Birds eat the moths they can see. Antibiotics kill the bacteria they can reach. In both cases the survivors are not random: they carry alleles that suit the conditions, and they pass those alleles on. Repeated over many generations this changes a whole population. Adaptation as a process and antibiotic resistance are Extended content.
Natural selection: there is genetic variation within a population; organisms produce many offspring; there is a struggle for survival, including competition for resources; individuals that are better adapted to the environment have a greater chance of surviving and reproducing; they pass on their alleles to the next generation. Extended: over many generations this makes populations more suited to their environment, a process called adaptation.
Fig. 1: the five steps of natural selection; each turn of the cycle is one generation
Section 01
Peppered moths on the bark.
The sim starts with 40 moths on a tree trunk, 10% of them dark. A bird eats the moths it can see. Press Run 1 generation to watch it hunt, then Run 10 to jump ahead, and watch the graph of % dark moths. Then switch the bark from clean (pale) to sooty (polluted) and run again.
(a) Genetic variation: moth colour is inherited; some moths have the allele for dark wings and some for pale wings.
(b) Many offspring: each generation produces more moths than can survive.
(c) Struggle for survival: birds hunt the moths; moths also compete for resources such as food and resting places.
(d) Better adapted survive and reproduce: on sooty bark the dark moths are camouflaged, so fewer are eaten and more of them breed.
(e) Alleles passed on: the survivors pass the allele for dark wings to their offspring, so the percentage of dark moths rises each generation. On clean bark the opposite happens.
Examiner trap
The moths do not turn dark because the bark is sooty, and they do not choose to change. The dark allele was already in the population; the environment only changes which moths survive to breed.
Section 02
Antibiotic resistance.
Extended. In the Petri dish a large colony of 150 bacteria contains just one (orange) that a random mutation has made resistant to an antibiotic. Press Add antibiotic, then Let them grow, and repeat. Watch the green and orange lines on the graph.
Variation by mutation: a random mutation gives one bacterium an allele for resistance.
Selection: the antibiotic kills the non-resistant bacteria; the resistant ones survive. The antibiotic is the selection pressure.
Reproduction: the survivors divide quickly with little competition and pass on the resistance allele.
Result: after repeated use the population is a resistant strain, and the antibiotic no longer works. MRSA is a well-known example.
Slowing it down: use antibiotics only when they are needed (never for viral infections) and complete the full course.
Examiner trap
The antibiotic does not cause the resistance mutation. The mutation happens at random; the antibiotic only selects the bacteria that already carry it.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the moth sim on sooty bark, which moths does the bird eat most often?
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After you run 10 generations on sooty bark, what happens to the percentage of dark moths?
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Extended: in the Petri dish, what happens when the antibiotic is first added?
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Extended: why does the resistant (orange) line keep rising after each dose and grow?
Section 03
Adaptation over generations.
Extended. An adaptive feature belongs to one organism; adaptation is a change in a whole population.
Adaptation: the process, resulting from natural selection, by which populations become more suited to their environment over many generations.
Allele frequency: each generation, alleles that help survival become more common in the population and harmful ones less common.
Speed: bacteria reproduce every 20 minutes or so in good conditions, so they adapt in days; moths adapt over years; large mammals over many centuries.
Examiner trap
Individuals do not adapt during their lifetime in this sense. A single moth cannot change its wing colour; only the population changes, as alleles are passed on.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
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Which is the correct order of events in natural selection?
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A population of rabbits produces many more young than the area can feed. This leads directly to:
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Extended: which statement best describes adaptation?
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Extended: why should a patient finish a full course of antibiotics?
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Natural selection, mastered.
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