Biology  by Bradford
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IB BIOLOGY · THEME D · CONTINUITY AND CHANGE

How selection changes a population

Nobody plans evolution, and nothing has to try. Here is how variation, too many young and a changing world add up to a population that is different a few generations later, and how a pond of guppies shows the tug of war between surviving and finding a mate.

IB Biology D4.1SL and HL2 hours SL, 4 HL11 chaptersdry lab

Start learningRun the dry labTeach it

One population: jaws vary A drought: only tough seeds Large jaws crack them; slender starve The next generation: more large jaws slender jawslarge jaws beforeJ 40%j 60%afterJ 60%j 40%
Natural selection in one loop: the jaws vary, a drought leaves only tough seeds, the large-jawed beetles crack them and breed, and the next generation carries more of the large-jaw allele (pink pips and bar) than the last. No beetle changed: the population did.

01Learn

Eleven chapters, the last five for higher level. Read one, play its figure or tap its parts, and make each prediction before you open the answer.

  1. 1Evolution has a mechanism
  2. 2Variation to select from
  3. 3More offspring than the world can feed
  4. 4Who survives, who breeds
  5. 5Choosing a mate
  6. 6Two pressures, tested
  7. 7The gene pool HL
  8. 8Selection shifts allele frequencies HL
  9. 9Three ways selection can act HL
  10. 10What a population would be like if it were not evolving HL
  11. 11Artificial selection and its side effects HL

D4.1.1 Natural selection drives evolution

Evolution has a mechanism

Long before Darwin, many naturalists suspected that species change over time. What nobody had was a convincing explanation of how. Jean-Baptiste Lamarck, in the early 1800s, suggested that animals change by using or not using their parts, and pass those changes on. A giraffe stretches for leaves, its neck lengthens, and its calf is born with a longer neck. It sounds sensible, and it is wrong (chapter 4 shows why).

In 1859 Charles Darwin published a different mechanism, natural selection. Individuals in a population differ. More young are born than can survive. The ones whose traits suit the conditions leave more offspring, and the traits they carry become commoner. Nothing has to try, and nothing is planned.

The process never stops. Every generation of every population is being sorted, over billions of years, and the accumulated result is the biodiversity of life on Earth.

Darwin's idea replacing Lamarck's is a classic paradigm shift. That is the nature of science: an explanation is kept while it fits the evidence, and replaced when a better one arrives.

To keep the logic of natural selection in order, use the mnemonic below. ICE AGE is a teaching aid, not an IB term. The next three chapters take its letters one at a time.

Remember it The logic of natural selection: ICE AGE

  • IcyInherited variationthe population varies, and the variation is heritable
  • CubsCompetitionmore young are born than resources can support
  • EatEnvironmental pressuresome survive and breed more than others
  • AnythingAdaptationshelpful traits are selected for
  • GrowingGenes (alleles)allele frequencies change from one generation to the next
  • EnormousEvolutionthe population has changed

Four claims about natural selection. Each one is a common slip. Decide, then check.

Claim 1: "Natural selection gives organisms what they need to survive."

Claim 2: "Natural selection acts for the good of the species."

Claim 3: "Organisms adapt to their surroundings by trying to."

Claim 4: "The fittest are the strongest, healthiest and fastest."

Watch out Three more slips. Individuals do not evolve: a population does, across generations. Mutations do not happen because they are needed: they are chance changes (chapter 2). Natural selection does not make perfect organisms: it sorts what variation is on offer, under today's conditions, so every organism is a compromise.
Lamarck's ideaearly 1800suse and disuseDarwin's idea1859natural selectionreaches upneck stretchesinherited, he saidnot inheritednecks varylong necks feed bestand leave more younglong necksnow commonerreplaced Lamarcka paradigm shift
  1. Lamarck's idea: a giraffe stretches for leaves, its neck lengthens, and it passes the change on.
  2. In fact the calf hatches with an ordinary neck: a change gained in life is not inherited (chapter 4).
  3. Darwin's idea starts with variation: the necks in a population are not all the same.
  4. Long necks feed best in the lean season and leave more young. The alleles go with them.
  5. Generations later, long necks are commoner. The explanation was replaced: a paradigm shift.
Follow one population through the six letters, one at a time, then drag the bar back to look again.

D4.1.2 Where variation comes from (I in ICE AGE)

Variation to select from

Selection can only choose between things that differ. In any sexually reproducing population, individuals differ in their heritable traits because they differ in their alleles. Two processes keep supplying them.

Mutation is the only source of brand new alleles. A copying slip, or damage to DNA, changes the base sequence of a gene, and the changed version is a new allele. Mutations happen by chance, with no regard for need. Most do nothing noticeable, some are harmful, and a few turn out to be useful under some conditions. Only a mutation in the cells that make gametes can be passed on.

Sexual reproduction makes new combinations of the alleles that already exist. In meiosis, crossing over swaps pieces between a pair of chromosomes, and independent assortment sends each pair's chromosomes to the gametes at random (a person can make more than eight million different chromosome mixes this way alone, before any crossing over). Fertilisation then joins two gametes that met by chance. So no two siblings, apart from identical twins, have the same set of alleles.

Put the two together. Mutation adds alleles to the population's stock. Sexual reproduction shuffles that stock into endless new individuals. Without both, there would be nothing for selection to act on, and no change.

Watch out A mutation is not a response to a problem. A bacterium does not mutate because an antibiotic is near, and a beetle does not mutate because the winter is cold. The mutation arrives by chance, and the environment decides whether it helps.

Where does the very first copy of a new allele come from?

Mutation: a new alleleallele 1GACTTGCAATGCnew alleleGACTTACAATGCcopying errorone base differs, so the allele differsSexual reproduction: new combinationsABaba pair of chromosomescrossing over swaps piecesABAbaBabnew combinations
  1. A gene is a sequence of bases. This version is one allele of the gene.
  2. A copying error changes one base. The changed sequence is a new allele: mutation makes new alleles.
  3. In meiosis, crossing over swaps pieces between a pair of chromosomes that carry the alleles A, B and a, b.
  4. The gametes carry combinations that neither chromosome had before: sexual reproduction makes new combinations of alleles.

D4.1.3 D4.1.4 Overproduction and pressures (C and E in ICE AGE)

More offspring than the world can feed

Most species make far more offspring than ever survive. A frog lays hundreds or thousands of eggs, and an oak drops thousands of acorns. If every one grew up and bred, the population would explode. It does not, because the place where it lives can support only so many.

The carrying capacity is that limit. It is set by whatever runs short first: food, water, shelter or nesting sites, space, and for plants light and mineral nutrients. When there are more young than the resources can supply, they compete. Individuals of the same species need the same things, so competition within a species is especially sharp, and the ones that win leave more offspring.

Not every pressure is about running short. An abiotic factor such as a heatwave, a hard frost, a drought or a flood can kill or spare individuals according to their traits. These are density-independent factors: a frost that kills half of a sparse patch of seedlings kills about half of a crowded patch too. A seedling with an allele that lets it tolerate cold has a better chance of surviving the frost and setting seed.

Any factor that makes some individuals more likely to survive and reproduce than others is a selection pressure. Food, water, nest sites and cold are all selection pressures. In ICE AGE this chapter is C, for competition, and E, for the environmental pressure that sorts the winners from the rest.

Linking question What mechanisms minimise competition? Species can share a place without fighting over everything: one feeds high in the tree and one low, one by day and one by night, young and adults eat different foods (tadpoles and frogs), territories space individuals out, and seeds are carried away from the parent plant. Sharing out resources like this is the subject of B4.2 (ecological niches).

A late frost kills 60% of the seedlings in a sparse patch. How will a crowded patch of the same plant fare?

More young than places4 places with food4 adults, one to a place12 young12 young4 find a place8 find noneFood sets the limit, not the number born.Two patches of seedlings, then a cold snapsparse patchcrowded patchsparse patchcrowded patch2 of 4 die6 of 12 diethe same share in each patchfrost
  1. A habitat has four places with enough food, and four adults hold them.
  2. The adults raise twelve young: far more than the places can hold.
  3. The young compete. Only four find a place; eight find none. The limit is the food, not the number born.
  4. A different pressure: two patches of seedlings, one sparse and one crowded.
  5. A cold snap kills half of each. It is density-independent: the share lost does not depend on the crowd.

D4.1.5 D4.1.6 Fitness and heredity (A, G and E in ICE AGE)

Who survives, who breeds

Look closely at any population and the individuals differ in how well they are suited to their surroundings. Each of these helpful differences is an adaptation. In a crowd competing for the same limited food, the individuals with the better adaptations are more likely to survive, and to breed. That is natural selection through competition within a species, which is the case you are asked to study.

What selection really measures is fitness. Fitness has two parts: the survival value of a genotype (how likely it is to live long enough to breed) and its reproductive potential (how many offspring it tends to leave). An animal that lives to a great age but leaves no young has fitness of zero, because none of its alleles reach the next generation.

One more condition has to hold. The trait must be heritable. Only differences in alleles are handed down. A body-builder's muscles, a scar, a clipped hedge or a bird that grew large on extra food all arise from what happened in life, and are not encoded in the base sequence of the genes in the gametes. They die with the individual and are not evolution.

Put the pieces in ICE AGE order. Adaptations help some individuals survive and breed (A). Their alleles become commoner in the next generation (G). The population has changed (E). Then the whole cycle starts again with the new generation.

Watch out Fitness is not fitness training. Nothing you gain by exercise, diet or injury changes the alleles you pass on, and a long life is worth nothing to evolution unless it comes with offspring.
Linking question How do intraspecific interactions differ from interspecific interactions? Intraspecific interactions are between individuals of one species, such as competing for the same food or mates, and they drive selection between the alleles in one gene pool. Interspecific interactions are between different species: predation, parasitism, mutualism and competition between species. Each can act as a selection pressure on the other species. C4.1 (populations and communities) takes both further.

A cat lives 15 years but raises no surviving kittens. A second cat lives 6 years and raises 8 kittens that all breed. Which has the higher fitness?

A weight-lifter trains for years and builds large muscles. Her child is born. What can you say about the child's muscles?

Fitness counts young that breedyears livedyoung that bred8none35The middle bird lived least and has the highest fitness.Acquired traits stay behindGACTAGCTparent's DNA basesgrew large on extra foodGACTAGCTchick's DNA baseshatches normal size
  1. Three birds, and how many years each lived.
  2. And how many young each raised that reached breeding age.
  3. The bird that lived least has the highest fitness. Survival matters only because it lets an animal reproduce.
  4. A parent grew large on extra food. Its DNA bases are the same as before: the size was acquired.
  5. The chick inherits the same bases and hatches a normal size: an acquired trait is not heritable.
Change the ground, then switch off heritable: with no inheritance the population stops changing, however many beetles are eaten.
ICEAGElarge jawsslender jawsa wet yearHow common each version of the jaw gene islarge-jaw versionslender-jaw versionwhere it startedgeneration 1ICEAGEdrought: only tough seedsHow common each version of the jaw gene islarge-jaw versionslender-jaw versionwhere it startedgeneration 1ICEAGEthree dry years onHow common each version of the jaw gene islarge-jaw versionslender-jaw versionwhere it startedgeneration 1generation 2generation 3generation 4
  1. Generation 1: the beetles' jaws vary, slender to large, and each carries two copies of the jaw gene (the pips: pink for large, green for slender). Which letter is this?
  2. A drought leaves only tough seeds: who cracks them, and who starves? Predict before you run it.
  3. Three dry years on. What has happened to the population, and to any one beetle?

D4.1.7 Sexual selection

Choosing a mate

Surviving is not enough: an animal has to find a mate. In many animals that is a contest of its own, and the traits that win it can be different from the ones that help you survive. Sexual selection is selection for success in getting mates.

It comes in two forms. In intrasexual selection members of one sex, usually males, compete with each other for access to mates: stags fight, and males display at one another. In intersexual selection one sex, usually females, chooses among the other.

A trait that wins mates is often a sign of overall fitness. A glossy coat, bright colour, loud call or large antler is hard to grow and keep up without good health and a good diet. A chooser that prefers it is, in effect, choosing a mate in good condition, and her offspring inherit the alleles that built the display and the allele for choosing it.

The male birds of paradise show it best. In many species the males have spectacular plumage, and perform elaborate displays and dances to the females that come to watch and pick. Each generation, the males with the finest displays leave the most young, and the plumage gets more extreme.

But the display has costs. Long feathers hinder flight, bright colour is easier for predators to see, and displaying takes energy and time that could go into feeding. A trait of this kind spreads only while the extra young it brings outweigh the extra risk. Natural selection and sexual selection can pull in opposite directions, and the next chapter is a real experiment on that tug of war.

Example Frogs. In many species the males call to attract females, and females tend to be drawn to louder calls. Calling costs energy, and it also tells predators where the caller is.
Watch out Sexual selection is not "survival of the fittest" in the sense of the strongest staying alive. A male that is eaten the day after mating has still passed on his alleles, and one that lives for years but never mates has not.

A male's bright tail makes him easier for predators to catch. How can the trait still spread through the population?

Males differ in plume lengthfemaleshe chooses thelongest plumeyoungsiredextra youngextra risk
  1. Four males of one species, with plumes of different lengths.
  2. A female looks them over, and chooses the longest plume.
  3. The male she chooses sires the most young, and they inherit his alleles for a long plume.
  4. But a long plume has a cost: it spreads only while the extra young outweigh the extra risk.

D4.1.8 Modelling selection: Endler's guppies

Two pressures, tested

Wild guppies (Poecilia reticulata) live in streams in Trinidad. The males carry spots of orange, black and shimmering colours, and the females are plain. Females tend to prefer the more colourful males, which is sexual selection pushing towards brighter males. But a bright male is also easier for a predator to see, which is natural selection pushing towards duller males.

The balance depends on the predators. Streams with dangerous predators have duller males, and streams with few predators have more colourful ones. That is a pattern, not a proof: streams differ in many other ways too.

John Endler tested it as a scientist should, by controlling everything except the thing he suspected. He set up ponds with the same starting guppies, the same food and the same conditions, and changed one thing, the predator: none, a weak predator, or a dangerous one. Then he followed the males' spots over many generations. The experiment models the streams: it is simpler than nature, so one pressure can be isolated and measured.

His result, in the direction that matters: where the predator was dangerous, the males came to have fewer and duller spots. Where there was no predator, or only a weak one, they ended up more colourful than they started. Predation is a strong selection pressure, strong enough to reverse what sexual selection builds.

Reading data like this Ask four questions of any graph of mean spots per male against generations. Where does each pond start (the same place, if the start was controlled)? Which way does each line move? Do the lines separate as the generations pass? And which pond is the control, the baseline the others are compared with? Differences between ponds, not changes inside one, are what show selection.
Watch out A single male does not change colour in response to the predator. The change is in the population, across generations, as males with some alleles leave more young than others. Check which one a result is claiming.

In the pond with the dangerous predator, males end up with fewer spots. What is the best explanation?

The dry lab at the end of this guide lets you run the ponds yourself, then change the predator, the strength of mate choice and the number of generations.

Streams with fierce predatorsStreams with few predatorsmales duller, few small spotsmales more colourfulEndler's test: the same start in three pondsno predatorthe controlweak predatorkillifish typestrong predatorpike cichlid typemore colourmore colourless colourYou changepredator levelYou measurespots on the malesYou keep the samestart fish, food, warmth
  1. In the wild, males in streams with fierce predators are duller than males in streams with few.
  2. Endler set up three ponds with the same starting guppies: no predator, a weak predator and a dangerous one.
  3. He changed one thing, the predator, measured the males' spots, and kept everything else the same.
  4. After the generations: more colour with no or a weak predator, less colour with the dangerous one.

D4.1.9 D4.1.10 The gene pool and allele frequencies

The gene pool

Natural selection happens to individuals, but evolution is a change in the population. To describe that change you need a way to describe a whole population's genes at once. A gene pool is all the genes and their different alleles in a population. Picture every allele of every individual tipped into one bag.

The useful number to take from a gene pool is an allele frequency: how common each allele is. Count the copies of one allele, and divide by the total number of alleles of that gene in the population. A diploid individual has two alleles of each gene, so a population of 10 has 20. Frequencies of all the alleles of a gene add up to 1, or 100%.

Two populations that are geographically isolated, with few or no individuals breeding across the gap, build up different gene pools. Their allele frequencies differ, because the pressures on each side differ and no alleles pass between the two. Comparing allele frequencies between populations is a way to see how far apart their gene pools have become.

Real allele frequencies are stored in public databases of human populations, and you are asked to search one. Your teacher will name the database your class uses. A search like this works with any of them.

Your search, in six steps
  1. Open the database your teacher names, and search for a gene or a variant. A good choice is the sickle-cell allele of the haemoglobin beta gene (HBB; the variant is called rs334).
  2. Find the table of allele frequencies by population.
  3. Choose two populations from different parts of the world, for instance one from West Africa and one from northern Europe.
  4. Write down the frequency of the same allele in each.
  5. Compare them. Which is higher, and by roughly how much?
  6. Suggest a reason. The sickle allele protects heterozygotes against malaria, so where malaria has been common its frequency has been pushed up.

No database to hand? The pattern you should find is that the allele is absent or very rare in the northern European populations and common in several West African ones.

A population of 50 diploid individuals has 30 copies of allele B in its gene pool. What is the frequency of B?

Population 1: ten individualseach individual carries two allelesgene pool: 20 allelesPopulation 1: share of each allele0.70.3allele Aallele ashare of all alleles = allele frequencyseaPopulation 2, cut off from population 10.40.6
  1. Ten individuals, each carrying two alleles of one gene: allele A in green, allele a in pink.
  2. Tip every allele into one jar: the gene pool, 20 alleles, 14 of them A and 6 of them a.
  3. The allele frequency of A is 14 out of 20, which is 0.7. The frequencies of A and a add to 1.
  4. A second population, cut off from the first, has different frequencies: 0.4 and 0.6.

D4.1.11 Selection changes allele frequencies

Selection shifts allele frequencies

Darwin explained evolution without knowing how traits are inherited. The missing piece came later, with the study of genes: Mendel's results on inheritance were rediscovered in the early 1900s, and then DNA was found to carry the code. Biologists joined Darwin's natural selection to genetics in what is now called neo-Darwinism.

That gives a sharper definition. Evolution is a change in the allele frequencies of a gene pool over generations. Mutation adds new alleles, and natural selection is a process that changes how common each one is: individuals differ in their heritable traits, so they differ in how many offspring they leave, and so the alleles that go with the successful traits become more common in the gene pool.

The figure runs one population by hand. Twenty beetles have two alleles for jaw strength: J (the dominant one) lets a beetle crack tough seeds, and j does not. A drought leaves only tough seeds. Beetles with two j alleles cannot feed and do not breed. The survivors carry 20 J out of 32 alleles, so the frequency of J has risen from 0.5 to 0.625. No beetle changed. The gene pool did.

Two points to carry away. Selection acts on phenotypes, the traits that individuals show, but evolution is read in alleles, in the population. And an allele does not vanish just because the trait it causes is disadvantageous: here j is still carried by the surviving Jj beetles.

Watch out The frequency of an allele can change without any one individual changing at all. Do not write "the beetles evolved"; write "the allele frequency in the population changed".

In the beetle example, the J share of the gene pool was 0.5 before the drought and 20 of the 32 alleles of the survivors. What is it among the survivors?

Generation 0: twenty beetlesDrought: only tough seeds are left. The four jjbeetles cannot crack them and starve.Alleles in generation 0: 20 J and 20 jJjp = 0.5Alleles in the 16 survivors: 20 J and 12 jJjp = 0.625dashed line: where the J share startedThe survivors breed, so the next generation inherits this pool:J has risen from 0.5 to 0.625. The population has evolved.
  1. Twenty beetles, each with two alleles of a jaw gene. Across the population, J and j are equally common: p = 0.5.
  2. A drought leaves only tough seeds. The four jj beetles cannot feed and do not breed.
  3. Count the alleles of the 16 survivors: 20 J and 12 j. The frequency of J is now 0.625.
  4. The survivors breed, so their gene pool is the next generation's. The allele frequency has changed: evolution.
The same six letters, now with the allele bar labelled in allele frequencies.

D4.1.12 Directional, stabilising and disruptive selection

Three ways selection can act

A trait such as beak depth varies in a population, and most birds sit near the middle. Which individuals are favoured depends on the pressure, and the shape of the whole distribution shows which of three patterns is acting.

In directional selection one extreme is favoured, so the whole population shifts towards it. Antibiotic resistance in bacteria is the standard case: the resistant extreme survives, and resistance alleles become more common.

In stabilising selection the middle is favoured and both extremes are removed. Human birth weight is the classic example: very small and very large babies have lower survival than average ones, so the range narrows around the middle.

In disruptive selection both extremes are favoured and the middle is removed. Imagine a bird on an island with only very hard seeds and very soft seeds: deep beaks crack the hard, shallow beaks pick up the soft, and middling beaks do neither well. The population may split into two forms.

All three change allele frequencies. This is the point students most often miss for stabilising selection, because the average does not move. But the alleles that push the trait to the extremes are removed, so their frequencies fall, and the alleles for the middle form rise. Any selection that sorts individuals by a heritable trait leaves the gene pool changed.

Remember it The three kinds of selection

  • DriveDirectionalone end is removed and the mean shifts
  • SqueezeStabilisingboth ends are removed and the range narrows
  • DivideDisruptivethe middle is removed and two groups emerge
Watch out Stabilising selection is not "no selection". The average stays the same because selection is at work: it keeps removing the extremes. Remember that all three kinds change allele frequencies.

Babies of very low and very high birth weight are less likely to survive than babies of average weight. Which kind of selection is this?

Directional: one end is removedremovedshare of alleles for small, medium and large beaksbeforeafterStabilising: both ends are removedremovedremovedshare of alleles for small, medium and large beaksbeforeafterDisruptive: the middle is removedremovedshare of alleles for small, medium and large beaksbeforeaftersmallmediumlargebeak depthnumber of birdsbeforeafter
  1. Directional: one tail of the curve is removed, and the whole curve shifts. Alleles for large beaks become commoner.
  2. Stabilising: both tails are removed and the curve narrows. Alleles for the extremes become rarer.
  3. Disruptive: the middle is removed and the curve splits in two. Alleles for both extremes become commoner.
The same population cut three ways, with the allele frequencies shifting beneath the curve in every case.

D4.1.13 D4.1.14 The Hardy-Weinberg equation

What a population would be like if it were not evolving

To tell whether a population is evolving you need something to compare it with. The Hardy-Weinberg equation gives it: the genotype frequencies you would expect in a population in genetic equilibrium, where the allele frequencies stay the same from one generation to the next.

For a gene with two alleles, let p be the frequency of one (A) and q the frequency of the other (a). They are the only alleles, so p + q = 1. With random mating, any egg can meet any sperm, so the genotype frequencies are the square of that: p² + 2pq + q² = 1, where p² is AA, 2pq is Aa and q² is aa. The figure shows why the middle term is double: there are two ways to be a heterozygote.

You can use it two ways. From the allele frequencies, work out the genotypes. Or, if one genotype is known, work back to the alleles. The one you can usually see is the homozygous recessive, whose frequency is q² : take the square root to get q, then p = 1 − q.

The equation holds only if five conditions are met. If real data do not fit it, one of them is failing, and the population is evolving.

Remember it The five Hardy-Weinberg conditions

  • LargeLarge populationbig enough that chance does not shift the frequencies
  • RatsRandom matingno mate choice by genotype, no inbreeding
  • MakeMigration: noneno alleles arrive or leave
  • MostMutation: noneno new alleles appear
  • SpaghettiSelection: noneevery genotype survives and breeds equally well
Worked example In a population of 200 beetles, 18 are green, which is a recessive trait. q² = 18/200 = 0.09, so q = 0.3 and p = 0.7. The heterozygotes are 2pq = 2 × 0.7 × 0.3 = 0.42, which is 84 beetles. Check: AA = 0.49 (98 beetles), Aa = 0.42 (84), aa = 0.09 (18); 98 + 84 + 18 = 200.

A recessive condition affects 1 person in 2500. Roughly how many people in the population are carriers (heterozygous)?

In a population, the allele frequencies of A and a are both 0.5, but the genotypes seen are AA 0.40, Aa 0.20 and aa 0.40. What do you conclude?

Random mating: any egg can meet any spermeggs: p = 0.6q = 0.4sperm:p = 0.6q = 0.4p²pqpqq²0.360.240.240.16AA = p²Aa = 2pqaa = q²p² + 2pq + q² = 10.36 + 0.48 + 0.16 = 1If 16% show the recessive trait, q² = 0.16,so q = 0.4 and p = 1 − 0.4 = 0.6
  1. Egg and sperm alleles meet at random. If the frequency of A is p = 0.6 and of a is q = 0.4, each edge of the square is split in those proportions.
  2. The four areas are the four ways eggs and sperm can combine: p², pq, pq and q².
  3. AA is p² = 0.36, Aa is 2pq = 0.48 and aa is q² = 0.16. Together they make 1.
  4. Working back: if 16% show the recessive trait, q² = 0.16, so q = 0.4 and p = 0.6.
Aatwo discs: one individual100 individuals: p = 0.60, q = 0.40predicted, from pcounted36.036AAp²48.048Aa2pq16.016aaq²A recessive condition in 1 in 2500 peopleaaq² = 1 ÷ 2500 = 0.0004qq = √0.0004 = 0.02pp = 1 − q = 1 − 0.02 = 0.98Aa2pq = 2 × 0.98 × 0.02 = 0.0392, about 1 in 25.5AAp² = 0.98² = 0.9604Assumes the population meets the Hardy-Weinberg conditions.
  1. One gene, two alleles: p is the frequency of A, q of a, and p + q = 1. Here p = 0.60.
  2. Hardy-Weinberg predicts the genotypes from p and q: p² + 2pq + q² = 1. When every condition is met, the counts fit.
  3. It works backwards too. From the aa frequency, 1 in 2500: q = 0.02, p = 0.98, and carriers 2pq = 0.0392, about 1 in 26.
Move p and q, then switch off each condition in turn and see how the population compares with the prediction.

D4.1.15 Artificial selection

Artificial selection and its side effects

Artificial selection is natural selection with a person choosing the parents. Farmers and breeders pick the individuals with the traits they want, the biggest cobs, the best milkers, the gentlest dogs, and let only those breed. The alleles that go with the chosen trait become commoner in each generation. It can change a population quickly, because a breeder's choice can be very strict. The A4.1 guide has the examples of crops and domestic animals, and its finches dry lab lets you try selection on a bench, so they are not repeated here.

The key word is deliberate. Humans sometimes change the environment without meaning to, and that can be a selection pressure too. Take an antibiotic. In a huge population of bacteria, mutation has by chance made a few cells resistant. The antibiotic kills the susceptible cells and leaves the resistant ones, which divide with no competitors. The resistance alleles go from rare to common.

Nobody chose the resistant bacteria for breeding, and nobody wanted the result. The antibiotic acted as a selection pressure on variation that already existed, so this is natural selection, an unintended consequence of a human action, not artificial selection. It matters in medicine: the more an antibiotic is used, the stronger the pressure, so using antibiotics only when they are needed slows the spread of resistance.

Compare Artificial selection: a person decides which individuals breed, for a trait they want, and the change is the aim. Natural selection by a human action: the environment decides who survives and breeds, nobody meant it, and the change is a side effect. The mechanism, heritable variation sorted by survival and reproduction, is the same in both.
Watch out The resistance did not appear "because" of the antibiotic, and the bacteria did not learn to resist. The mutation was already there, or arose by chance. The antibiotic only changed who survived.

Farmers spray a crop with an insecticide. After some years most of the pests resist it. Which kind of selection has taken place?

Artificial selection: people choose the parentsgeneration 1generation 2generation 3The breeder lets only the biggest cobs breed.A dish of bacteria meets an antibioticmostly susceptible, one resistantantibioticsurvivors are resistantand go on to multiply
  1. A crop varies in the size of its cobs. Each plant's size is partly heritable.
  2. The breeder picks the largest and lets only those breed.
  3. Repeat for a few generations: the cobs get steadily bigger, as the alleles for size become commoner.
  4. A dish of bacteria with one resistant cell among many susceptible ones.
  5. Add an antibiotic: the susceptible cells die, the resistant one multiplies. Nobody chose it, so this is natural selection.

02Dry lab: Endler's guppy ponds

Four ponds of guppies side by side: no predator, a weak predator, a strong predator and a control. Choose how fussy the females are, run the generations, and watch the males' spots rise or fall as sexual selection and predation pull in opposite directions.

Your questionWhat decides how colourful male guppies become? Do females choosing the brightest males always win, or does it depend on the predator? Start with one pond and one change, then compare it with a control pond.
What you can changeThe predator in each pond (none, weak, strong, or the control, where mating is random and nothing is selected), how strongly the females prefer colourful males (none, weak, strong), the number of generations (10, 20 or 30) and the spots the males start with (6, 9 or 12). Change one thing at a time and keep the rest the same.
What to recordFor each run: the settings, the mean number of spots per male each generation, the mean colour area, how many of the plainest and showiest males survived, and how many young each fathered. Repeat each set-up at least three times: the runs scatter, as real populations do. The coach under the bench helps you plan, process and evaluate.
Pond Ano predatorsPond BkillifishPond Cpike cichlidPond Dcontrol: random pairsmale (spots counted)femaleGeneration 0 of 20pike cichlid, a strong predatorkillifish, a weak predatorThe settingsPonds: A no predators, B killifish, C pike cichlid, D control (no predators, random pairs)Kept the same: strong mate choice, founders with 9 spots on average, 20 generations, 200 guppies a pondRecord each generation: mean spots per male, colour area,survival of the plainest and showiest males, young per maleResultsPredatorPondSpots, gen 0gen 20ChangePer generationno predatorskillifishpike cichlidcontrol
Guppy ponds. Four ponds start from the same stock of guppies. Each has its own predator, or none; the control has no predators and pairs females with males at random. Run them for 20 generations and record the mean spots per male.

IA coach

Write it up as an investigation

Your results from this dry lab can be written up the way an IB scientific investigation is: four criteria, each worth 6 marks. Tick each line as your write-up does it. The coach on the page checks these as you go; it never gives a mark.

1 Research design

A focused question set in its biology; your choices of what to measure, how, over what range and how often, explained rather than listed; what you kept the same and how; safety, ethics and the environment; a method someone else could follow and repeat.

  • My question names what I changed.
  • It gives the range I used, with units.
  • It names what I measured and how.
  • It names the organism or system.
  • My background explains, with the biology, why the two should be linked.
  • I cite my sources, and each reference can be traced.
  • If I give a hypothesis, it predicts and explains.
  • Every variable has its How, Why and Units.
  • For each thing kept the same, I say how I kept it so.
  • I list at least three things I kept the same.
  • I name the apparatus that set what I changed, and the instrument that measured.
  • I test enough values.
  • My values spread wide enough.
  • I run at least three trials at every value.
  • I run a control.
  • I explain why these values, this many and this often.
  • Someone else could repeat my method.
  • I deal with safety, ethics and the environment.

2 Data analysis

Raw data recorded clearly, with titles, units, uncertainties and the same decimal places down a column; what you saw as well as what you measured; processing that suits the question, shown with a worked example; the spread shown; a graph a reader can trust.

  • Each table has a numbered title that says what it shows.
  • Every heading has its quantity and its unit.
  • Every raw data heading has its uncertainty.
  • Each uncertainty is realistic for the instrument.
  • Each uncertainty has the same decimal places as its readings.
  • I record what I saw as well as what I measured.
  • I show one worked example of my processing, and it is right.
  • I say why I left out any trial.
  • My error bars show a spread my repeats can support.
  • My graph has a numbered title and labelled axes with units.
  • I say what the error bars show.
  • A straight line, if I draw one, has its R².
  • A statistical test, if I run one, has its hypotheses and suits the data.

3 Conclusion

Your question answered from the processed data, with numbers quoted and the spread taken into account; the biology that explains the pattern; a comparison with published work; no claim bigger than the data.

  • I answer my question in a sentence, naming both variables.
  • I quote my processed numbers.
  • I say how sure the spread lets me be.
  • I say whether my hypothesis is supported.
  • I explain the pattern with biology.
  • I compare my result with published work, and cite it.
  • I claim no more than my data show.

4 Evaluation

Weaknesses and limitations of this method (not slips in carrying it out), with evidence from your own data, the effect each had on the results, which mattered most and why, and a realistic improvement for each.

  • I give at least three weaknesses or limitations.
  • Every row is filled in.
  • No slips in carrying it out: weaknesses of the method itself.
  • Each improvement is specific: which instrument, how many, what changes.
  • Each effect says what it did to my data.
  • At least two rows point to evidence in my own data.
  • At least one limitation: what this experiment cannot tell me.
  • I say which mattered most, and why.
  • I deal with what stands out in my data.
  • My improvements improve this investigation, not a new one.

Variables

VariableHow?Why?Units
Independent: predator in the pond
Dependent: rate of change in mean spots per male
Kept the same: strength of mate choice (how much females favour colourful males)
Kept the same: starting mean spot number of the founders
Kept the same: number of generations
Kept the same: number of guppies each pond holds (200)
Kept the same: the ponds: size, gravel, plants, light and food
Kept the same: where the founders came from (one mixed stock)

Evaluation

Weakness or limitationWhich?Evidence in my dataEffect on my resultsImprovement

Data generated with the guppy ponds dry lab, biologybybradford.com (a simulation; the values are illustrative). These checks catch what is missing. They cannot judge how good it is: that is what your teacher does.

Find out

  • What does mate choice do on its own? Set strong mate choice and run the pond with no predator (A) beside the control pond (D, where nothing is selected) for 20 generations, three times. Which way does the mean number of spots move in each?
  • What does predation do? Keep everything the same and compare ponds A, B and C: no predator, a weak one and a strong one. Run each three times. Where does each line end, and in what order?
  • Which pressure wins? In the strong predator pond (C), change mate choice from none to weak to strong. At what point does colour stop falling, or start to rise?
  • Does the start matter? Start the ponds with 6, then 9, then 12 spots per male. Do the strong predator ponds end up in the same place?
  • How hard does each pressure push? Open the Two pushes graph. Which pressure pushes harder in each pond, and does the mean follow the stronger one?

Record

  • For every run: the pond, the predator, the strength of mate choice, the starting spots and the number of generations.
  • Each generation: the mean spots per male and the mean colour area. Note how many of the plainest and the showiest third of males survived, and how many young each fathered, when the bench shows them.
  • Change one setting at a time, and read every pond against the control.
  • Repeat each set-up at least three times and take a mean. Chance is part of the model, as it is of real ponds, so no two runs match exactly.
  • Use the live graph to compare the ponds, or download the CSV and plot your own with error bars.

Pond A has no predator, and the females strongly prefer colourful males. Which way should the mean number of spots per male move over the generations?

You keep the females' preference exactly as it was, but add a strong predator (pond C). The mean number of spots now falls. What does the difference between the two ponds show?

Why does the lab ask you to run each set-up at least three times, when one run already gives a line?

03Check

A fresh paper from the exam bank on this guide's topics each time: past-paper style questions, marked as you go, and saved to your Exam practice progress when you are signed in.

Exam practice

Exam questions on D4.1, marked

Start 15 marks of exam questionsYour exam practice

D4.1 revision mapD4.1 exam questionsD4.1 multiple choice quizD4.1 higher level quizD4.1 pub quizD4.1 Tarsia puzzleD4.1 dominoesBack: A4.1 evolution and speciationLinked: C4.1 populations and communitiesLinked: B4.2 ecological niches

04Downloads

Take the guide with you, or into a classroom with no internet.

DOCXNote templateFor students to fill in alongside the guide: a page per chapter with cues, gaps for the key terms and a summary box. Edit it in Word or Google Docs (A4). 2.3 MB PDFNote template, to printThe same note template, ready to print (A4). 2.4 MB PDFRevision guideThis guide as a printed book: every chapter, every step of every figure, the key terms, the predictions with answers at the back, and a link to the Check. 7.4 MB HTMLThe guide, offlineOne file that opens in any browser with no internet: the figures, the animations and the dry lab all work. 1.5 MB

The whole IB Biology revision guide, every guide written so far in one PDF.

05Teacher resources

The guide as three one-hour lessons, and three more at higher level. Each has a plan to print (the timings, what to project and where to stop, the activities with what they need, what students get wrong) and slides to teach it from, in the same order, with the teacher's moves in the notes.

LessonCoversFocusChaptersLesson planSlides
1Variation, pressure and ICE AGE D4.1.1 to D4.1.460 min Natural selection as the mechanism of evolution, and its first three ingredients: variation, overproduction and the pressures that sort the winners 1, 2, 3 Lesson planPDF 701 KB SlidesPPTX 3.9 MB
2Fitness, heritability and choosing a mate D4.1.5 to D4.1.760 min Fitness as survival and reproduction, why only heritable traits are selected, and sexual selection as a pressure of its own 4, 5 Lesson planPDF 668 KB SlidesPPTX 4.1 MB
3Endler's guppies D4.1.860 min Interpreting data from a controlled experiment on sexual and natural selection, and planning one of your own 6, dry lab Lesson planPDF 598 KB SlidesPPTX 557 KB
4Gene pools and neo-Darwinism HL D4.1.9 to D4.1.1160 min Describing a population by its allele frequencies, and defining evolution as a change in them 7, 8 Lesson planPDF 688 KB SlidesPPTX 3.6 MB
5Three kinds of selection, and Hardy-Weinberg HL D4.1.12 to D4.1.1460 min Directional, stabilising and disruptive selection, and using the Hardy-Weinberg equation and its conditions 9, 10 Lesson planPDF 721 KB SlidesPPTX 2.5 MB
6Artificial selection, and putting it together HL D4.1.1560 min Artificial selection against natural selection, antibiotic resistance as natural selection, and a consolidation of the whole subtopic 11 Lesson planPDF 608 KB SlidesPPTX 462 KB
PDFEvery lesson planAll six plans in one PDF, with this overview at the front. 1.5 MB KEYNote template, filled inThe class’s note template with every gap filled. 2.3 MB

The note template, the revision guide and the offline copy are in Downloads, for you and the class.

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How this was made. The course structure, the activities and the teaching choices come from my own classroom, my presentations and my years of teaching IB and AP Biology. I use Claude, an AI, to help write and build the interactive guides, lesson plans, slides and practice questions, and to draw the diagrams and code the interactives. I direct the work and review it against the IB and AP specifications before it goes live.