Theory of Evolution (AQA GCSE Biology): Revision Note

Darwin's theory of evolution by natural selection

• Charles Darwin, as a result of observations on a round-the-world expedition, backed by years of experimentation and discussion and linked to developing knowledge of geology and fossils, proposed the theory of evolution by natural selection

• Darwin’s theory of natural selection states that:

  • individuals in a species show a wide range of variation caused by differences in genes

  • individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce

  • alleles that code for advantageous characteristics are therefore passed to offspring at a higher rate than those that code for characteristics less suited to survival

  • over many generations these beneficial characteristics become more common in the population and the species changes, or evolves

• This idea of natural selection became known as survival of the fittest

• Darwin published his ideas in his famous book, On the Origin of Species (1859)

Natural selection example: snails

• Within the population of snails there is variation in shell colour

  • Normal varieties of shell colours in this snail species are black or grey

• Chance mutations lead to a small number of snails having a white shell

  • This ‘small number’ is shown in the second diagram where there are fewer white shelled snails than black or grey shelled snails

• The white shelled snail(s) survive longer because they are better camouflaged and so are less likely to be seen by predators and eaten

  • This is the ‘survival of the fittest’, a term used to explain why some organisms succeed in the competitive struggle for survival against other members of their population

• As they survive longer they get more opportunities to reproduce and so the allele for white shells is passed onto offspring more frequently than the alleles for black or grey shells

• Over generations this is repeated until the majority of snails in the population have white shells

Natural selection example diagram

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Natural selection example: peppered moth

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Acceptance of evolution

• The theory of evolution by natural selection was only gradually accepted because:

  • There was much controversy surrounding these revolutionary new ideas

  • The theory challenged the idea that God made all the animals and plants that live on Earth

  • There was insufficient evidence at the time the theory was published to convince many scientists

  • The mechanism of inheritance and variation was not known until 50 years after the theory was published

  • The theory of evolution by natural selection developed over time and from information gathered by many scientists

Lamarck's theory of evolution

• Another theory of evolution, developed at the start of the 19th century (before Darwin announced his theory), was that of French scientist Jean-Baptiste Lamarck

• Lamarck’s theory was based mainly on the idea that changes that occur in an organism during its lifetime can be inherited

• His theory involved two main ideas:

  • a characteristic that is used frequently by an organism becomes better and stronger, whereas a characteristic that isn't used gradually disappears

  • the beneficial characteristics that are used frequently (and are improved as a result) are passed to offspring

• For example, Lamarck suggested that:

  • Giraffes had a short-necked ancestor that would frequently stretch its neck to reach the high branches so it could feed on the leaves

  • This repeated stretching could very slowly elongate the giraffe's neck and that this would be passed to the giraffe’s offspring

  • Over time and many generations, the giraffe would evolve to have the very long neck it has today

• It is now known that in the vast majority of cases this type of inheritance cannot occur and that Lamarck’s ideas were incorrect

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