14.2 Evolution by Natural Selection

A free GCSE Biology lesson for AQA (8461) from OBA School: the theory of evolution, the steps of natural selection, and how new species form. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe the theory of evolution by natural selection
• Explain natural selection in steps, using the words variation, competition, survival and reproduction
• Apply natural selection to new examples and data
• Explain how two populations can become separate species

Key words

• Evolution: A change in the inherited characteristics of a population over time, through natural selection. It may produce new species.
• Natural selection: Individuals best suited to their environment are more likely to survive, breed and pass on their alleles.
• Allele: A version of a gene. A new allele first appears through a mutation.
• Competition: Organisms struggling for the same limited resources, e.g. food, water, space or mates.
• Species: A group of organisms that can interbreed to produce fertile offspring.
• Speciation: The formation of a new species.

The theory of evolution

The theory of evolution says that all species of living things have evolved from simple life forms that first developed more than three billion years ago.

Charles Darwin put forward the idea of natural selection to explain how evolution happens. He published it in 1859. Today it is supported by evidence from fossils, DNA and the way bacteria become resistant to antibiotics.

Key fact: Populations evolve, over many generations. A single animal or plant cannot evolve during its own life.

Natural selection step by step

Natural selection is the mechanism of evolution. Learn it as five linked steps – exam mark schemes give a mark for each one.

• Variation: individuals in a population show genetic variation. New alleles appear by mutation.
• Competition: organisms produce more offspring than the environment can support, so they compete for food, space and mates, and face predators and disease.
• Survival: individuals with characteristics best suited to the environment are more likely to survive ("survival of the fittest" – fittest means best suited, not strongest).
• Reproduction: survivors breed and pass on the alleles for the useful characteristic to their offspring.
• Over many generations, the useful allele becomes more common in the population.

Exam tip: Mutations are random. An organism does not mutate "because it needs to". The environment only selects which existing variants survive.

Applying the steps to an example

A beetle species has pale and dark forms. Most were pale, which matched the pale sand of their beach. Then a volcanic eruption covered the beach in black ash.

Worked example: explaining Figure 2

There was variation: some beetles were already dark because of an allele that arose by mutation.
On black ash, birds spotted pale beetles more easily, so dark beetles were more likely to survive.
Dark beetles bred and passed on the allele for dark colour, so it became more common.
Increase = 90 − 5 = 85 percentage points (fastest between generations 4 and 5).

Answer: natural selection favoured the dark allele, so it became common

Worked example: how fast bacteria evolve

A bacterium with a mutation giving antibiotic resistance divides every 20 minutes. If none die, how many resistant bacteria are there after 4 hours?
4 hours = 240 minutes = 12 divisions. Number = 2¹² = 4096

Answer: 4096 – this is why resistance can spread so quickly

How new species form

If two populations of one species are separated (for example by a mountain range, a river or the sea), they live in different conditions. Natural selection favours different alleles in each population, and mutations differ too.

Over a very long time, the two populations may become so different in phenotype that they can no longer interbreed to produce fertile offspring. They have then formed two new species.

Worked example: testing whether two groups are one species

Two groups of frogs mate, but none of the young can ever breed. Are they one species?
No. A species can interbreed to produce fertile offspring. These offspring are not fertile, so the frogs are two separate species.

Answer: two separate species

Common mistakes

• Saying an animal changes or mutates "because it needs to" or "to survive". Mutations are random; the environment selects.
• Saying an individual evolves. Populations evolve over many generations.
• Saying antibiotics make bacteria resistant. The resistant ones were already there; the antibiotic kills the others.
• Missing out the final step: survivors must reproduce and pass on their alleles.

Revision questions

Try these, then check your answers below each one.

Question 1. According to the theory of evolution, all species evolved from simple life forms that first developed how long ago? (Choose from: About 3000 years ago, More than 3 billion years ago, About 3 million years ago, About 300 000 years ago.)

Answer: More than 3 billion years ago

Working: Simple life forms first developed more than three billion years ago.

Question 2. True or false? Antibiotics cause bacteria to mutate so that they become resistant.

Answer: False

Working: False. Resistant bacteria arise by random mutation. The antibiotic kills the non-resistant ones, so the resistant ones survive and multiply.

Question 3. Which is the best definition of a species? (Choose from: A group of organisms with the same number of chromosomes, A group of organisms that can interbreed to produce fertile offspring, A group of organisms that look the same, A group of organisms that live in the same habitat.)

Answer: A group of organisms that can interbreed to produce fertile offspring

Working: Members of a species can interbreed to produce fertile offspring. Looks and habitat are not enough.

Related lessons

• Previous lesson: 14.1 Variation
• Next lesson: 14.3 Selective Breeding
• In this chapter: 14.4 Genetic Engineering
• In this chapter: 14.5 Cloning
• In this chapter: 14.6 Adult Cell Cloning

Continue in the full course

This lesson is section 14.2 (AGCBH1402) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.