AQA GCSE Biology: Extinction and Antibiotic Resistance

Species can die out completely, and bacteria can evolve to resist the drugs we use against them. This guide covers the causes of extinction and how antibiotic resistance develops and spreads, which is evolution happening fast enough to watch. The link between them is natural selection acting on populations.

Extinction

Extinction is when no living individuals of a species remain. It can be caused by:

• new predators or hunting;
• new diseases;
• better-adapted competitors;
• long-term environmental changes, such as ice ages, drought or habitat destruction;
• catastrophic events, such as asteroid strikes or volcanic eruptions.

Underneath all five is one idea. A species survives by being suited to its surroundings, and natural selection can only change a population over many generations. When the surroundings change faster than the population can adapt, the species dies out.

That explains why a catastrophic event is so destructive. An asteroid strike changes the climate in a matter of months, which is far quicker than selection can work on a large animal that takes years to breed. It also explains why species with small populations or slow reproduction are the most vulnerable, and why a species that is already restricted to one habitat has nowhere to go when that habitat is destroyed.

Antibiotic-resistant bacteria

Bacteria evolve resistance to antibiotics by natural selection, and this happens quickly because they reproduce so fast.

• Random mutations produce some bacteria that are resistant.
• An antibiotic kills the non-resistant bacteria.
• The resistant bacteria survive and reproduce.
• The resistant population grows and spreads.

The order of those steps is where most marks are lost. The mutation comes first, and it is random. The antibiotic does not cause resistance and does not teach bacteria anything. It kills everything that is not already resistant, which leaves the resistant ones with no competition and plenty of room to multiply.

Phrases like "the bacteria became resistant so they could survive" or "the bacteria adapted to the antibiotic" describe an organism changing itself on purpose, and they will not earn the mark. The population changes because the individuals that happened to carry a useful mutation were the ones that lived to reproduce.

Bacteria are a good illustration of this because they reproduce roughly every twenty minutes, so a change that would take a mammal thousands of years takes them a few days.

MRSA is a well-known example of an antibiotic-resistant bacterium. It is a particular problem in hospitals, where antibiotics are used heavily, and where many people are already ill and so less able to fight an infection off.

Slowing down resistance

The development of resistance can be slowed in several ways:

• avoiding unnecessary prescriptions of antibiotics;
• always completing a course of antibiotics;
• restricting the use of antibiotics in agriculture.

This matters because developing new antibiotics is slow and costly, so we struggle to keep pace with new resistant strains.

Each of the three has a reason behind it. Unnecessary prescriptions, including for viral infections, which antibiotics cannot treat, expose bacteria to the drug for no benefit, and every exposure is another round of selection. Completing a course matters because stopping early kills only the most sensitive bacteria and leaves the slowest-dying ones alive, and those are exactly the individuals most likely to carry partial resistance and pass it on. Agricultural use, where antibiotics have been given to healthy livestock to promote growth, selects for resistant bacteria that can then reach people.

How this fits together

Extinction and antibiotic resistance both show natural selection shaping populations. The final part of Topic 6 is about how we organise and name the huge variety of living things, in the classification guide.