NATURAL SELECTION AND EVOLUTION

A fossil is the preserved remains or trace of a dead organism.

Fossils can be formed in several ways:

• Dead animals and plants can be preserved in amber - hardened tree resin - peat bogs, tar pits, or in ice.
• Casts or impressions, such as foot prints, can be covered by layers of sediments. These eventually become rock, so preserving the casts.
• Hard body parts, such as bones, shells and leaves, can be covered by layers of sediments. Over time the parts are gradually replaced by minerals.

-Certain environmental conditions drastically slow down the decaying process, helping to preserve the tissues. Examples of this are:

• insufficient oxygen eg when an organism becomes trapped in amber
• low temperatures eg when an organism becomes frozen in a glacier
• high soil acidity eg when an organism falls into a peat bog

The basic idea behind the theory of evolution is that all the different species have evolved from simple life forms.

The theory of evolution states that evolution happens by natural selection. Here are the key points:

• Individuals in a species showa wide range of variation.
• This variation is because of differences in genes.
• Individuals with characteristics mostsuited to the environmentare more likely to survive and reproduce.
• The genes that allowed the individuals to be successful arepassed to the offspring in the next generation.

Individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed to the next generation. Given enough time, a species will gradually evolve.

Before the industrial revolution in Britain, most peppered moths were of the pale variety. This meant that they were camouflaged against the pale birch trees that they rest on. Moths with a mutant black colouring were easily spotted and eaten by birds. This gave the white variety an advantage, and they were more likely to survive to reproduce.

Airborne pollution in industrial areas blackened the birch tree bark with soot. This meant that the mutant black moths were now camouflaged, while the white variety became more vulnerable to predators. This gave the black variety an advantage, and they were more likely to survive and reproduce. Over time, the black peppered moths became far more numerous in urban areas than the pale variety.

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

The main features in his theory are that:

• individuals compete for limited resources
• individuals in a population show natural variation
• individuals with characteristics best suited to their environment are more likely to survive to reproduce
• 'successful' characteristics are inherited

Species unable to compete successfully eventually become extinct.

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

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 centred on two ideas:

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

His theory stipulated that a characteristic which is used more and more by an organism becomes bigger and stronger. One that is not used disappears eventually. Any characteristic of an organism that is improved through use is passed to its offspring.