B2 Revision Cards Natural Selection

Charles Darwin and Natural Selection

Over 150 years ago Charles Darwin wrote his theory of natural selection.
- This explained how evoulution might happen.
- It says that if animals and plants are better adapted to their environment, they and the following genarations are more likely to survive.
He didn't know exactly how adaptions were passed on. When organisms reproduce, their genes are passed on to the next generation.

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The modern version can be summarised like this:
- Within any species there is a variation.
- Organisms produce far more live young than will survive, so their is competition for limited recources such as food.
- Only the best adapted will survive, which is called survival of the fittest.
- Those that suvive pass on successful adaptations for the next generation in their genes.

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Over time, the changes produced by natural selection may result in a new species.
- This only happens if different groups of organisms cannot mate for a long time.
- The organisms might be prevented from mating because they live in different areas. This is called geographical isolation.
  - They might be prevented from mating because of behavioural isolation.
- If each group evolves differently they might over time become different enough to be classified as separate species.

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Natural selection is difficult to study because it usually takes thousands of years to see the effect. Some examples have been studied over shorter time spans:
- More and more bacteria are developing resistance to antibiotics.
- Peppered moths are dark or pale in colour. Dark moths are better camouflaged in polluted areas, so more of them survive.

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At first many people disagreed with Darwin's ideas:
- Some people thought he did not have enough evidence to back up his theory.
- Many people disagreed because they thought God created all species.
Now Darwin's theoryis much more widely accepted. This is because:
- It explains lots of observations.
- It has been discussed and tested and tested by a wide range of scientists.

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There have been other attempts to explain evoulution.
- Before Darwin, Jean Baptiste de Lamarck had a different theory, called the law of required characteristics. This said, for exmaple, that giraffes aquired long necks to feed, and this characteristic was passed on.
As we have discovered more about how genes are passed on, theories like Lamarck's have been proved incorrect and Darwin's theory has become more widley accpected.

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