Darwin's Theory of Evolution by Natural Selection (Edexcel IGCSE Biology: Double Science)

• Charles Darwin proposed the theory of evolution by natural selection
  • He came up with this theory as a result of observations from a round-the-world expedition, years of experimentation and his knowledge of geology and fossils

• Evolution can be defined as the change in the frequency of a phenotype in a population over many generations
• Darwin’s theory, very simply, is:
  • 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
  • Therefore these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival
  • Over many generations, these beneficial characteristics become more common in the population and the species changes (the species 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)

The 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

Examples of evolution by natural selection

Natural selection illustrated by snail shell colour

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