Variation refers to the differences in the characteristics of individuals within a population.

1The genes each individual inherits.

2The environments each individual is exposed to.

Charles Darwin was a 19th century biologist, who is credited with developing and proposing the theory of evolution.

Natural selection is the idea that in each generation, the individuals with the most favourable characteristics are most likely to survive and reproduce, therefore passing on their genes.

It is the idea that 'nature' is 'selecting' the fittest individuals. 

Evolution is the process by which the inherited characteristics of a population change over time due to natural selection.

In some cases this may result in the formation of a new species.

The individuals with the most favourable characteristics are most likely to survive.

Speciation is the evolutionary process by which populations evolve to become distinct species.

Continuous variation is where a characteristic changes gradually over a range of values, such as height, body mass, and skin colour.

These characteristics are usually influenced by multiple genes (polygenic).

Discontinuous (or 'discrete') variation is where a characteristic has distinct forms, with no overlap.

For example, eye colour is discontinuous because you have either blue, green, or brown eyes.

These characteristics are usually only influenced by one or two genes.