6 higher-level sections hidden.
Educerie · IB Diploma · Biology
Theme D Continuity and change · D4.1 Natural selection
What you must be able to do
| You must be able to | Level | What it looks like in the exam |
|---|---|---|
| Explain natural selection as the mechanism of evolution; explain why Darwin's theory was a paradigm shift from Lamarckism | SL, HL | "Distinguish between Lamarckian and Darwinian explanations" (3 marks) |
| Explain how mutation and sexual reproduction generate variation | SL, HL | "Outline two sources of variation" (2–4 marks) |
| Explain overproduction of offspring and competition for limited resources, with examples of resources | SL, HL | Part of a 6–8 mark natural selection answer |
| Explain abiotic factors, including density-independent ones, as selection pressures | SL, HL | "Suggest how a severe frost could act as a selection pressure" (2 marks) |
| Explain differences in survival and reproduction, intraspecific competition and fitness | SL, HL | "Define fitness" (1 mark); within an extended response |
| Explain why only heritable traits can evolve | SL, HL | "Explain why a trait acquired during life is not inherited" (2 marks) |
| Explain sexual selection, with birds of paradise as the example | SL, HL | "Explain how sexual selection leads to bright plumage" (3 marks) |
| Interpret data from Endler's guppy experiments | SL, HL | Paper 1B: describe, calculate and explain from a graph (4–6 marks) |
| Define the gene pool | HL only | Definition, 1 mark |
| Compare allele frequencies in geographically isolated populations, using databases | HL only | Paper 1B: interpret a table of allele frequencies (3 marks) |
| Explain changes in allele frequency by natural selection (neo-Darwinism) | HL only | "Explain how natural selection changes allele frequencies" (4 marks) |
| Distinguish directional, disruptive and stabilising selection | HL only | Graphs, 3 marks; "distinguish between…" |
| Use the Hardy–Weinberg equation to calculate allele and genotype frequencies | HL only | Calculation, 3–4 marks |
| State the Hardy–Weinberg conditions and explain what a failure to fit means | HL only | "Suggest reasons why the population is not in equilibrium" (2 marks) |
| Distinguish artificial selection from natural selection | HL only | "Explain why antibiotic resistance is an example of natural selection" (3 marks) |
Before you start
You need A4.1 (evolution as a change in heritable characteristics, and Lamarckism), D3.2 (alleles, genotype, dominance, and how meiosis shuffles alleles), and the idea of a population from C4.1. Evolution and speciation were described in A4.1; this page supplies the mechanism.
1The idea in one paragraph
Individuals in a population differ, and much of that difference is inherited. Populations produce far more offspring than can survive, so individuals compete for what the environment supplies. Those whose inherited traits suit the environment best survive longer and leave more offspring. Their alleles become commoner in the next generation. Repeat that over thousands of generations and the population changes: that is evolution by natural selection. The selection pressure can be food, predators, temperature or the choices of mates. At HL, you describe the same process as changing allele frequencies in a gene pool, classify it into three patterns, calculate frequencies with the Hardy–Weinberg equation, and compare it with the selection humans do deliberately.
2Natural selection, and why Darwin's idea was a paradigm shift
Natural selection is the process by which individuals with heritable traits that make them better suited to their environment survive and reproduce more than others, so that those traits become commoner in later generations. It is the main mechanism of evolution. It operates continuously, in every population, generation after generation, and it has been operating for over three billion years. Everything from bacteria to blue whales is the product of that accumulated change, which is why natural selection explains the biodiversity of life on Earth.
Nature of science: a paradigm shift. By the middle of the nineteenth century, many naturalists accepted that species had changed over time. The question was how. The best-known answer was Lamarckism: organisms change during their lifetime by using or not using body parts, and pass those acquired changes to their offspring. Darwin's theory, published in 1859, gave a different mechanism: variation arises first, at random with respect to need, and the environment then selects among it. The evidence Darwin assembled was so convincing that it replaced Lamarck's explanation. A paradigm shift is a change in the fundamental framework of assumptions and methods within which scientists work, so that questions are asked and answered in a new way. After Darwin, biologists no longer asked "how does an animal change itself to fit its surroundings?" but "which variants survive and reproduce?"
Figure 1 puts the two explanations side by side for one trait.
3Where variation comes from
Natural selection cannot create variation. It can only act on variation that already exists. Two processes supply it.
Mutation is a change in the base sequence of DNA. A mutation in a gene creates a new allele. Most mutations are neutral or harmful; a few happen to be beneficial in a particular environment. Mutation is the only source of genuinely new alleles, and only mutations in the cells that give rise to gametes are passed on.
Sexual reproduction does not create new alleles. It creates new combinations of alleles, in three ways from D2.1 and D3.2:
- crossing over in prophase I of meiosis swaps sections between homologous chromosomes, making new combinations on a single chromosome;
- random orientation of homologous pairs in metaphase I makes gametes with different mixtures of maternal and paternal chromosomes;
- random fertilisation joins any one of a parent's vast number of possible gametes with any one of the other parent's.
So every offspring of sexual reproduction, apart from identical twins, is genetically unique. Mutation supplies the raw material; sexual reproduction shuffles it into new combinations for selection to test.
4Overproduction and competition
Every species can produce more offspring than the environment can support. A single oak tree may drop thousands of acorns in a good year; a salmon lays thousands of eggs; even a slow-breeding mammal, if every young survived and bred, would multiply without limit. Yet most populations stay roughly the same size over time. So most offspring die before they reproduce.
The limit is set by resources. The carrying capacity is the maximum population size that an environment can support over a long period. Figure 2 shows the gap between what a population could produce and what the environment allows.
Resources that limit carrying capacity include:
- food, for animals, and for predators the supply of prey;
- water, especially in dry habitats;
- light, for plants in a crowded forest;
- space: nesting sites for birds, territories for many animals, burrows, or places for sessile organisms to attach;
- mineral nutrients in the soil, for plants.
When individuals need the same limited resource, they compete for it. Competition between members of the same species is intraspecific competition, and it is the most intense kind, because members of a species need exactly the same things. The winners are not the strongest in any general sense; they are those whose traits happen to suit the current conditions.
5Abiotic factors as selection pressures
A selection pressure is any feature of the environment that affects which individuals survive and reproduce. Not all are living. Abiotic (non-living) factors act too: temperature, drought, flooding, salinity, soil pH.
Many abiotic pressures are density-independent: they kill a proportion of the population whatever its size. A hard frost kills individuals whether the population is large or small. But it does not kill at random. Individuals whose inherited physiology makes them more tolerant of cold are more likely to survive it. After an unusually severe winter in the southern United States in 2014, for example, the green anole lizards that survived could tolerate lower temperatures than the populations studied before it: the storm had removed the least cold-tolerant individuals. If cold tolerance is heritable, the next generation carries more of the alleles for it.
Other density-independent examples: an unusually hot summer selects for heat tolerance in fish of shallow ponds, and a long drought selects for plants with deeper roots or more water-efficient leaves.
6Differences in survival and reproduction: fitness
Put sections 3 to 5 together and you have natural selection, set out as a chain in Figure 3.
The phrase "survival of the fittest" misleads, because what counts is not survival but reproduction. An individual that lives a long time but leaves no offspring contributes nothing to the next generation. Fitness is the ability of an individual, or of a genotype, to survive and reproduce in a particular environment, measured by how many offspring it contributes to the next generation compared with others. Fitness has two parts:
- survival value: the chance of living to reproductive age;
- reproductive potential: how many offspring can be produced once there.
Fitness is always relative and always tied to an environment. The pale mouse of Figure 1 is fit on pale sand and unfit on dark soil. When the environment changes, fitness changes with it.
Natural selection: heritable variation + overproduction + competition → differential survival and reproduction → the alleles of the fitter individuals become commoner.
7Only heritable traits evolve
A trait can only change in a population across generations if it is heritable, which means it is encoded in the base sequence of genes that are passed on in gametes.
Characteristics acquired during an individual's lifetime because of the environment are not encoded in the base sequence, so they are not inherited. A sprinter's trained leg muscles, a plant stunted by poor soil, the scar on a lion's face from a fight: none of these alters the DNA in the gametes, so the offspring start without them. This is precisely the point on which Lamarck was wrong, and it is why a natural selection answer must use the words heritable or genetic at the variation step. "Some giraffes had longer necks" is not enough; "some giraffes inherited alleles for longer necks" is.
8Sexual selection
Not every selection pressure comes from the physical environment or from predators. In many animals, individuals compete for mates, and the traits that win mates are selected even when they are costly for survival. This is sexual selection.
It works in two main ways.
- Mate choice. One sex, usually the female, chooses among members of the other. Physical and behavioural traits act as signals of overall fitness: only a healthy, well-fed male with few parasites can grow a large, bright, symmetrical display and keep it in good condition. A female that chooses such a male is likely to have offspring that inherit his good alleles. Over generations, the preferred trait becomes more exaggerated.
- Competition between members of one sex. Males fight or display to each other for access to females, as red deer stags do with their antlers in the autumn rut. Traits that win contests are selected.
Birds of paradise, the guide's example, show sexual selection at its most extreme. The males of many of the forty or so species, in New Guinea and nearby, have elaborate plumage: long tail wires, iridescent breast shields, fans of feathers. They perform complex courtship displays, some clearing a patch of forest floor as a stage. Females are much plainer and choose among displaying males. A widely accepted explanation for why it went so far is that fruit is plentiful and predators are few in their forests, so the survival cost of showy plumage is relatively low, while the reproductive gain for a male that is chosen is large. Selection by females, repeated over many generations, has produced the extravagant plumage and displays.
The general point: a trait can reduce survival and still spread, if it increases reproduction by more.
9Modelling selection: Endler's guppies
The guppy is a small freshwater fish from the streams of Trinidad. Males are covered in coloured spots, some bright orange and blue; females are plain. In the 1970s John Endler noticed that males in different streams differed in how colourful they were, and that the difference matched the predators present:
- in lower stretches of streams with a dangerous predator, the pike cichlid (Crenicichla), males had fewer and less conspicuous spots;
- in upper stretches, above waterfalls, where the only predator was a small killifish (Rivulus) that rarely eats adult guppies, males were much more colourful.
Two pressures pull in opposite directions. Sexual selection by females favours brighter males. Natural selection by predators favours duller males that are harder to see. The balance depends on which pressure is stronger.
The experiment. Endler tested this by controlling the selection pressure. He set up large artificial ponds in a greenhouse, with gravel on the bottom, and stocked them with guppies from many sources, so the populations started with plenty of variation. After the guppies had bred for some months, he added a strong predator to some ponds, a weak predator to others, and no predator to the rest. He counted the spots on males at intervals over more than a year, which spans many guppy generations. Figure 4 shows invented data in the pattern he found.
Reading it. With no predator or with the weak predator, the number of spots rose over time: sexual selection by females dominated. With the strong predator, it fell: conspicuous males were eaten before they could breed, and natural selection dominated. The starting populations were the same; only the selection pressure differed, so the difference is caused by it. Endler also found that spot size tended to match the gravel size where predators were present, making males less visible against the background.
He then did the same in the wild, moving guppies from a high-predation site to a stream section with only Rivulus. Within a couple of years the males there had become noticeably more colourful. A field experiment like this, with an unmanipulated site for comparison, confirms that the laboratory result applies in nature.
Why the experiments are strong evidence. They are controlled: the same starting population, one variable changed. They are replicated: several ponds per treatment. They are predictive: the result was predicted before the experiment from the field pattern. And the change was measured in a heritable trait over generations, not within individuals.
10HLThe gene pool and allele frequencies
SL students can skip to section 16.
A gene pool consists of all the genes, and all their different alleles, present in a population. For any one gene, the allele frequency is the proportion of all the copies of that gene in the population that are a particular allele. If a population of 500 diploid individuals carries 1,000 copies of a gene, and 300 of them are allele b, the frequency of b is 300 ÷ 1,000 = 0.3.
Geographically isolated populations have different allele frequencies. Populations separated by distance or barriers do not exchange alleles, and they experience different mutations, different selection pressures and different chance events. So their gene pools drift apart. Human populations show this clearly, and online databases let you look it up: genome browsers such as Ensembl show the frequency of an allele in each population sampled by large sequencing projects, and gnomAD reports frequencies grouped by ancestry. Two examples you can check:
- ABO blood group alleles. The frequency of the Iᴮ allele is higher in much of Central and South Asia than in Western Europe, and the i allele reaches very high frequencies in many Indigenous populations of South America.
- Lactase persistence. The allele that keeps the lactase gene switched on into adulthood, so adults can digest milk, is common in north-western Europe and in some pastoralist populations of Africa, and rare in East Asia. Its distribution matches a long history of dairy farming, which is consistent with natural selection in herding populations.
A Paper 1B question will give you such a table and ask you to compare frequencies with figures and suggest reasons.
11HLNatural selection changes allele frequencies
Darwin developed the theory of evolution by natural selection without knowing how inheritance worked. In the twentieth century, biologists combined his theory with Mendelian genetics and population genetics. That synthesis is called neo-Darwinism, and it restates natural selection in the language of alleles:
Evolution is a change in allele frequencies in a gene pool. Natural selection changes them because individuals with some alleles survive and reproduce more than individuals with others.
Suppose a moth population carries a dominant allele D for dark wings and a recessive allele d for pale wings. The trees darken, and pale moths (dd) are eaten more often than dark ones. Each generation, dd moths leave fewer offspring, so fewer d alleles pass on. Figure 5 models the result, with dd moths leaving 80 offspring for every 100 left by dark moths.
The dark allele starts rare, at a frequency of 0.05, and rises steadily. Notice the shape: the change slows as d becomes rare, because most of the remaining d alleles are hidden in heterozygotes (Dd), which look dark and are not selected against.
12HLDirectional, disruptive and stabilising selection
For a trait that varies continuously, such as body size, selection can act in three patterns. Figure 6 shows each as a change in the distribution.
- Directional selection favours one extreme. The mean shifts in one direction. Example: resistance to an antibiotic in a bacterial population; the dark moths of section 11.
- Stabilising selection favours the intermediate and acts against both extremes. The mean stays the same and the range narrows. Example: human birth mass, where very small and very large babies have historically had lower survival.
- Disruptive selection favours both extremes and acts against the intermediate. The distribution can split into two peaks. Example: the black-bellied seedcracker, an African finch, in which birds with small bills feed efficiently on soft seeds and birds with large bills on hard seeds, while intermediate bills do neither well.
The guide makes one point explicit: all three types change allele frequencies. Even stabilising selection, which leaves the mean where it was, removes the alleles that produce extreme phenotypes, so their frequencies fall.
13HLThe Hardy–Weinberg equation
Take one gene with two alleles. Let p be the frequency of the dominant allele and q the frequency of the recessive one. Every copy of the gene is one or the other, so:
p + q = 1
If individuals mate at random, the chance of an offspring receiving two dominant alleles is p × p, two recessive alleles q × q, and one of each p × q or q × p. So the genotype frequencies are:
p² + 2pq + q² = 1, where p² = frequency of AA, 2pq = frequency of Aa, q² = frequency of aa
Figure 7 shows where each term comes from.
Working from a phenotype. You usually cannot tell AA from Aa by looking, but you can count aa, because it has its own phenotype. So start from q². Suppose 1 in 10,000 babies in a population is born with an autosomal recessive condition such as PKU (D3.2).
The result surprises most people: a condition affecting 1 in 10,000 is carried, unseen, by about 1 in 50. Recessive alleles hide in heterozygotes.
Working from counts. A population of 1,000 has 490 AA, 420 Aa and 90 aa individuals.
The observed counts equal the expected ones, so the population fits Hardy–Weinberg proportions.
14HLThe Hardy–Weinberg conditions, and what a misfit means
The equation predicts genotype frequencies that stay the same generation after generation, a genetic equilibrium, but only if five conditions hold:
- the population is very large, so chance events do not change allele frequencies;
- mating is random, with no preference for particular genotypes;
- there is no mutation, so no alleles are created or changed;
- there is no migration: no individuals, and so no alleles, move into or out of the population;
- there is no natural selection: all genotypes survive and reproduce equally well.
Real populations rarely meet all five, which is the point. If observed genotype frequencies do not fit p² + 2pq + q², one or more conditions is not being met, and the population may be evolving.
Take a second population of 1,000: 560 AA, 280 Aa, 160 aa. The allele frequencies are the same as before, p = (1,120 + 280) ÷ 2,000 = 0.7 and q = 0.3, so the expected numbers are again 490, 420 and 90. But there are far fewer heterozygotes than expected and more of both homozygotes. That pattern points to non-random mating, for example individuals mating with close relatives, or with others of the same phenotype, or plants fertilising themselves. A deficit of one homozygote instead would point to selection against that genotype.
15HLArtificial selection
Artificial selection is selection by deliberate human choice. Breeders choose which individuals of a crop plant or a domesticated animal will reproduce, on the basis of traits they want, and repeat the choice generation after generation. Dairy cattle have been selected for milk yield, wheat for large seed heads on short, strong stems, and dog breeds for size, shape and behaviour. The outcome is the same as natural selection, a change in allele frequencies, but the selection pressure is a person's choice.
The distinction is about intent, and the guide makes it sharply with antibiotic resistance. When a doctor prescribes an antibiotic, nobody intends to breed resistant bacteria. But the antibiotic kills susceptible bacteria and the few with a resistance allele survive and multiply, so the resistance allele becomes commoner. The selection pressure comes from a human action, but no one chose the resistant bacteria to breed. It is an unintended consequence, and so it is natural selection, not artificial selection. The same applies to pests evolving resistance to pesticides and weeds to herbicides.
16Linking questions
How do intraspecific interactions differ from interspecific interactions? Intraspecific interactions are between members of one species, which need exactly the same resources, so competition is intense and it is the main engine of natural selection (section 4); mate choice, the basis of sexual selection, is intraspecific too. Interspecific interactions, such as predation, are between species; in the guppy streams, predation by Crenicichla is the interspecific pressure acting against the intraspecific one of female choice.
What mechanisms minimise competition? Differences in niche between species (B4.2), territories that spread individuals out, and disruptive selection, which can push a population towards two forms that use different resources, as with the seedcracker bills.
17Where marks are lost
Writing Lamarck by accident. "The mice turned pale because they needed camouflage" or "the bacteria became resistant to survive" describes need driving change. Selection acts on variation that was already there.
Leaving out "heritable". Variation must be genetic for selection to change the next generation. Write "inherited" or "genetic" at the variation step.
Treating fitness as strength or health. Fitness is survival and reproduction, measured by offspring left, relative to others, in a particular environment.
Saying individuals evolve. Individuals are selected; populations evolve. A bacterium does not become resistant; a resistant one survives.
Missing the balance in Endler's experiment. Colour is favoured by sexual selection and opposed by predation. An answer that mentions only one pressure misses the point of the experiment.
Calling antibiotic resistance artificial selection. It is natural selection: the outcome was not chosen.
HL: taking the square root of the wrong term. Start from q², the recessive phenotype, and take its square root to get q. The dominant phenotype is p² + 2pq, not p².
HL: saying stabilising selection does not change allele frequencies. It removes alleles for extreme phenotypes, so their frequencies fall.
18Draw it right
- Selection graphs: frequency on the vertical axis, the trait (with units) on the horizontal. Draw the original distribution dashed and the new one solid, and label the selected extreme(s) with arrows.
- Directional: the peak moves; stabilising: same peak, narrower and taller; disruptive: two peaks, with the middle reduced.
- Population growth versus carrying capacity: time on the horizontal axis, population size vertical; label K as a dashed horizontal line.
- Data from Endler-type experiments: quote values with units, give the direction of change for each treatment, and name the selection pressure responsible.
- HL, Hardy–Weinberg: write p + q = 1 and p² + 2pq + q² = 1, state which term you start from, and give answers as decimals (0.42) or as numbers of individuals, as asked.
19Try it
Marks in brackets. Answers and marker's notes are at the end.
Q1. Explain how natural selection can lead to a change in the characteristics of a population. 5 marks
Q2. A mouse loses the tip of its tail in a trap. Explain why its offspring will be born with complete tails. 2 marks
Q3. In an experiment of the kind Endler carried out, guppy populations were kept in three sets of ponds (invented data). The table shows the mean number of spots per male.
| Pond treatment | Start | After 5 months | After 14 months |
|---|---|---|---|
| No predator | 10.0 | 11.0 | 12.4 |
| Weak predator (Rivulus) | 10.0 | 10.8 | 12.1 |
| Strong predator (Crenicichla) | 10.0 | 9.2 | 8.1 |
(a) Calculate the percentage change in the mean number of spots in the strong-predator ponds over 14 months. 2 marks
(b) Explain the difference between the no-predator and strong-predator ponds. 3 marks
(c) Suggest why the ponds were all stocked from the same mixed population. 1 mark
Q4. Explain how sexual selection can lead to the evolution of elaborate plumage in male birds of paradise. 3 marks
Q5 (HL). In a population of 800 snails, 72 have yellow shells, which is the phenotype of the homozygous recessive genotype. Assuming the population is in Hardy–Weinberg equilibrium, calculate the number of snails that are heterozygous. 3 marks
Q6 (HL). Explain why the evolution of antibiotic resistance in bacteria is an example of natural selection rather than artificial selection. 3 marks
20In one breath
Natural selection is the mechanism of evolution, working continuously for billions of years and producing Earth's biodiversity; Darwin's theory replaced Lamarck's inheritance of acquired characteristics, a paradigm shift. Mutation makes new alleles and sexual reproduction makes new combinations. Populations overproduce offspring, so individuals compete for food, water, light and space, and abiotic pressures such as frost remove the least tolerant. Individuals whose heritable traits suit the environment survive and reproduce more, so their alleles become commoner; fitness is that relative success in leaving offspring. Acquired characteristics are not in the base sequence, so they are not inherited. Sexual selection favours traits that win mates, like the plumage of birds of paradise, even at a cost to survival. Endler's guppies showed that predators make males duller and female choice makes them brighter, depending on which pressure is stronger. HL: evolution is a change in allele frequency in a gene pool (neo-Darwinism); isolated populations differ in allele frequencies; selection can be directional, stabilising or disruptive, and all three change allele frequencies; p + q = 1 and p² + 2pq + q² = 1 hold only in a large, randomly mating population with no mutation, migration or selection, so a misfit shows a condition is broken; and artificial selection is deliberate, while antibiotic resistance, being unintended, is natural selection.
Answers
Q1. There is variation within the population, and some of it is heritable; the variation is produced by mutation, which forms new alleles, and by sexual reproduction, which forms new combinations; more offspring are produced than the environment can support; individuals compete for limited resources such as food; individuals with heritable traits better suited to the environment are more likely to survive (they have greater fitness); they reproduce more and pass on their alleles; so the frequency of those alleles, and of the traits, increases in the next generation; repeated over many generations, the characteristics of the population change. any five. An answer in which individuals change because they need to is Lamarckian and scores at most 1.
Q2. The loss of the tail tip is an acquired characteristic, caused by the environment during the mouse's life; it does not change the base sequence of the genes in its gametes, so the alleles passed on still code for a complete tail. 1 for acquired or not genetic, 1 for no change to the DNA in the gametes.
Q3. (a) (8.1 − 10.0) ÷ 10.0 × 100 = −19% (a 19% decrease). M1 for the change divided by the starting value, A1 for −19% with the sign or the word decrease. (b) Without predators, males with more spots are preferred by females, so they mate more and pass on their alleles, and spot number rises (10.0 to 12.4); with the strong predator, males with more spots are more conspicuous and more likely to be eaten before breeding, so fewer spots are selected and spot number falls (10.0 to 8.1); so the direction depends on whether sexual selection or natural selection by predation is the stronger pressure. 1 for sexual selection with the data, 1 for predation with the data, 1 for the balance between the two. (c) So that any difference between treatments could be caused only by the selection pressure, not by differences in the starting populations, and so that there was plenty of variation for selection to act on. 1 mark
Q4. Females choose which males to mate with, choosing males with the most elaborate plumage and displays; elaborate plumage is a signal of overall fitness, since only healthy males with plenty of food can grow and maintain it; chosen males father more offspring, which inherit the alleles for elaborate plumage; over many generations, the frequency of these alleles rises and the plumage becomes more extreme. any three. An answer that says males grow feathers to attract females, without choice and inheritance, scores 1.
Q5 (HL). q² = 72 ÷ 800 = 0.09; q = √0.09 = 0.3; p = 1 − 0.3 = 0.7; 2pq = 2 × 0.7 × 0.3 = 0.42; number heterozygous = 0.42 × 800 = 336. M1 for q² = 0.09 and q = 0.3, M1 for p = 0.7 and 2pq = 0.42, A1 for 336. Taking q = 0.09 scores 0 for the first mark but can earn follow-through for the method.
Q6 (HL). Artificial selection is the deliberate choice by humans of which individuals breed, for traits they want; no one chooses resistant bacteria to breed: resistance is an unintended consequence of using an antibiotic; the antibiotic acts as an environmental selection pressure, killing susceptible bacteria while those with a resistance allele survive and reproduce, so the allele frequency rises, which is natural selection. 1 for artificial selection as deliberate, 1 for resistance as unintended, 1 for the mechanism of selection by the antibiotic.
Educerie · written from the published IB Diploma Programme Biology guide, first assessment 2025, section D4.1 Natural selection. Original text, examples and questions. Diagrams drawn by Educerie. Last reviewed 25 September 2026.
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