3 31 Evolution & 3.33 Antibiotic resistance

3.31 Describe the process of evolution by means of natural selection.

Evolution is the change in the form of organisms. OR a change in the frequency (how many) of Alleles.

Natural Selection is the mechanism of evolution and was first proposed by Charles Darwin.

In the following example i am going to demonstrate a change in the frequency of alleles by natural selection:


The Eg I am using is Staphlococcus Aureus (SA) which is a Skin Infection and Lung Infection. It can be killed by the antibiotic called Methecilline (M). 


Basically the original form of SA is susceptible to M. However this infection is known to Random Mutation which makes it able to break down M.


Because it can now break down M it is no longer killed by this antibiotic. This is the resistant form of SA


This is an example of evolution because there are now 2 forms of the organism Staphlococcus Aureus (SA)


When antibiotics are applied to the population of SA, the form which is susceptible are decreased, and the form which are resistant increase. This is evolution again because it follows the second definition we have of evolution which is the change in the frequency of Alleles.


There are 2 feature to be noticed here. First the Random Mutation, and the second the Non-Random selection.


Random Mutation produces the form of SA which is resistance to antibiotic M.


Non-Random Selection due to the antibiotic which is selecting the" form of SA which is resistance to antibiotic M" to survive. And for the "form of SA which is susceptible to antibiotic M" to be selected and killed.


These two components here are the classic features of the process of natural selection.

3.33 Understand how resistance to antibiotics can increase in bacterial populations.

Returning to the example of Staphlococcus Aureus (SA) in [3.31], which is a skin or lung infection.  People infected with this can be treated with Methecilline Antibiotic which will kill the SA.

Type which can be killed by antibiotic is the ‘Susceptible SA’.

And a random mutation caused a Resistant form of this infection. ‘Resistant SA’.

The problem here is that the antibiotic is killing the susceptible form of this infection which causes an increase in the resistant form. This is a serious problem because it means the antibiotic no longer works.