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Charles Darwin is famous for his ideas about evolution. The theory of evolution explains the origins of all species of animal, plant, fungi, bacteria and viruses.
Darwin explained how evolution works, basing his ideas about three observations: variation, the struggle for existence and the survival of the fittest.
We are all slightly different: taller or shorter, fatter or thinner, different hair colours and so on. Some of this variation is caused by the environment in which we live. For example, if we eat too much we get fatter and if we don’t eat enough we will get thinner. Another example that is easy to understand is hair length: it continually grows, but only gets long if you don’t cut it. This environmetal variation will not have much evolutionary effect, because it is not passed on the the next generation.
Hair colour is an example of genetic variation (excluding the vain people who dye or bleach their hair). Eye colour is also inherited (of course there are also those who wear coloured contact lenses: more vanity). You can read about genetics and the inheritance of blood groups on this site. Gregor Mendel explaind how he thought that genetic inheritance worked. The idea about evolution is that each generation may be slightly different;
Struggle for Existence
We all need to find shelter, find food, escape from predators and find a mate.
All living things need to find places that are not too hot or too cold, not too dry or too wet. We need to get food: plants need sunlight, moisture and nutients from the soil. Plants get eaten by herbivores: if grass grows faster than it gets eaten by cows, sheep or rabbits it will survive, but if there are too many herbivores it will die out. This is overgrazing. We don’t usually have to worry about getting killed and eaten by lions and tigers, but we do have to worry about London buses: look both ways before crossing the road!
Eventually we may die of old age, but before we do, we need to pass our genes onto another generation.
Survival of the fittest
So we are all slightly different and struggle to survive and reproduce. That is fine if there is no evolutionary pressure: easy to find somewhere to live, easy to find food, no predators to kill us, and easy to find a husband or wife. We will all survive and become parents, grand parents or even great grand parents.
Evolution occurs when there is evolutionary pressure. Perhaps there is not enough food for everyone or too many predators, perhaps the climate is too extreme. The result of evolutionary pressure is that some individuals do not survive, some survive but do not reproduce. It is only the fittest that contribute to the next generation.
The word “fittest” does not have to mean fittest in the sense of PE. The fittest might be the individuals who run the fastest to escape a predator, but it might be the ones best at hiding from their predators.
Some butterflies have a nasty taste and bay even be poisonous, so they are avoided by birds. Insect eating birds learn to avoid the unpleasant tasting butterflies.
Hedgehogs and porcupines have spines that stop carnivores catching them.
Toads have a bitter tasting mucus on their skins that dogs, cats and foxes don’t like.