Every population produces more young than the environment can support. Those best suited to the conditions are most likely to survive and breed, so their alleles become commoner. Chance also plays a part, especially when populations are small.
Populations have the capacity to produce many more offspring than can survive, so individuals compete for limited resources such as food, mates and territory. Because of variation, some individuals have alleles that make them better adapted to the conditions. In this struggle for existence they are more likely to survive and reproduce and pass on their alleles, so the frequency of those alleles rises in the next generation. Environmental factors act as selection pressures that may be stabilising (the mean is favoured), directional (one extreme is favoured) or disruptive (both extremes are favoured). Allele frequencies also change by chance: genetic drift, the founder effect and the bottleneck effect all have the greatest effect in small populations. Antibiotic resistance is directional selection in action.
Fig. 1: the three forces of natural selection acting on a continuous feature; the dashed curve is the population before selection
Section 01
Overproduction, competition, survival.
Choose stabilising, then press Run 10 generations and compare the yellow histogram with the grey starting population. Reset and repeat for directional and disruptive.
Overproduction: populations can produce many offspring (a cod lays millions of eggs), far more than the environment can support.
Competition: resources such as food, water, light, nesting sites and mates are limited, so there is a struggle for existence; predators, disease and harsh conditions also kill many.
Variation: individuals differ, partly because they carry different alleles.
Selection: individuals whose alleles give a phenotype best adapted to the conditions are most likely to survive to reproduce.
Inheritance: they pass on their advantageous alleles, so the allele frequency increases over many generations.
Type
What is favoured
Effect on the distribution
Example
stabilising
the intermediate (mean) phenotype
mean unchanged, range narrows
human birth mass
directional
one extreme
mean shifts in one direction
antibiotic resistance; larger body size as climate cools
disruptive
both extremes over the intermediate
two peaks may form
two food sources of very different sizes
Examiner trap
Organisms do not change in order to survive, and an individual does not evolve. Variation already exists; the environment selects among it, and it is the population's allele frequencies that change.
Section 02
Chance: drift, founders and bottlenecks.
With genetic drift chosen, set Population size to 30 and press Run 60 generations: watch the five lines wander. Repeat at 500. Then try founder effect and bottleneck.
Genetic drift: random changes in allele frequency because which individuals happen to reproduce, and which gametes fuse, is partly chance. In small populations drift is large and an allele can be lost (frequency 0) or fixed (frequency 1) within a few generations; in large populations it has little effect.
Founder effect: a few individuals colonise a new area, such as an island. Their alleles are a chance sample of the original gene pool, so the new population has different allele frequencies and less genetic variation.
Bottleneck effect: a catastrophe, disease or hunting reduces a population to a few survivors. Genetic diversity is lost, and when numbers recover the variation does not, as in the northern elephant seal and the cheetah.
Selection versus drift: selection changes allele frequencies in a consistent direction because some alleles give an advantage; drift changes them at random, regardless of advantage.
Examiner trap
Drift is not caused by an allele being harmful. A neutral or even an advantageous allele can be lost by chance in a very small population.
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
You choose stabilising and run 10 generations. How does the yellow histogram compare with the grey starting population?
Quick check+10 XP
Which option in the simulator makes the mean body size shift steadily in one direction?
Quick check+10 XP
In genetic drift with Population size 30, some lines reach 0 or 1 before 60 generations. What does a line reaching 1 mean?
Quick check+10 XP
You compare drift at Population size 30 and at 500. What do you see, and why?
Section 03
Antibiotic resistance: selection you can watch.
Antibiotic resistance follows the same logic as directional selection in the simulator, but in bacteria, which can divide every 20 minutes.
Variation: a random mutation in a bacterium produces an allele that gives resistance, for example a gene for an enzyme that breaks down penicillin (penicillinase). The mutation is not caused by the antibiotic.
Selection pressure: when the antibiotic is used, susceptible bacteria are killed; resistant bacteria survive.
Reproduction: survivors reproduce by binary fission with little competition, passing on the resistance allele, so its frequency rises rapidly.
Spread between bacteria: resistance genes are often carried on plasmids, which can pass to other bacteria, even of other species, by conjugation (horizontal gene transfer).
Making it worse: overuse of antibiotics, using them for viral infections, not finishing a course and routine use in farm animals all increase the selection pressure.
Examiner trap
Bacteria do not become resistant because they are exposed to the antibiotic or get used to it. The resistant mutants were already present; the antibiotic simply removes their competitors.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which statement correctly describes natural selection?
Exam style+20 XP
A small population of lizards reaches an island after a storm. Thirty years later, a rare allele in the mainland population is common on the island. What is the most likely explanation?
Exam style+20 XP
Which correctly explains how a population of bacteria becomes resistant to an antibiotic?
Exam style+20 XP
Seed-eating birds on an island feed on either very small or very large seeds; medium seeds are rare. What type of selection on beak size is expected?
Skill unlocked
Natural selection, drift and resistance, mastered.
This skill is now recorded on your profile. Keep the chain going.