15.3 Accepting Darwin's Ideas

A free GCSE Biology lesson for AQA (8461) from OBA School: why the theory of natural selection was resisted, and the evidence that changed minds. 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

• Give the reasons why Darwin’s theory was only gradually accepted
• Describe the evidence that has built up to support evolution by natural selection
• Explain how a successful prediction supports a scientific theory
• Use data on resistance to show evolution happening today

Key words

• Theory: A scientific explanation that is supported by a large body of evidence and can be used to make predictions.
• Evidence: Observations and measurements that can be used to support or reject an idea.
• Peer review: Checking of a scientist’s work by other experts in the same field before it is published.
• Fossil: The remains or traces of an organism from many thousands of years ago, found in rocks.
• Resistance: The ability of an organism to survive a chemical, such as an antibiotic or poison, that would normally kill it.

Why people objected

When Darwin published On the Origin of Species in 1859, many people refused to accept it. There were three main reasons:

• It challenged religious beliefs; What it means Most people believed God had created every species in its present form. The idea that humans shared ancestors with apes upset many people.
• Not enough evidence; What it means Many scientists did not think Darwin had enough evidence. Few fossils of in-between forms had been found.
• No mechanism for inheritance; What it means Nobody knew how variation was caused or how characteristics were passed on. Genes and DNA were not discovered until more than 50 years later.

Exam tip: For the “mechanism” reason, say that the way characteristics are inherited and the cause of variation (genes, mutations) were not known at the time.

Building up the evidence

Over the next 150 years, evidence from many areas of science supported Darwin’s theory.

• Genetics: after Mendel’s work was rediscovered in 1900, genes explained how characteristics are inherited. Mutations explained where new variation comes from.
• Fossils: many more fossils have been found, including forms that link major groups, such as feathered animals with features of both reptiles and birds.
• DNA: comparing DNA shows how closely species are related, and the patterns match those from fossils.
• Evolution we can watch: organisms that breed quickly evolve in a few years. Bacteria become resistant to antibiotics, insects to pesticides and rats to the poison warfarin.

Predictions that came true

A good theory lets scientists make predictions that can be tested. Evolution predicts that fossils of in-between forms should exist, in rocks of the right age.

Worked example: a successful prediction

Fish appear in the fossil record before four-legged land animals. Scientists predicted that animals part-way between the two should be found in rocks about 375 million years old.
They searched rocks of that age in northern Canada and in 2004 found a fish-like animal with a flat head, neck and sturdy fins with wrist-like bones.

Answer: the prediction was correct, so confidence in the theory increased

New findings are published in journals after peer review, so other scientists can check the methods and repeat the work. When many independent studies agree, a theory becomes widely accepted. Today the theory of evolution by natural selection is accepted by almost all biologists.

Evolution happening now

Worked example: rats and warfarin

A pest control company surveys rats on farms where the poison warfarin is used. At the start, 2 in every 100 rats are resistant. After 8 years, 85 in every 100 are resistant.
Increase = 85 − 2 = 83 percentage points.
Explanation: a few rats had a mutation giving resistance. Warfarin killed the others, so resistant rats survived and bred, passing on the allele.

Answer: resistance rose by 83 percentage points in 8 years – natural selection in action

Common mistakes

• Saying Darwin knew about genes. Nobody knew the mechanism of inheritance until after 1900.
• Thinking a theory is “just a guess”. A scientific theory is backed by a lot of evidence and makes testable predictions.
• Saying the poison causes the resistance mutation. The mutation was already present; the poison selects for it.
• Giving only “religion” as a reason. Lack of evidence and no known mechanism were just as important for scientists.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these was not a reason why many people rejected Darwin’s theory in 1859? (Choose from: It challenged the belief that God made all living things, There was not enough evidence to convince many scientists, The way characteristics are inherited was not known, It was based on DNA evidence that people did not trust.)

Answer: It was based on DNA evidence that people did not trust

Working: DNA was not understood until the mid-1900s. The real reasons were religion, lack of evidence and no known mechanism of inheritance.

Question 2. True or false? Darwin knew that genes were responsible for passing on characteristics.

Answer: False

Working: False. Genes were not understood until more than 50 years after Darwin published.

Question 3. What is peer review? (Choose from: Checking of scientific work by other experts before it is published, Repeating your own experiment three times, Voting by the public on whether a theory is true, Comparing your results with a friend’s.)

Answer: Checking of scientific work by other experts before it is published

Working: Peer review means other experts check the methods and conclusions before publication.

Related lessons

• Previous lesson: 15.2 Theories of Evolution
• Next lesson: 15.4 Evolution and Speciation
• In this chapter: 15.1 The History of Genetics
• In this chapter: 15.5 Evidence of Evolution
• In this chapter: 15.6 Fossils and Extinction

Continue in the full course

This lesson is section 15.3 (AGCBH1503) 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.