🌍 Changes to the environment
The climate may become hotter, colder, wetter or drier. Food or water may run out. If the animals or plants cannot survive the new conditions, the species dies out.
Understanding Genetics and Evolution » Extinction and Resistant Bacteria
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.6.3.6, 4.6.3.7
- What extinction is and the factors that can cause it
- Why bacteria can evolve so quickly
- How antibiotic resistant strains such as MRSA arise and spread
- How we can slow down the rise of resistant strains
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseWhat is extinction?
Every species on Earth is a group of living things that can breed together. Some species survive for millions of years. Others disappear. Extinction happens when there are no individuals of a species left alive anywhere. Once a species is extinct, it can never come back.
Species do not die out for no reason. Something changes, and the species cannot cope with the change. You need to be able to describe the factors that may contribute to the extinction of a species.
Key terms:
- Extinction: when there are no remaining individuals of a species still alive.
Factors that can cause extinction
Remember that natural selection favours the individuals that are best suited to their environment. A species is in danger when the environment changes faster than the species can adapt. Here are the main factors.
🦟 New predators
A new predator may arrive in an area. The prey may have no defence against it, so too many are eaten and none are left.
🦠 New diseases
A new pathogen may infect a species. If most individuals cannot fight it off, the whole species can be wiped out.
⚔ New competitors
A new species may compete for the same food, water or space. If the new competitor is better at getting these, the original species may not get enough to survive.
☄ A single catastrophic event
A volcanic eruption, a huge fire or an asteroid hitting the Earth can change the environment very suddenly. Many species cannot survive such a rapid change.
🏭 Human activity
People can cause extinction by hunting, destroying habitats, polluting the environment and bringing in new species.
Worked example
A small island bird has no natural predators, so it has never needed to hide or fly away. Cats are then brought to the island by people. This is a new predator. The birds cannot escape, fewer survive to breed each year, and eventually none are left. Several factors can act together, and here a human action led to a new predator.
Why do bacteria evolve so fast?
Bacteria are tiny, single-celled organisms. Many of them reproduce at a fast rate, so a new generation can be produced in a very short time. Evolution happens over generations. The more generations there are, the faster a population can change. This means bacteria can evolve rapidly.
Remember that a mutation is a change in the genetic material. Mutations of bacterial pathogens produce new strains. Most mutations make no difference, but sometimes a mutation makes a strain resistant to an antibiotic.
Key terms:
- Strain: a different type of the same species of bacteria, produced by a mutation.
- MRSA: a strain of bacteria that is resistant to antibiotics.
How resistant strains spread
Here is the step-by-step story. It is natural selection happening in a short time.
- A mutation produces a new strain of the bacterial pathogen that is resistant to an antibiotic.
- The antibiotic is used. It kills the non-resistant bacteria, but the resistant bacteria are not killed.
- The resistant bacteria survive and reproduce, so the population of the resistant strain rises.
- The resistant strain spreads from person to person. People are not immune to it, and there is no effective treatment.
⚠ Why it is a problem
An infection by a resistant strain may not respond to the usual antibiotic. A person can stay ill for longer, and the strain can pass to other people.
🧠 MRSA
MRSA is an example of a bacterium that is resistant to antibiotics. It is a well-known example of the problem in hospitals, because it is hard to treat.
Reducing resistant strains
We cannot stop mutations, but we can slow down the rate at which resistant strains develop. There are three main actions.
- Doctors should not prescribe antibiotics inappropriately. For example, they should not give them for non-serious infections, or for viral infections. Antibiotics do not work on viruses, so using them only gives bacteria more chances to become resistant.
- Patients should complete their course of antibiotics. This means all the bacteria are killed, so none survive to mutate and form resistant strains.
- The agricultural use of antibiotics should be restricted. Using antibiotics less on farms gives bacteria fewer chances to become resistant.
New antibiotics are not the answer
You might think we can just make new antibiotics. But the development of new antibiotics is costly and slow. It is unlikely to keep up with the emergence of new resistant strains. That is why using the antibiotics we already have carefully is so important.
Common mistakes
Do not say that a person becomes resistant to antibiotics. It is the bacteria that become resistant. Do not say that the bacteria 'get used to' the antibiotic or 'learn' to resist it. Resistant bacteria already exist because of a mutation, and the antibiotic simply leaves them alive to reproduce.
Exam-style question
(a) Describe two factors that may contribute to the extinction of a species. [2 marks]
(b) Explain how a strain of bacteria can become resistant to an antibiotic and spread through a population. [4 marks]
(c) Give one way to reduce the rate at which resistant strains develop. [1 mark]
Model answer
(a) Any two from: a new predator (1); a new disease (1); a new competitor (1); changes to the environment (1); a catastrophic event such as a volcanic eruption (1); human activity such as hunting (1).
(b) A mutation produces a new strain of the bacteria that is resistant to the antibiotic (1). The antibiotic kills the non-resistant bacteria but not the resistant ones (1). The resistant bacteria survive and reproduce, so the population of the resistant strain rises (1). The strain spreads because people are not immune to it and there is no effective treatment (1).
(c) Any one from: doctors should not prescribe antibiotics for non-serious or viral infections (1); patients should complete their course of antibiotics (1); the agricultural use of antibiotics should be restricted (1).
Exam tip
In a question about resistance, put the steps in order: mutation, antibiotic kills the others, resistant bacteria survive and reproduce, strain spreads. Each step is a mark.