WEIT 2012 Chap5
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.
From these observations, he comes to the fact that the individuals that have the best traits to live will be more likely to survive and they will be able to have more offspring than the others. This is what he called natural selection.
Is natural selection based on chance ?
While learning Darwinism, many people understand that "everything happens by chance", but it is wrong.
If natural selection was only based on chance, many species would have go extinct.
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.
What is artificial selection ?
Artificial selection is a process which has the same result that natural selection but through a shortest way.
It is a process that consists of a transformation of a plant or an animal into an ameliorated and more adapted form in order to satisfy human's desires in a shortest time than nature selection can.
The most obvious example is the evolution of dogs. All the species have different shapes, temperaments, colours, and sizes however they descend from a single ancestral species which is the Eurasian gray wolf. Of course they evolved because of natural selection over thousand of years but they can't be as different as they are now only through this natural selection. Some dogs for example can run faster than others or are bigger than others. This is not due to natural selection but to artificial selection. For example, because people needed faster dogs in order to win some competitions, breeders changed some traits of the dogs in order to give a size that permits it to run faster than is ancestor.
It shows that artificial selection is a shortest way to obtain what humans want and what nature can make only over thousand of years.
Good start!--Pierre.brawand 15 mars 2012 à 17:34 (CET)
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