, focus on which phenotype has the highest fitness.
This diagram is ideal for exam revision because it shows how different forms of selection change the distribution of phenotypes in a population. Source
HL only: Hardy–Weinberg equilibrium
If a population is in genetic equilibrium, allele frequencies stay constant from generation to generation.
Use p and q for two allele frequencies.
p + q = 1
p² + 2pq + q² = 1
Meanings:
p² = frequency of homozygous dominant genotype
2pq = frequency of heterozygous genotype
q² = frequency of homozygous recessive genotype
If one genotype frequency is known, you can calculate allele frequencies using the same equations.
HL only: Hardy–Weinberg conditions
For Hardy–Weinberg equilibrium to hold, the required conditions must be maintained:
Large population
Random mating
No mutation
No migration / gene flow
No natural selection
If observed genotype frequencies do not fit Hardy–Weinberg expectations, then one or more conditions are not being met.
Common exam conclusion: the population is evolving if equilibrium is not maintained.
Checklist: can you do this?
Explain natural selection using the sequence: variation → selection pressure → differential survival/reproduction → inheritance → change in allele frequency.
Distinguish between natural selection, sexual selection, and artificial selection.
Interpret data from John Endler’s guppy experiments.
Identify whether selection shown in a graph is directional, stabilizing, or disruptive.
Use the Hardy–Weinberg equations to calculate allele frequencies and genotype frequencies.
Common exam mistakes to avoid
Saying individuals evolve instead of populations evolve.
Saying traits appear because organisms need them.
Forgetting that the trait must be heritable.
Using fitness to mean strength instead of reproductive success.
Describing natural selection without mentioning allele frequency change.
Confusing natural selection with artificial selection.
Forgetting that sexual selection can favour traits that may reduce survival but increase mating success.
One-line exam model answer
Natural selection is the process by which individuals with advantageous heritable traits survive and reproduce more successfully than others, causing the alleles for those traits to increase in frequency in the population over generations.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.