Natural selection: how it works, with examples

Key ideas

Darwin and Wallace argued that natural selection follows from a few facts about living things (OpenStax Biology 2e, 18.1):

Variation. Individuals in a population differ in their traits.
Inheritance. Many traits are passed from parents to offspring through genes.
Overproduction. More offspring are born than the environment can support, so they compete for resources.
Differential survival and reproduction. Individuals with helpful traits survive and reproduce more, so their alleles become more common in the next generation.

Adaptation is an inherited trait that helps an organism survive and reproduce in its environment.
Fitness in biology means how many offspring an individual leaves that survive to reproduce. It does not mean strength or speed.
Evolution is a change in allele frequencies in a population over generations.

Evidence for evolution
Evidence	What it shows	Example
Fossils	Life forms changed over time, in order	Fossil series of horses and of human ancestors
Homologous structures	Same bones, different uses: shared ancestry	Human, dog, bird and whale limbs
Vestigial structures	Leftover parts that no longer have their old job	Hind leg bones in whales, wings on flightless birds
DNA and proteins	Closer relatives have more similar sequences	The same genetic code in nearly all life
Observed change	Selection seen happening today	Antibiotic-resistant bacteria

Worked examples

Example 1: antibiotic resistance

Problem Explain, using natural selection, why an antibiotic that worked well 30 years ago no longer kills many infections.

Variation: in a large bacterial population, a few cells happen to carry a mutation that makes them resistant.
Selection: the antibiotic kills the susceptible cells. The resistant ones survive.
Reproduction and inheritance: survivors divide and pass on the resistance allele.
Repeated use over many generations makes resistant bacteria the majority. The antibiotic did not create the mutation. It selected for one that already existed.

Answer Resistant bacteria survived and reproduced, so resistance became common

Example 2: one generation of selection, with numbers

Problem A population has 400 brown beetles and 100 green beetles on brown soil. Birds eat 50% of the green beetles and 10% of the brown ones. What percent of the survivors are green?

Before: green is $100 \div 500 = 20\%$ of the population.
Survivors.
$\text{green: } 100 \times 0.50 = 50 \qquad \text{brown: } 400 \times 0.90 = 360$
Share of survivors that are green.
$\frac{50}{50 + 360} = 0.122$

Answer 12.2% green (down from 20%)

Example 3: Hardy-Weinberg (AP Biology)

Problem In a population in Hardy-Weinberg equilibrium, 16% of individuals show a recessive trait (aa). What are the allele frequencies, and what percent are carriers (Aa)?

The Hardy-Weinberg equations: $p$ and $q$ are the frequencies of alleles A and a.
$p + q = 1 \qquad p^2 + 2pq + q^2 = 1$
The aa share is $q^2 = 0.16$ , so $q = \sqrt{0.16} = 0.4$ and $p = 1 - 0.4 = 0.6$ .
Carriers.
$2pq = 2(0.6)(0.4) = 0.48$

Answer q = 0.4, p = 0.6, and 48% are carriers

Common mistakes and how to fix them

Saying organisms change because they need to. Giraffes did not stretch their necks into longer ones. Fix: the variation was already there, and the environment selected it.
Saying individuals evolve. An individual's genes do not change during its life in a way it passes on. Populations evolve as allele frequencies shift across generations.
Thinking fittest means strongest. Fitness is reproductive success. A small, quiet animal that leaves many offspring is fit.
Thinking selection makes perfect organisms. It can only work with existing variation, and what helps can change when the environment changes.

Practice questions

Which of these is NOT part of how natural selection works?
1. Individuals in a population vary
2. Many traits are inherited
3. More offspring are born than can survive
4. Individuals change their genes because they need to
Show answer
Answer: Individuals change their genes because they need to
Natural selection acts on variation that is already present. Organisms cannot change their genes on purpose to fit the environment.
A population has 120 dark mice and 80 light mice on dark lava rock. Owls eat 25% of the light mice and 5% of the dark mice. What percent of the survivors are dark?
Show answer
Answer: 66%
Survivors: dark $120 \times 0.95 = 114$ , light $80 \times 0.75 = 60$ . Dark share: $114 \div 174 = 0.655$ , about 66% (up from 60%).
In a population in Hardy-Weinberg equilibrium, 9% show the recessive phenotype. What fraction are heterozygous carriers?
1. 0.09
2. 0.21
3. 0.42
4. 0.49
Show answer
Answer: 0.42
$q^2 = 0.09$ , so $q = 0.3$ and $p = 0.7$ . Carriers: $2pq = 2(0.7)(0.3) = 0.42$ . 0.49 is $p^2$ , the homozygous dominant share.
In biology, what does fitness mean?
1. How strong or fast an organism is
2. How long an organism lives
3. How many offspring it leaves that survive to reproduce
4. How well it fights off disease
Show answer
Answer: How many offspring it leaves that survive to reproduce
Strength, speed and long life only count if they lead to more surviving offspring.
A human arm, a bat wing and a whale flipper have the same set of bones. What are they called, and what do they show?
1. Homologous structures, showing common ancestry
2. Analogous structures, showing common ancestry
3. Vestigial structures, showing lost functions
4. Homologous structures, showing they have the same job
Show answer
Answer: Homologous structures, showing common ancestry
Homologous structures share a bone layout inherited from a common ancestor, even though they now do different jobs.

Frequently asked questions

Is natural selection the same as evolution?

No. Evolution is any change in allele frequencies over generations. Natural selection is one cause of it, and the only one that produces adaptations. Others are genetic drift (random change, strongest in small populations), gene flow (migration) and mutation.

Is evolution just a theory?

In science, a theory is a well-tested explanation supported by many lines of evidence, not a guess. The theory of evolution by natural selection is supported by fossils, DNA comparisons, homologous structures and changes observed today, such as antibiotic resistance.

Why is the Hardy-Weinberg equation useful if real populations are never in equilibrium?

It is a baseline. It predicts what allele frequencies would be if nothing were changing them: no selection, mutation, migration or drift, with random mating in a very large population. If real data differ from the prediction, something is causing evolution.

Sources

OpenStax Biology 2e, 18.1 Understanding Evolution, accessed October 1, 2026
OpenStax Biology 2e, 19.1 Population Evolution, accessed October 1, 2026
OpenStax Biology 2e, 19.3 Adaptive Evolution, accessed October 1, 2026

Key ideas

Worked examples

Common mistakes and how to fix them

Frequently asked questions

Sources

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