Survival of the fittest
- Natural selection was first described as 'survival of the fittest' by Charles Darwin.
- Reporduction is wasteful- animals/plants always produce more offspirng than the enviroment can hold.
- Individual species in any organism show lots of variation because of differences in genes inherited.
- Only offspring with genes best suited to the enviroment will stay alive and breed successfully. This is natural selection.
- E.g. Rabbits with the best overall eyesight, sharpest hearing and the longest legs will be the most likely to escape from a predator.
- They will therefore pass on these useful genes to their offspring.
- The slower and less alert rabbits will get eaten and their genes become increasingly less likely to be passed on.
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Natural selection in action
- Malpeque Bay, Canada has very large oyster beds.
- In 1915 fishermen noticed a few small flabby oysters with puss-filled blisters among their normal, healthy catch.
- By 1922 the oyster beds were almost empty, the oysters had been wiped out by a new disease.
- A few oysters had a mutation which made them resistant to the disease and these oysters were the only ones to survive and breed.
- A new population of oysters had been born and the whole population were resistant to the new disease because the oysters that were originally resistant bred to give these new oysters.
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The part played by mutation
- New forms of genes cause changes in existing genes, this is known as mutations.
- Mutations occour naturally through mistakes when copying DNA when the cells divide.
- Mutations introduce more variety to the genes of a species and this is very important to survival.
- Occassionally mutations have a positive effect because it produces an adaptation to make the organism better suited to its enviroment.
- Whether the adaptation is useful or not it will be passed on to the next generation.
- The mutant gene will gradually become more common in the population and it will cause the species to evolve.
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