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15.4 Evolution and Speciation

GCSE Biology · AQA · Chapter 15: Genetics and Evolution

A free GCSE Biology lesson for AQA (8461) from OBA School: alfred Russel Wallace and how isolation leads to new species. 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 Wallace’s contribution to the theory of evolution by natural selection
  • Define a species using the idea of fertile offspring
  • Explain step by step how isolation can lead to the formation of a new species
  • Analyse simple data comparing two isolated populations

Key words

  • Species: A group of organisms that can interbreed to produce fertile offspring.
  • Speciation: The formation of a new species.
  • Isolation: Separation of two populations of a species so they can no longer breed together.
  • Geographical barrier: A physical feature such as a river, sea, mountain range or desert that keeps populations apart.
  • Fertile: Able to reproduce. Infertile offspring cannot have offspring of their own.
  • Warning colouration: Bright colours or patterns that warn predators an animal is poisonous or tastes bad.

Alfred Russel Wallace

Alfred Russel Wallace (1823–1913) was a British naturalist. He collected thousands of animal specimens in the Amazon rainforest and then in the islands of South-East Asia (the Malay Archipelago).

  • He worked out the idea of evolution by natural selection independently of Darwin.
  • In 1858 he sent Darwin an essay describing it. Their ideas were presented together to scientists that year, as joint writings.
  • This pushed Darwin to publish On the Origin of Species the next year, in 1859.
  • Wallace gathered evidence from around the world. He is best known for his work on warning colouration in animals and for his theory of speciation.

Example: Wallace thought that bright, easy-to-see caterpillars survive because birds learn they taste bad. This is natural selection in action: the colour is an adaptation.

What is a species?

Members of a species can breed together and their offspring are fertile. Two different species may sometimes mate and have young, but the young are usually infertile.

Worked example: is it the same species?

A horse and a donkey can mate. Their young, a mule, is healthy but almost always infertile.
Because the offspring cannot breed, horses and donkeys are different species.

Answer: different species – the offspring are not fertile

How new species form

New species form when populations become isolated and then change in different ways:

  • 1. Isolation. A barrier such as a new river, a rising sea level or a mountain range splits a population into two. They can no longer interbreed.
  • 2. Genetic variation. Each population contains a range of alleles; new mutations also occur in each one.
  • 3. Different conditions. The two areas differ (climate, food, predators), so different characteristics are useful in each.
  • 4. Natural selection. In each area, individuals with useful characteristics survive and breed, passing on their alleles. Different alleles become common in each population.
  • 5. New species. After many generations the populations are so different that they can no longer interbreed to produce fertile offspring.

Exam tip: Speciation needs isolation + variation + different selection pressures + many generations. The final test is that the populations cannot produce fertile offspring together.

Using data on isolated populations

Worked example: beak depth on two islands

A scientist measures beak depth (mm) of five birds on each of two islands.
Island A: 8.2, 8.6, 8.4, 8.9, 8.4. Mean = 42.5 ÷ 5 = 8.5 mm.
Island B: 10.1, 10.6, 10.3, 10.0, 10.5. Mean = 51.5 ÷ 5 = 10.3 mm.
Percentage difference compared with A = (10.3 − 8.5) ÷ 8.5 × 100 = 21% (2 s.f.).

Answer: birds on island B have beaks about 21% deeper – perhaps suited to harder seeds

A difference in appearance alone does not prove the birds are separate species. Scientists would need to show they cannot breed together to produce fertile offspring.

Common mistakes

  • Defining a species as organisms that “can breed together”. They must produce fertile offspring.
  • Missing out isolation. Without a barrier, the populations keep interbreeding and mixing their alleles.
  • Saying the barrier causes the mutations. Mutations happen randomly; the barrier just stops alleles being shared.
  • Thinking Wallace copied Darwin. He reached the same idea independently, and they were presented together in 1858.

Revision questions

Try these, then check your answers below each one.

Question 1. Alfred Russel Wallace is best known for his work on which two topics? (Choose from: Warning colouration and speciation, DNA structure and genes, Antibiotics and bacteria, Classification and naming.)

Answer: Warning colouration and speciation

Working: Wallace is best known for warning colouration in animals and his theory of speciation.

Question 2. True or false? Wallace and Darwin’s ideas on natural selection were presented together to scientists in 1858.

Answer: True

Working: True. Their joint writings were presented in 1858; Darwin published his book in 1859.

Question 3. Which is the best definition of a species? (Choose from: Organisms that can interbreed to produce fertile offspring, Organisms that look the same, Organisms that live in the same habitat, Organisms that can mate with each other.)

Answer: Organisms that can interbreed to produce fertile offspring

Working: A species is a group whose members can interbreed to produce fertile offspring.

Related lessons

Continue in the full course

This lesson is section 15.4 (AGCBH1504) 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.

 

 

 

 

 

 

 

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