15.2 Theories of Evolution

A free GCSE Biology lesson for AQA (8461) from OBA School: lamarck’s idea of acquired characteristics, Darwin’s theory of evolution by natural selection. 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 what the theory of evolution says about how life on Earth developed
• Describe Lamarck’s theory and explain why it was rejected
• Describe the work Darwin did and the theory he published
• Explain evolution by natural selection step by step for a new example
• Compare Lamarck’s and Darwin’s explanations of the same change

Key words

• Evolution: A change in the inherited characteristics of a population over time, through natural selection, which may result in the formation of new species.
• Variation: Differences between individuals of the same species. Variation can be genetic, environmental or both.
• Mutation: A change in the DNA. Mutations happen all the time and create new alleles; most have no effect.
• Natural selection: The process by which individuals with characteristics best suited to their environment are more likely to survive and breed, passing on their alleles.
• Acquired characteristic: A feature an organism develops during its life, such as bigger muscles or a scar. It is not caused by its genes.
• Adaptation: A feature that helps an organism survive in its environment.

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 3 billion years ago. Living species are related through common ancestors. Two scientists who tried to explain how evolution happens were Lamarck and Darwin.

Lamarck’s theory

Jean-Baptiste Lamarck was a French biologist (1744–1829). He suggested that:

• a part of the body that is used a lot gets bigger and stronger, and a part that is not used gets smaller;
• these changes, made during an organism’s lifetime, are passed on to its offspring. This is the inheritance of acquired characteristics.

Worked example: a Lamarckian explanation

Lamarck would say a wading bird stretched its legs to keep its feathers dry in deeper water. Its legs grew a little longer during its life, its chicks inherited the longer legs, and so on, until the species had very long legs.

Answer: the change happens in the individual first, then is inherited

Why it was rejected: characteristics acquired during life do not change the DNA in sex cells (gametes), so they cannot be inherited. For example, a person who builds big muscles at the gym does not have children with bigger muscles. In the 1880s, August Weismann cut the tails off mice for many generations, yet every new generation was born with normal tails. Once genes and DNA were understood, Lamarck’s mechanism was rejected.

Darwin and natural selection

Charles Darwin (1809–1882) sailed around the world on HMS Beagle from 1831 to 1836. He collected and studied plants, animals and fossils, especially on the Galápagos Islands, where similar species differed from island to island. Back home he read widely, bred pigeons and experimented with plants. In 1859 he published On the Origin of Species.

Worked example: explaining a change in stages

A population of snails lives in a wood where thrushes eat them. Shell colour varies from pale to dark brown because of different alleles.
On the dark leaf litter, pale snails are seen and eaten more often. Dark snails are more likely to survive and breed.
They pass on the alleles for a dark shell, so more of the next generation are dark.
Over many generations, dark shells become the most common colour in the wood.

Answer: variation → selection → survival and breeding → alleles passed on → over many generations

Exam tip: Always use the words variation, survive, breed (reproduce), alleles / genes passed on and many generations in a natural selection answer.

Comparing the two theories

• Where variation comes from; Lamarck Organisms change during life by using or not using body parts; Darwin Variation already exists in the population (we now know it comes from mutations and sexual reproduction)
• What is passed on; Lamarck Acquired characteristics; Darwin Inherited characteristics (alleles) of the survivors
• Who changes; Lamarck Each individual changes; Darwin The population changes; individuals do not evolve
• Status today; Lamarck Rejected; Darwin Widely accepted, supported by lots of evidence

Common mistakes

• Writing that organisms change “because they need to” or “to survive”. Mutations are random; selection then favours some of them.
• Saying individuals evolve. Individuals survive or die; it is the population that changes over many generations.
• Saying the environment causes the useful mutation. The variation was already there before conditions changed.
• Leaving out reproduction. Surviving is not enough – the organism must breed to pass on its alleles.
• Thinking “survival of the fittest” means the strongest. It means the best suited to the environment.

Revision questions

Try these, then check your answers below each one.

Question 1. What was Lamarck’s idea about how evolution happens? (Choose from: Inheritance of acquired characteristics, Natural selection of random variation, Formation of species by isolation, Creation of each species separately.)

Answer: Inheritance of acquired characteristics

Working: Lamarck proposed that characteristics acquired during a lifetime (e.g. from using a body part) are inherited.

Question 2. True or false? A woman who builds large muscles at the gym will pass larger muscles on to her children.

Answer: False

Working: False. Bigger muscles are acquired; they do not change the DNA in her eggs, so they are not inherited.

Question 3. In which year did Darwin publish On the Origin of Species? (Choose from: 1809, 1831, 1859, 1953.)

Answer: 1859

Working: 1859. (1809 is the year Darwin was born; 1831 is when the Beagle voyage began.)

Related lessons

• Previous lesson: 15.1 The History of Genetics
• Next lesson: 15.3 Accepting Darwin's Ideas
• In this chapter: 15.4 Evolution and Speciation
• 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 15.2 (AGCBH1502) 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.