Evolution can be a change in the form of organisms
Evolution can be a change in the frequency (how many) of alleles.
Natural selections is the mechanism of evolution and was first proposed by Charles Darwin.
A bacteria staphlococcus aureus which can lead to skin and lung infection is introduced.
The original form of such bacteria is sustained to be kill by methecilline, which is a type of antibiotic. (They are susceptible to the antibiotic.)
What happens is that a random mutation to the genome of the bacteria allowed us a characteristic of 'breaking down methecilline.' This means that it is no longer killed by the antibiotic. This new form is called the Resistant form, MRSA.
[Refer to definition number 1 of evolution]
Because the MRSA is resistant to the antibiotic, they became increasingly common (increase in frequency of the allele)
[Refer to definition number 2 of evolution]
Two features of natural selection (process not a thing):
1. Random mutation - produce MRSA form
2. Non-random selection - due to anti-biotic which is selecting the MRSA to survive and MSSA to be selected and killed