Mutations in genetic code occur naturally leading to different alleles in genes. (1 mark)
This leads to variation within a species. (1 mark)
Variation provides some individuals with traits which mean they are more capable of surviving in their specific environment, this is known as a selective advantage. (1 mark)
Those with the selective advantage are more likely to survive and reproduce than those without it. (1 mark)
This means that more offspring will be born with the advantageous allele and will out-compete those without it. (1 mark)
Over thousands of years this process will continue and gradually alter the traits of a species in order to make them more able to survive in their environment.