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B13.2 Evolution by Natural Selection

GCSE Combined Science · AQA · Chapter B13: Variation and Evolution

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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

  • State the theory of evolution by natural selection
  • Describe evolution as a change in the inherited characteristics of a population over time
  • Explain natural selection in steps: variation, competition, survival, reproduction, many generations
  • Apply natural selection to new examples and data
  • Explain how two populations can become two new species

Key words

  • Evolution: A change in the inherited characteristics of a population over time through natural selection. It may result in a new species.
  • Natural selection: Individuals best suited to their environment are more likely to survive, breed and pass on their alleles.
  • Common ancestor: An earlier species from which two or more later species evolved.
  • 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.

The theory of evolution

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

Evolution is a change in the inherited characteristics of a population over time through natural selection, which may result in the formation of a new species. Charles Darwin published the idea of natural selection in 1859. It is now supported by evidence such as fossils and antibiotic-resistant bacteria (see B14).

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

Natural selection step by step

  • Variation: individuals in a population show a wide range of genetic variation. New alleles arise by mutation.
  • Competition: more offspring are produced than the environment can support, so they compete for food, space and mates, and face predators and disease.
  • Survival: individuals with characteristics most suited to the environment are more likely to survive ("fittest" means best suited, not strongest).
  • Reproduction: survivors breed and pass on the alleles for the useful characteristic to their offspring.
  • 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 variants survive.

Applying the steps to an example

A species of snail has plain and banded shells. On open grassland, plain snails were well camouflaged and banded snails were rare. Then farmers planted hedges, and the snails now live among stripy shadows and stems, where thrushes hunt them.

Worked example: explaining the graph

There was variation: some snails already had banded shells, because of an allele that first arose by mutation.
In the hedges, thrushes spotted plain snails more easily, so banded snails were more likely to survive.
Banded snails bred and passed on the allele for bands, so it became more common each generation.
Increase = 91 − 6 = 85 percentage points over 10 generations.

Answer: natural selection favoured the banded allele

How new species form

If two populations of one species are separated, for example on two islands, they live in different conditions. Natural selection favours different alleles in each population, and different mutations happen in each.

If the two populations become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species.

Worked example: one species or two?

A horse and a donkey can mate and produce a mule, but mules are infertile. Are horses and donkeys the same species?
No. A species can interbreed to produce fertile offspring. Mules cannot breed, so horses and donkeys are separate species.

Answer: two separate species

Exam tip: For new species you need: separation of populations, different alleles selected (and different mutations), and a very long time, until fertile offspring can no longer be produced.

Common mistakes

  • Saying an animal mutates "because it needs to" or "to survive". Mutations are random; the environment selects.
  • Saying an individual evolves. Populations evolve over many generations.
  • Missing out the last steps: survivors must reproduce and pass on their alleles.
  • Defining a species as organisms that "can breed". The offspring must be fertile.

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: More than 3 billion years ago, About 6000 years ago, About 65 million years ago, About 3 million years ago.)

Answer: More than 3 billion years ago

Working: Simple life forms first developed more than three billion years ago, and all species evolved from them.

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

Answer: False

Working: Mutations are random. A few bacteria already had a resistance allele; the antibiotic killed the others, so the resistant ones survived and multiplied.

Question 3. Which is the best definition of a species? (Choose from: 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 place, A group of organisms that eat the same food.)

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

Working: A species is a group of organisms that can interbreed to produce fertile offspring.

Related lessons

Continue in the full course

This lesson is section B13.2 (AGCSC1302) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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