Evolution

Evolution is an observation, combined with explanations of that observation. The "fact of evolution" is the observation that families of living things change from one generation to the next. The "theory of evolution" is the explanations of why they change.

In your family, your grandparents are one generation. Your parents, aunts, and uncles are another, and you, your brothers, sisters, and cousins are a third. Even if you looked at photographs of all these people at the same age, you wouldn't find very many people who looked exactly the same. This is the fact of evolution: Your family, like all groups of living things, is changing from generation to generation in small ways, and these changes can add up over many generations and lead to very big differences.

The theory of evolution tries to explain all the patterns found in the entire history of living things. One part of evolution that explains a lot of what we see is natural selection. Natural selection says that since children aren't exactly like their parents, some of them might have some slight advantage that makes them better able to live where they live. Since they have an advantage, these ones are more likely to survive. Children are more like their parents than other unrelated individuals. So the ones with a slight advantage are likely to have children with a slight advantage, or might even end up with some children with a bigger advantage. After many generations in the same place, the great-great-great-great-etc. grandchildren are likely to be really good at living where they live.

A good example is the wing of a bird. Some birds, like penguins, lived near the ocean, and over time their wings evolved into more of a flipper so that they were better suited for swimming. Other birds, like vultures, lived where there was little food, so they had to search for it over long distances. Their wings evolved for soaring so that they wouldn't use as much energy when flying for long periods of time. But other birds, like ostriches, took to the ground, and didn't need their wings much. Their wings evolved simply to show off and make themselves appear larger.

These changes can happen very fast in the smaller, simpler types of living things. For example, many bacteria (a type of "germ") that cause disease can no longer be killed with some of the antibiotic medicines. But these medicines have only been in use about eighty years, and used to work. The bacteria evolved so that they aren't affected by these types of medicine anymore. .

Theory

A modern elephant, with almost no hair. It lives in the heat of Africa

A mammoth - a hairy elephant that lived in the frozen north. They died out 4,500 years ago - this is a model!

There are two main ways that evolution works. Natural selection is probably the easiest to learn. It works like this:

1. In most living things, parents have more offspring (children) than the food and shelter available where they live can sustain. There will be a 'struggle to survive'. Other possible problems might be some of the offspring getting eaten, or only some of them getting to have children.
2. Not all the offspring will be identical.
3. Some of the differences between the offspring can be passed to their children in turn. These are genetic differences.
4. If these genetic differences in some way help the offspring survive and have children, they're more likely have more children. If it hurts their chances, they will probably have fewer or even no children.
5. Since the parents with the helpful genetic differences have more children, more of the children in the next generation have the helpful genetic differences. The parents with the harmful genetic differences may not breed at all, making the harmful genetic differences get lost.
6. After many generations of this, each of the new children will end up with many helpful genetic differences, and few of the harmful ones.

The other way is called genetic drift. Some genetic differences just don't really matter much, like eye colour. Blue or brown or green eyes are all equally good. Since they don't matter, whether they get passed to offspring is just down to chance. It's possible that everyone in one smaller group could end up with only one of the possibilities. This means that all that group's offspring will have just that one possibility for whatever that genetic difference involved. Sometimes, though, these differences can become important later. Let's say that what an organism used to eat is no longer available. Those chance differences may mean that they're unable to eat one new food, but are more able to eat another. But another group might have ended up with the other possibility, and so end up eating the first food. This can cause two groups that began the same to end up very different.

Two groups that start the same can also become very different if they live in different places. Elephants who ended up living in very cold parts of the world became the very hairy mammoths.

When two groups that started the same become different enough, they can become two different species. Evolution predicts that all living things started off the same, but then split off into different groups over billions of years.

Controversy

A number of people oppose and disagree with the evolution idea for a number of reasons, mainly religious. (See creationism)

Other Websites

• The English Wikipedia has an article called Introduction to evolution that covers more details.

• Understanding Evolution - a guide prepared by the University of California at Berkeley
• Howstuffworks.com — How Evolution Works
• The Big Picture on Evolution by the Wellcome Trust.

Videos about evolution

• Evolution - A video series created by the American "Public Broadcasting Service" (PBS).
• Carl Sagan's Evolution of Life Explained
• Carl Sagan's Natural Selection Explained