Natural selection: B1 7.3

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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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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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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