Natural Selection

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Natural Selection

"Survival of the fittest". That is the bit that everyone remembers about natural selection. But what does it really mean?

The world inhabited by living things is very harsh. They have to compete for everything such as light, food, shelter and mates.

All this and they have to avoid getting eaten!

Only the best suited will survive long enough to reproduce, and so pass on their genes to the next generation.

Organisms try to ensure that their offspring have advantages in survival. Many strategies are used to ensure this; some produce huge numbers of them so that at least a few will survive, others try to protect and shelter them for a long time - and care for them through school!

But where did these ideas about natural selection originate?

Charles Darwin is the one who is credited with coming up with the idea of natural selection, although there were others too, notably Alfred Wallace.

Darwin noted four things from his observations:

  1. All organisms produce an abundance of offspring, many more than is necessary.
  2. There is a fairly constant population size over time for any particular organism
  3. Within a species there is a wide range of features, due to different alleles
  4. Some variations are passed on and inherited by the offspring

From these Darwin decided that all organisms struggle for survival and so have to produce many offspring to ensure that some do survive.

He also concluded that for a species to survive, the best (or 'fittest') of them must survive long enough to reproduce and pass on their genes.;

As usual, someone else had a different idea.

Jean Baptiste de Lamarck had the idea that plants and animals evolve features according to how much they use them.

Lamarck's idea means that giraffes evolved long necks by reaching up for leaves in trees. At one time they would have had short necks but then they got longer.

Why didn't all the other animals get longer necks too?

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An experiment that disproved Lamarck's idea was that mice who had their tails cut off - sound's like a nursery rhyme - still gave birth to baby mice with tails!

If Lamarck had been right the parent's wouldn't use their tails and neither would their offspring.

Mutations are alterations in the DNA sequence of genes.

Most mutations are harmful and do not benefit the organism, they give rise to things like inherited diseases

However occasionally a mutation occurs which gives an organism an advantage over the others within its species. It gives a better chance of survival.

This is quite often seen in bacteria which can develop antibiotic resistance and therefore survive our attempts to kill them.

Other examples could include the development of different shape teeth that enable other types of food to be eaten when the traditional food source is scarce, or stronger legs that could allow you to run faster and avoid getting eaten.

You could think about any characteristic and think about how it might have arisen.

Why do some animals 'die out' and become extinct? The dinosaurs, hairy mammoth, dodo and many others are all extinct.

There are 3 ways in which extinction can occur:

1. A rapid environmental change to which they can't adapt, this could be a natural change such as flooding or manmade such as the building of a city.

2. A new threat may occur, perhaps a new predator or disease kills them.

3. Failure to compete successfully against a new species.

While some organisms will die out others will increase in number. Those that can cope do so by gradually developing characteristics that suit the new conditions over many generations.

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