Describe the Darwinian theory of natural selection (GCSE/A-Level)

Every individual of every species living on the planet has DNA, made up of sections called genes. Mutations to these genes occur over time, resulting in a change to an individual's behaviour or physiology. This can sometimes be advantageous to the survival of the individual such that it is better able to adapt and compete to produce fertile offspring in a challenging environment.Natural selection therefore occurs when chance mutations enable individuals to be better adapted to their environment, increasing their probability of survival, hence passing the advantageous mutation on to future generations. Over time, the proportion of the species with the advantageous allele increases, and thus the species is said to have evolved