WEIT 2012 Chap5

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THE ENGINE OF EVOLUTION

What is natural selection ?

Darwin realises two things:
• The different species produce an excessive number of offspring. As the resources are limited, they can't all survive.
• In the diverse populations, individuals vary in many heritable traits.

What are the conditions for natural selection ?

Natural selection needs three conditions:
• First of all the changing population has to be variable. For example, if the colour of the coat is changing, the population has to be diverse.
• Then, some of the variations that already exist has to come from changes in the genes. Imagine that mice with dark coat are more often eaten than the ones with light ones. If the change weren't in the genes, the process will be random and they will still be dark mice who will still be eaten more often. But if the change was in the genes, natural selection will allow more light ones to born and they will leave longer. That is why variations in the genes are needed.
• Finally, the genetic variation must allow individuals to have more offspring as others.

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