view updated


Wikipedia contributors


Evolution is a scientific idea used by biologists. It explains how living things changed over a long time, and how they have come to be the way they are.

The Earth has been around for a very long time. By doing research on the layers of rock, we can find out about its past. That kind of research is called historical geology.

We know that living things have changed over time, because we can see their remains in the rocks. These remains are called 'fossils'. So we know that the animals and plants of today are different from those of long ago. And the further we go back, the more different the fossils are. How has this come about? Evolution has taken place. That evolution has taken place is a fact, because it is overwhelmingly supported by many lines of evidence. At the same time, evolutionary questions are still being actively researched by biologists.

Comparison of DNA sequences allows organisms to be grouped by how similar their sequences are. In 2010 an analysis compared sequences to phylogenetic trees, and supported the idea of common descent. There is now "strong quantitative support, by a formal test", for the unity of life.

The theory of evolution is the basis of modern biology. "Nothing in biology makes sense except in the light of evolution".


A) Fossils Show that Change Has Occurred

Evolution of horses

The ancestors of our horses lived in forests.

The evolution of the horse family (Equidae) is a good example of the way that evolution works. The oldest fossil of a horse is about 52 million years old. It was a small animal with five toes on the front feet and four on the hind feet. At that time, there were more forests in the world than today. This horse lived in woodland, eating leaves, nuts and fruit with its simple teeth. It was only about as big as a fox.[19]

About 30 million years ago the world started to become cooler and drier. Forests shrank; grassland expanded, and horses changed. They ate grass, they grew larger, and they ran faster because they had to escape faster predators. Because grass wears teeth out, horses with longer-lasting teeth had an advantage.

For most of this long period of time, there were a number of horse types (genera). Now, however, only one genus exists: the modern horse, Equus. It has teeth which grow all its life, hooves on single toes, great long legs for running, and the animal is big and strong enough to survive in the open plain. Horses lived in western Canada until 12,000 years ago, but all horses in North America became extinct about 11,000 years ago. The causes of this extinction are not yet clear. Climate change and over-hunting by humans are suggested.

So, scientists can see that changes have happened. They have happened slowly over a long time. How these changes have come about is explained by the theory of evolution.

B) Geographical Distribution

This is a topic which fascinated both Charles Darwin and Alfred Russel Wallace.[21][22][23] When new species occur, usually by the splitting of older species, this takes place in one place in the world. Once it is established, a new species may spread to some places and not others.

C) Common Descent

When biologists look at living things, they see that animals and plants belong to groups which have something in common. Charles Darwin explained that this followed naturally if "we admit the common parentage of allied forms, together with their modification through variation and natural selection".

D) Artificial Selection

Charles Darwin lived in a world where raising animals and domesticated crops were vitally important. In both cases farmers selected for breeding individuals with special properties, and prevented the breeding of individuals with less desirable characteristics. The eighteenth and early nineteenth century saw a growth in scientific agriculture, and artificial breeding was part of this.


Evolution mainly works by natural selection. What does this mean? Animals and plants which are best suited to their environment will, on average, survive better. There is a struggle for existence. Those who survive will produce the next generation. Their genes will be passed on, and the genes of those who did not reproduce will not. This is the basic mechanism which changes a population and causes evolution.

Natural selection explains why living organisms change over time to have the anatomy, the functions and behaviour that they have. It works like this:

1) All living things have such fertility that their population size could increase rapidly for ever.

2) We see that the size of populations does not increase to this extent. Mostly, numbers remain about the same.

3) The food and other resources are limited. Therefore, there is competition for food and resources.

4) No two individuals are alike. Therefore, they will not have the same chances to live and reproduce.

5) Much of this variation can be inherited. Parents pass such traits to the children through their genes.

6) The next generation can only come from those that survive and reproduce. After many generations of this, the population will have more helpful genetic differences, and fewer harmful ones. Natural selection is really a process of elimination. The elimination is being caused by the relative fit between individuals and the environment they live in.



Adaptation is one of the basic phenomena of biology. Through the process of adaptation, an organism becomes better suited to its habitat.

Adaptation is one of the two main processes that explain the diverse species we see in biology. The other is speciation (species-splitting or cladogenesis). A favorite example used today to study the interplay of adaptation and speciation is the evolution of cichlid fish in African rivers and lakes.

When people speak about adaptation they often mean something which helps an animal or plant survive. One of the most widespread adaptations in animals is the evolution of the eye. Another example is the adaptation of horses' teeth to grinding grass. Camouflage is another adaptation; so is mimicry. The better adapted animals are the most likely to survive, and to reproduce successfully (natural selection).

An internal parasite (such as a fluke) is a good example: it has a very simple bodily structure, but still the organism is highly adapted to its particular environment. From this we see that adaptation is not just a matter of visible traits: in such parasites critical adaptations take place in the life cycle, which is often quite complex.