Theory of Evolution

Origins of life

Living organisms on Earth today, and the chemicals which work together to keep them alive, are extremely complex, even in single-celled micro-organisms. Some people find it hard to accept that such complexity could have evolved through natural selection. Some religious people believe that all living things on Earth were made by God, or that life was begun by God but then evolved through natural selection. We will probably never be absolutely certain about how life began, as no one was there to observe it. But scientists must base their theories on evidence.

Lamarck

 

Darwin was not the only person to develop a theory of evolution. Jean-Baptiste Lamarck was a French scientist who developed an alternative theory at the beginning of the 19th century. His theory involved two ideas. These are:

• the law of use and disuse
• the law of inheritance of acquired characteristics

His theory stated that a characteristic which is used more and more by an organism becomes bigger and stronger, and one that is not used eventually disappears. Any feature of an organism that is improved through use is passed to its offspring.
This table summarises the two different explanations for giraffes having long necks.

Darwin's theory of evolution

Charles Darwin (1809 - 1882)

Charles Darwin was an English naturalist who studied variation in plants and animals during a five-year voyage around the world in the 19th century. He explained his ideas on evolution in a book called, 'On the Origin of Species', published in 1859.

Darwin's ideas caused a lot of controversy, and this continues today, because they can be seen as conflicting with religious views about the creation of the world and the creatures in it.

The basic idea behind the theory of evolution is that all the different species have evolved from simple life forms. These simple life forms first developed more than 3 billion years ago (the Earth is about 4.5 billion years old). The timeline below shows some of the key events in the evolution of life on Earth, from the first bacteria to the first modern humans.

Selective breeding

Natural selection and selective breeding can both produce changes in animals and plants. The difference between the two is that natural selection occurs in nature, but selective breeding only occurs when humans intervene.
Selective breeding is a process where we choose the characteristics we want in an animal. We then breed together a male and female that show some of those characteristics. From the offspring produced we select those that show the characteristic the most, and breed them together.
This process is repeated over many generations, each time selecting and breeding together those animals that have the characteristics we are looking for. Over a large number of generations, this can produce some surprising results.

All of these dogs were selectively bred from a common ancestor

Farmers have used selective breeding for centuries to increase milk yield in cattle, produce larger eggs from chickens and obtain more grain from wheat.

Causes and effects

Mutations are changes that can occur in genes. These changes are random and can be caused by background radiation and chemicals that we come into contact with, for example the chemicals in cigarette smoke.
Sometimes these changes can be so severe that the cell dies, sometimes the cell can divide uncontrollably and become cancerous, and sometimes the changes are small and the cell survives. Very rarely the changes may even be beneficial to us and produce new and useful characteristics.

Passing on mutations

If these changes occur in normal body cells, the changes are lost when we die. But if the changes occur in our sex cells such as sperm and ova, there is the possibility that the changes in the gene will be passed onto the next generation.
It is when these changes are passed on to the next generation that natural selection can either ensure that they are selected if they are useful, or disappear from the gene pool if they are not.