Natural Selection and Antibiotic Resistance

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.38 — Darwin's theory of evolution by natural selection
• 3.39 — How antibiotic resistance increases in bacterial populations and why that matters

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Natural selection is often described as a theory about change, but it is easier to follow as an argument about who leaves offspring. It has four steps, each of which is an observation rather than a speculation, and the conclusion follows from them whether you like it or not. That structure is worth learning as a structure, because every question on this topic is really asking you to run it on a new example.

The argument, in the four steps every answer needs

1. VARIATION. The individuals in a population are not identical, and some of those differences are genetic — the result of mutation, meiosis and random fertilisation.
2. COMPETITION. More offspring are produced than can survive, so individuals compete for limited resources such as food, water, light, territory and mates, and face predators and disease.
3. SURVIVAL OF THE BEST ADAPTED. The individuals whose characteristics happen to suit those conditions are more likely to survive long enough to reproduce.
4. INHERITANCE. Because those characteristics are genetic, the survivors pass the alleles for them to their offspring — so the proportion of the population carrying those alleles increases in the next generation.
5. REPEAT that over many generations and the population changes. Over a very long time, and with populations separated from one another, it produces new species.

Now the single most important thing on this page, and the one examiners test relentlessly. The variation comes FIRST. Individuals do not develop useful characteristics because their environment demands them — mutations are random, as the last lesson established, and they occur whether or not they would be useful. What the environment does is decide which of the existing variations gets passed on. Selection is a filter, not a designer.

Think of it like a sieve, not a sculptor

A sculptor decides what the finished shape will be and works towards it. A sieve does nothing of the kind: it holds whatever happens to be too large to pass through, and if nothing of that size was put in, nothing of that size comes out. Natural selection is the sieve. It cannot produce a characteristic that was not already present as a variation, it has no target in mind, and it does not improve anything — it simply removes what does not pass. Everything that looks designed is the accumulated result of many generations of sieving on variation that arrived at random.

The same argument, running fast — antibiotic resistance

1. In any large population of bacteria, RANDOM MUTATION produces a few individuals with an allele that makes them resistant to a particular antibiotic. This happens before the antibiotic is ever used.
2. When the antibiotic is given, it kills the non-resistant bacteria. The resistant ones survive.
3. The survivors now face no competition from the bacteria that have been killed, so they reproduce — and bacteria reproduce very rapidly, dividing as often as every twenty minutes.
4. Their offspring inherit the resistance allele, so the proportion of resistant bacteria in the population rises.
5. Repeat this whenever the antibiotic is used, and the whole population eventually becomes resistant. The antibiotic no longer works, and infections caused by that strain become very difficult to treat.

Notice what did NOT happen there. The antibiotic did not cause the resistance, and the bacteria did not become resistant in response to it. The resistant bacteria existed before the antibiotic arrived; all the antibiotic did was remove their competition. That distinction is the difference between a Grade 9 answer and a Grade 6 one on this topic, and it is the same point as the sieve.

Bacteria make the process visible for two reasons. They reproduce extraordinarily quickly, so hundreds of generations pass in a few days rather than a few centuries. And a population of them is enormous — a single infection can contain billions of individuals — so even a mutation with a very low probability per division is almost certain to occur somewhere in the population. Rarity stops being a protection when the numbers are that large.

Why this matters, and what reduces it

1. Strains resistant to several antibiotics at once — MRSA is the familiar example — leave very few treatments available, and infections that were routine become dangerous again.
2. Doctors should not prescribe antibiotics for viral infections, because antibiotics do not affect viruses and the prescription selects for resistance in the patient's harmless bacteria for no benefit at all.
3. Patients should COMPLETE the full course. Stopping early kills the most susceptible bacteria and leaves the more resistant ones alive, which is precisely the selection you were trying to avoid.
4. Antibiotic use in farming should be reduced, since the same selection happens in livestock and resistant bacteria do not stay where they are.
5. New antibiotics must keep being developed — but resistance to a new one will also arise in time, so this buys time rather than solving the problem.

One last observation about the shape of the argument. Nothing in it is specific to bacteria, or to antibiotics, or to any particular organism — it needs only variation, competition, differential survival and inheritance. That is why the same four steps explain a peppered moth becoming darker in an industrial region, a population of rats becoming resistant to a poison, and the beak of a finch on a dry island. If you can state the four steps and then attach the details of the example, you can answer any version of this question you are given.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Natural selection

The process by which individuals with characteristics best suited to their environment are more likely to survive and reproduce, so that the alleles for those characteristics become more common in the population over generations.

Learn this definition · Evolution

The change in the inherited characteristics of a population over successive generations, as a result of natural selection acting on genetic variation.

Statement 3.38 — the four steps, in the order marks are awarded

1. There is genetic VARIATION within a population, caused by mutation, meiosis and random fertilisation.
2. More offspring are produced than can survive, so individuals COMPETE for limited resources.
3. Individuals with characteristics best suited to the environment are more likely to SURVIVE and reproduce.
4. They pass the alleles for those characteristics on to their OFFSPRING.
5. Over many generations the proportion of the population carrying those alleles increases, and the population changes.

Statement 3.39 — antibiotic resistance, the five marking points

1. Random mutation produces a few bacteria that are resistant to the antibiotic — this happens BEFORE the antibiotic is used.
2. When the antibiotic is given, the non-resistant bacteria are killed and the resistant ones survive.
3. The survivors face no competition from those killed, and bacteria reproduce very rapidly.
4. Their offspring inherit the allele for resistance.
5. The proportion of resistant bacteria in the population increases, and eventually the antibiotic no longer works against that strain.

The distinction the whole topic turns on — and the commonest way to lose the marks

A. CORRECT: the resistant bacteria were already present through random mutation, and the antibiotic killed their competitors. Variation comes first; the environment selects.

B. WRONG: the bacteria became resistant because the antibiotic was used, or adapted in order to survive it. Nothing changes an organism because a change would be useful.

Why resistance matters, and what reduces its spread

• Strains resistant to several antibiotics, such as MRSA, leave very few treatments available and make routine infections dangerous
• Do not prescribe antibiotics for VIRAL infections — antibiotics do not affect viruses, and the prescription selects for resistance for no benefit
• COMPLETE the full course: stopping early kills the most susceptible bacteria and leaves the more resistant ones alive
• Reduce antibiotic use in farming, since the same selection happens in livestock
• Develop new antibiotics — though resistance to those will arise in time, so this buys time rather than solving the problem

Applying the four steps to any example — what to name in each

Variation

Name the characteristic that varies, and say the variation is genetic and arose by mutation.

Competition

Name what the individuals are competing FOR — food, light, territory, mates — or what is killing them.

Survival

Say which individuals are MORE LIKELY to survive and reproduce, and why that characteristic helps in these conditions.

Inheritance

State that the survivors pass the alleles to their offspring.

Over generations

State that the proportion of the population with the characteristic increases. Populations change; individuals do not.

The three classic examples, and the selection pressure in each

A. PEPPERED MOTHS: dark moths were rare until soot darkened the trees; on darkened bark they were less visible to predators, so more survived to reproduce. The variation existed first.

B. RATS AND WARFARIN, or bacteria and antibiotics: a few individuals already carried an allele conferring resistance; the poison or drug killed the rest, leaving the resistant ones to reproduce without competition.

Not this: Bacteria become resistant to an antibiotic because they are exposed to it.

This: Resistant bacteria arise by random mutation before the antibiotic is used. Exposure kills their competitors, which is why the resistant ones then increase — the antibiotic selects, it does not create.

Not this: Organisms evolve characteristics because they need them.

This: Variation arises randomly and without regard to need. Individuals that happen to be suited to the conditions survive and reproduce, so their alleles become more common. Nothing is aiming at anything.

Model answer [5 marks]

Explain how a population of bacteria becomes resistant to an antibiotic. [5]

Random mutation in some bacteria produces an allele giving resistance to the antibiotic, and this occurs before the antibiotic is used. When the antibiotic is given, the bacteria without the allele are killed while the resistant ones survive. The survivors now face no competition from the bacteria that have been killed, and they reproduce rapidly. Their offspring inherit the allele for resistance. The proportion of resistant bacteria in the population therefore increases, until eventually the antibiotic is no longer effective against that strain.

Model answer [4 marks]

Describe Darwin's theory of evolution by natural selection. [4]

There is genetic variation between the individuals in a population. More offspring are produced than can survive, so individuals compete for limited resources. Those whose characteristics are best suited to the environment are more likely to survive and reproduce, and they pass the alleles for those characteristics to their offspring. Over many generations the proportion of the population carrying those alleles increases, so the characteristics of the population change.

Model answer [3 marks]

Explain why patients are told to complete a full course of antibiotics. [3]

The bacteria in an infection vary in how easily the antibiotic kills them. Stopping the course early kills the most susceptible bacteria but leaves the more resistant ones alive, and those survivors then reproduce with no competition. This selects for resistance and makes the surviving population harder to treat, so completing the course kills the more resistant bacteria as well.

Mark-losing trap. The mutation comes FIRST. Never write that bacteria became resistant because of the antibiotic.

Mark-losing trap. Avoid 'they adapted to survive' and 'they needed to change'. Say variation, competition, survival, inheritance.

Mark-losing trap. Individuals do not evolve; POPULATIONS do. The proportion carrying an allele is what changes.

Mark-losing trap. Say 'more LIKELY to survive', not 'will survive'. Selection shifts probabilities, it does not guarantee outcomes.

Mark-losing trap. Antibiotics do not affect viruses, so prescribing them for a viral infection selects for resistance and achieves nothing else.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — In a population of bacteria, where does resistance to an antibiotic originally come from?
2. Grade 7 · Explain [2 marks] — A doctor prescribes antibiotics to a patient with a viral throat infection. Explain why this is a bad idea.
3. Grade 8 · Explain [5 marks] — Explain how a population of bacteria in a hospital becomes resistant to an antibiotic that used to treat it successfully. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [4 marks] — A population of 8.0 × 10⁸ bacteria contains 40 individuals carrying an allele for antibiotic resistance. An antibiotic is given which kills all the non-resistant bacteria. The survivors then divide every 20 minutes. Calculate the number of resistant bacteria present after 2 hours.
5. 9+ · Analyse [5 marks] — A farmer adds a low dose of antibiotic to livestock feed every day to prevent infections and increase growth. A vet warns that this is more dangerous than treating individual sick animals with a full dose. Select every statement that belongs in a full-mark explanation of why the vet is right.

The people behind this science

Two ways into the same idea — the one who built the argument, and the one who found the antibiotic and predicted its defeat. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Charles Darwin — the one who built the argument

The four steps on this page are his, and the way he assembled them is worth as much as the conclusion. He spent five years on the Beagle collecting observations, then more than twenty years accumulating evidence before publishing — partly because he understood how unwelcome the argument would be, and partly because he wanted the case to be overwhelming. He was pushed into print when Alfred Russel Wallace sent him an essay in 1858 containing essentially the same idea; On the Origin of Species followed in 1859. He never knew where variation came from, which was the largest hole in his own argument and the one this topic fills.

• “How did you get from finches and tortoises to a general argument?”
• “Why did you wait more than twenty years before publishing?”
• “What did you do when Wallace sent you the same idea?”
• “Which objection to your theory troubled you the most?”
• “What would you most like to have known about inheritance?”

Alexander Fleming — the one who found the antibiotic and predicted its defeat

Fleming noticed in 1928 that a mould contaminating one of his plates had killed the bacteria around it, and he named the substance penicillin. What makes him the right figure for this page is what he said afterwards. In his Nobel lecture in 1945 he warned that exposing bacteria to too little penicillin would make them resistant, and described exactly how a careless patient could pass a resistant strain to someone else. He was applying the argument on this page to his own discovery, and everything he predicted has happened.

• “What did you actually see on the contaminated plate?”
• “Why did you warn that bacteria would become resistant?”
• “What happens in a population of bacteria given too small a dose?”
• “Why is it so difficult to persuade people to finish a course?”
• “Did you expect resistance to arrive as quickly as it did?”

Carry on through the course

• Previous lesson: How Mutations Affect Proteins
• Next lesson: Populations, Ecosystems and Sampling with Quadrats
• Sexual and Asexual Reproduction — Two ways to make a new organism — one produces copies, the other produces something that has never existed before. Why any species would want both.
• Flowers, Pollination and Fertilisation — Two completely different solutions to one problem — a plant that cannot move has to get its male gamete to another plant's female gamete.
• Seeds and Germination — What a seed needs to start growing, what it lives on before it has a single leaf — and why light is not on the list.
• The Human Reproductive Systems — Two systems built around the same problem — making gametes and bringing them together — and the hormones that switch them on.
• The Menstrual Cycle — Four hormones, two organs and one repeating question — should the uterus lining be built up, held, or let go?
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification