• In any environment, the individuals that have the best adaptive features are the ones most likely to survive and reproduce
• This results in?natural selection:
• Individuals in a species show a?range of variation?caused by differences in genes
• When organisms reproduce, they?produce more offspring?than the environment is able to support
• This leads to?competition?for food and other resources which results in a?‘struggle for survival’
• Individuals with characteristics?most suited to the environment?have a higher chance of survival and?more chances to reproduce
• Therefore the alleles resulting in these characteristics are?passed to their offspring at a higher rate?than those with characteristics less suited to survival
• This means that in the next generation, there will be a?greater number of individuals?with the?better adapted variations?in characteristics
• This theory of natural selection was put forward by?Charles Darwin?and became known as?‘survival of the fittest’

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An example of natural selection

Natural selection illustrated by snail shell colour

• Within the population of snails there is variation in shell colour
• Normal varieties of shell colours in this snail species is black or grey (as evidenced by the first picture)
• Chance mutations?lead to a small number of snails / one snail having a white shell
• This ‘small number’ is shown in the second diagram where there are less white shelled snails than black or grey shelled snails
• The white shelled snail(s)?survive longer
• 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
• The reason the white shelled snail(s) survive longer is?because they are better camouflaged
• This means that they are?less likely to be seen by predators?and eaten
• 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

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

• ?If the environment?does not change, selection does not change
• This will favour individuals with the?same characteristics?as their parents
• If the environment?changes, or a chance mutation produces a?new allele, selection might now?favour individuals with different characteristics?or with the new allele
• So the individuals that survive and reproduce will have a?different set of alleles?that they pass on to their offspring
• Over time, this will bring about a?change in the characteristics of the species?- it will produce?evolution
• Evolution is defined as the?change in adaptive features of a population over time as a result of natural selection
• Natural selection results in a?process of adaptation, which means that, over generations, those features that are better adapted to the environment become more common
  • This is how?evolution?occurs
• This means populations of organisms become?better suited to their environment
• A good example of this is the?development of antibiotic resistance?by bacteria

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