Introduction
Okay, let's talk variation and selection, aka the chapter where biology finally becomes evolution. You know how no two people are exactly alike? That's variation. And you know how a desert cactus and an Arctic fox look nothing like each other? That's what happens when the environment spends millions of years picking winners. That's selection.
Here's the whole chapter in one sentence: differences show up by chance (variation), and the environment quietly keeps the ones that fit (selection). Get that and everything else, mutation, adaptation, natural selection, selective breeding, is just the details. And the one rule that trips everyone up: living things don't change themselves on purpose to survive. The change comes first, by luck. Let's get into it 🧬
Variation: Everyone's a Little DifferentE18.1
Variation is just the differences between individuals of the same species. Your whole class is human, but everyone's got different heights, hair, and everything else. That's variation, and it's the raw material evolution works with, no differences means nothing to select.
It comes from two places. Genetic variation is from the alleles (gene versions) you were born with, and it can be passed to your kids. Environmental variation is from your life, diet, exercise, sun, and it does not get passed on. Most things are a mix: your height is genes plus how well you ate growing up.
> 🔑 Key idea: variation = differences between individuals of the same species. Genetic variation is inherited (passed on); environmental variation is from your lifestyle (not passed on).
Worked Example: Name That ConceptE18.1
Problem Within a single species, the differences seen between one individual and another are known as which of the following: variation, mutation, selection, or adaptation?
Step 1 The definition here is literally "differences between individuals of the same species". Match it to the word: that's variation.
Step 2 Rule out the traps by their job: mutation is a cause of variation, selection is the process that acts on it, and adaptation is the result.
Step 3 Only one word means the differences themselves, so the answer is variation. 🎯
Continuous Variation: The Smooth RangeE18.1
Continuous variation gives a whole range of values with every in-between possible. Human height is the poster child: people aren't just "tall" or "short", they come in every height in between, most clustered in the middle. Body mass and hand span are the same vibe.
The cause? Many genes working together, plus the environment. Because loads of genes and your lifestyle all pitch in, you get a smooth spread. Plot it and you get a smooth bell-shaped curve, tallest in the middle.
> 🔑 Key idea: continuous variation = a smooth range (all in-betweens), caused by many genes + environment, drawn as a smooth curve.
Worked Example: The Herd DataE18.1
Problem A farmer records the body mass of a large herd of cattle and finds the values vary smoothly around a central mass, with every intermediate value present. What type of variation is this?
Step 1 The magic phrase is "vary smoothly … every intermediate value present". Every in-between value exists, so this is continuous variation.
Step 2 That fits the cause: many genes controlling mass, plus how much food each cow got. It's not discontinuous, because there are no separate groups with gaps.
Step 3 So body mass here is a continuously varying feature, and you'd draw it as a smooth curve. 🐄
Discontinuous Variation: Pick a BoxE18.1
Discontinuous variation gives a few separate groups with nothing in between, you're in one box or another. Human blood group is the classic: you're O, A, B or AB, and there's no "halfway" group. Tongue-rolling (yes or no) and pea seeds (round or wrinkled) are the same deal.
The cause is one gene, or just a few, with basically no environmental input. That's why the phenotypes fall into sharp categories. Plot it and you get a bar chart of separate bars, not a smooth curve.
> 🔑 Key idea: discontinuous variation = a few separate groups (no in-between), caused by one/few genes, drawn as a bar chart. Test it: can you be in between? No = discontinuous.
Worked Example: Tongue-Roll TestE18.1
Problem Human tongue-rolling ability is either present or absent, everyone is in exactly one of two categories, with no one partway between. What type of variation is this, and why?
Step 1 Two categories (can / can't) with nobody partway between screams discontinuous variation.
Step 2 The reason: it's a limited number of distinct phenotypes with no intermediates, controlled mostly by genes. Compare with height, where every in-between exists (that's continuous).
Step 3 Since there are no in-between values here, it's discontinuous. 👅
Mutation: Where New Alleles Come FromE18.1
A mutation is a change in the DNA, and here's the headline fact: mutation is the way new alleles are made. Sexual reproduction only remixes alleles that already exist, it can't invent a new one. Only mutation can. So mutation is the ultimate source of all genetic variation.
Mutations happen naturally at a low rate, but some things speed them up, these are mutagens. The two to know: ionising radiation (UV light, X-rays) and some chemicals (like the ones in tobacco smoke). More mutations = more new alleles, but also a higher cancer risk. A gene mutation itself is a random change in the base sequence of DNA, random meaning it happens by chance, not because it's needed.
> 🔑 Key idea: only mutation makes new alleles. Gene mutation = a random change in the DNA base sequence. Ionising radiation and some chemicals raise the mutation rate.
Worked Example: Making Something NewE18.1
Problem What is a way in which new alleles are formed?
Step 1 Recall the exact syllabus line: mutation is the way in which new alleles are formed. So the answer is mutation.
Step 2 Check the traps: meiosis and fertilisation only mix existing alleles (new combos, not new alleles); selection just changes how common an allele is.
Step 3 Only mutation makes a genuinely new allele, so mutation it is. 🆕
Adaptive Features: Built for the JobE18.2
An adaptive feature is an inherited feature that helps an organism survive and reproduce in its environment. Two words earn the marks: inherited (born with it, from genes, not gained in life) and helps it survive and reproduce (gives an advantage where it lives). Polar bear's thick fur, cactus spines, fish gills, all adaptive features.
Don't mix up adaptive features (the actual structures) with adaptation (the slow process, powered by natural selection, that produced them). And watch the classic trap: something you gain in your life (a suntan, gym muscles) is not an adaptive feature, because it isn't inherited.
> 🔑 Key idea: adaptive feature = inherited feature that helps survival + reproduction. Inherited from genes = counts. Gained in life = doesn't count.
Worked Example: Two Rabbits, One Cold DayE18.2
Problem Two rabbits live in a cold place. One has thick fur from the genes it was born with. The other has thin fur but was kept warm by a keeper. Which one has an adaptive feature for the cold?
Step 1 An adaptive feature must be inherited. Rabbit 1's thick fur came from its genes, so it can be passed to its babies, that's a real adaptive feature.
Step 2 Rabbit 2 is only warm because of the keeper (something outside it), not from its genes. That warmth can't be passed on, so it's not an adaptive feature.
Step 3 So the answer is rabbit 1. Inherited = adaptive; gained in life = not. 🐰
Xerophytes & Hydrophytes: Opposite Water ProblemsE18.2
Two plant examples with opposite problems. A xerophyte lives somewhere super dry (desert), so its whole design is about saving water: thick waxy cuticle, tiny or rolled leaves (or spines), sunken/hidden stomata, hairs to trap damp air, and water-storing stems. Every feature = less water lost.
A hydrophyte lives in or on water, so saving water is irrelevant, its problems are floating and gas exchange. Think big air spaces (buoyancy + moving gases), stomata on the top surface only (a floating water-lily leaf, because the bottom's on the water), and a thin or absent cuticle.
> 🔑 Key idea: xerophyte = dry habitat, everything saves water (waxy cuticle, rolled leaves, sunken stomata). Hydrophyte = water habitat, features for floating + gas exchange (air spaces, stomata on top only, thin cuticle).
Worked Example: Why Marram Grass Rolls UpE18.2
Problem Marram grass grows on dry, windy sand dunes. Its leaves roll up so the stomata are enclosed inside, and the stomata sit in pits. How do these features help it survive?
Step 1 The problem is too little water, and wind dries the plant out faster.
Step 2 Rolling the leaf hides the stomata inside, and the pits keep them out of the wind. Both trap a layer of damp air right next to the stomata.
Step 3 Damp air outside the stomata means the water-vapour gradient is smaller, so less water escapes by transpiration. Both features do the same job: trap damp air, lose less water. 🏜️
Natural Selection: Nature Picks the WinnersE18.3
Natural selection is how a species slowly changes to fit its home. Learn it as steps, examiners love the steps:
1. Variation – individuals differ, and some differences are inherited. 2. Overproduction + competition – too many offspring, so there's a struggle to survive. 3. The best-suited survive – "survival of the fittest" (fittest = best-suited, not strongest). 4. They breed – survivors pass on their helpful alleles. 5. Change over generations – the helpful allele becomes more common, so the group changes.
The differences were already there by chance. The environment just filters them. That slow build-up of "fit" is adaptation.
> 🔑 Key idea: natural selection = variation → competition → best-suited survive → they breed → helpful allele gets more common over generations. The variation comes FIRST, by chance.
Worked Example: Dark vs Pale BeetlesE18.3
Problem Insects live on dark tree bark. Some are dark, some pale, and birds eat the ones they can see most easily against the dark bark. How will the population change over time?
Step 1 Variation: some are dark, some pale (inherited). Selection pressure: birds spot the pale ones easily on dark bark, so more pale ones get eaten.
Step 2 The dark ones are camouflaged, so more survive, then breed and pass on the dark allele.
Step 3 Over generations the dark allele becomes more common, so the population becomes mostly dark. (They didn't turn dark on purpose, the dark ones just survived better.) 🪲
Antibiotic Resistance: Evolution in Fast-ForwardE18.3
Antibiotic-resistant bacteria are natural selection you can watch happen, and it's on the exam constantly. The steps:
1. A chance mutation makes one bacterium resistant, this happens before the antibiotic is used. 2. The antibiotic kills the non-resistant bacteria. The resistant one survives. 3. The survivor reproduces fast and passes on the resistance allele. 4. Over time most of the population is resistant, and the antibiotic stops working. Hello, superbug (like MRSA).
The antibiotic doesn't create the resistance, it just kills off everything that isn't resistant, leaving the resistant ones to take over. Overusing antibiotics just runs this process more often.
> 🔑 Key idea: mutation gives resistance BEFORE the antibiotic → antibiotic kills the non-resistant → resistant survivors breed → resistant strain takes over. The antibiotic selects, it doesn't cause the mutation.
Worked Example: Put It In OrderE18.3
Problem Put these events in order: (1) the resistant bacteria reproduce; (2) a chance mutation makes one bacterium resistant; (3) the antibiotic kills the non-resistant bacteria; (4) most of the population becomes resistant.
Step 1 The mutation comes first, before the antibiotic, because resistance is there by chance: that's (2).
Step 2 Then the antibiotic kills the non-resistant (3), and the resistant survivor reproduces (1).
Step 3 Finally most become resistant (4). So the order is 2 → 3 → 1 → 4. Never put the mutation after the antibiotic. 🔢
Selective Breeding: Humans Play NatureE18.3
Selective breeding (aka artificial selection) is when humans choose which organisms breed, to get features we want. Same idea as natural selection, but a person does the choosing instead of the environment. The three magic words:
Choose the individuals with the feature you want → Breed them together → Repeat over many generations (each time picking the best offspring).
We do it for crops (higher yield, bigger grain, disease resistance) and animals (more meat, more milk, faster growth). Miss out "repeat over many generations" and you drop marks, one round of breeding barely shifts anything.
> 🔑 Key idea: selective breeding = choose, breed, repeat over many generations. Humans pick the parents for features useful to us.
Worked Example: Bigger Chickens, PleaseE18.3
Problem A farmer wants chickens with a larger body mass. How could the farmer do this by selective breeding?
Step 1 Choose the largest chickens to be the parents.
Step 2 Breed them together, then from their offspring pick the largest again as the next parents.
Step 3 Repeat over many generations. Each round, the alleles for large body mass get more common, so the flock's average mass climbs. (Just feeding them more doesn't count, that changes size but isn't inherited.) 🐔
Key Points to Remember
- Variation = differences between individuals of the same species; genetic variation is inherited, environmental variation isn't.
- Continuous variation: a smooth range, all in-betweens (height, mass); many genes + environment; smooth curve.
- Discontinuous variation: separate groups, no in-between (blood group, tongue-rolling, seed shape); one/few genes; bar chart.
- Only mutation makes new alleles. Gene mutation = a random change in the DNA base sequence. Ionising radiation and some chemicals raise the mutation rate.
- Three sources of genetic variation: mutation (new alleles), meiosis and random fertilisation (new combinations).
- Adaptive feature = an inherited feature that helps survival and reproduction; adaptation is the process that makes them. Xerophytes save water; hydrophytes are built for water.
- Natural selection: variation → competition → best-suited survive → they breed → helpful allele becomes more common. Antibiotic resistance is natural selection in action.
- Selective breeding = choose, breed, repeat over many generations; humans pick the parents. Natural selection = the environment picks; artificial = humans pick.