Theory of Evolution (AQA GCSE Biology)

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

Natural selection occurs when individuals with phenotypes best suited for survival reproduce and pass on their advantageous alleles

Natural selection example: peppered moth

Another good example of natural selection is the evolution of the peppered moths

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 scientists 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

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

An example demonstrating Lamarck's ideas on evolution, which we now know are wrong