Introduction
Okay, let's talk about why nobody in your class looks exactly like anyone else, and how that fact is secretly the whole engine behind evolution. Every living thing is a little different from every other member of its species, and biology has a name for that: variation. It sounds simple, but it's the starting point for one of the biggest ideas in the whole course.
We're going to cover the two flavours of variation, where new alleles actually come from, how to spot an adaptive feature when a question describes some random organism you've never heard of, and the two big engines that filter variation over time: nature doing it (natural selection) and humans doing it on purpose (selective breeding). Once this clicks, questions about giraffes, drought-tolerant beans and antibiotic-resistant bacteria all start looking like the exact same question wearing a different costume. Let's go 🧬
Differences That CountC18.1
Variation is the differences between individuals of the same species. Height, mass, colour, all of it, no two individuals of one kind of organism are exactly alike. This matters because variation is the raw material everything else in this chapter runs on: without differences to choose between, selection would have nothing to do. Variation splits into two types by where it comes from. Genetic variation comes from the alleles you inherit, it's passed on to your kids. Environmental variation comes from your life, your diet, your climate, an injury, and it dies with you, it's not heritable.
> 🔑 Key idea: variation = differences within one species. Genetic variation gets passed on; environmental variation doesn't.
Worked Example: Same Species, Different StoryC18.1
Problem Variation in biology is best described as: differences between individuals of the same species, or the similarities every member of a species shares?
Step 1 Match the wording to the definition: variation = differences, not similarities.
Step 2 Also check it's within one species, not a comparison between two different kinds of organism.
Step 3 So the answer is the differences between individuals of the same species. If a question compares two different species, that's classification, not variation.
The Sliding ScaleC18.1
Continuous variation gives you a whole range of values, with every value in between possible: body mass, height, hand span. Nobody is "tall" or "short" only, there's a whole spread. It's usually caused by lots of genes acting together, plus your environment. Plot it and you get a smooth, bell-shaped curve, most people bunched in the middle, tailing off at both ends.
> 🔑 Key idea: continuous variation = a full range, every value possible, many genes + environment, smooth curve.
Worked Example: Curve or Boxes?C18.1
Problem Which of these is an example of continuous variation: body mass in cats, or ABO blood group in humans?
Step 1 Ask the one real question: can an individual have an in-between value?
Step 2 A cat's body mass can be any weight at all, light, heavy, everything between. Blood group is one of four fixed categories, no in-between.
Step 3 So body mass is the continuous one. Blood group is discontinuous, that's next.
Pick a BoxC18.1
Discontinuous variation sorts individuals into a small, fixed number of separate categories, no in-betweens allowed. ABO blood group (O, A, B, AB), tongue-rolling (you can or you can't), pea seed shape (round or wrinkled). It's usually caused by one gene, or just a few genes, with little or no effect from the environment. Plotted, it gives a bar chart of separate bars, not a curve.
> 🔑 Key idea: discontinuous variation = fixed categories, no in-betweens, one or few genes, little/no environment, bar chart.
Worked Example: Four Boxes, No BlendC18.1
Problem Every student in a class belongs to exactly one of four blood groups, A, B, AB or O, with no in-between values. What does this tell you about blood group?
Step 1 No in-betweens is the discontinuous signal, straight away.
Step 2 Discontinuous variation is controlled by genes only, with little or no environmental input.
Step 3 So blood group is discontinuous variation, caused by genes only. Having four categories instead of two doesn't change the answer, the test is always "are there in-betweens?"
Data DetectiveC18.1
To figure out which type of variation a feature shows, you measure it across a large group of individuals, tally the values, and plot the result. A smooth curve says continuous, separate bars say discontinuous. A tiny sample won't show you the real pattern, you need a big group before the shape becomes obvious. And remember: it's the feature, not the organism, that's continuous or discontinuous, the same plant can show one feature of each type at once.
> 🔑 Key idea: investigate with a large sample, plot the frequencies, and classify feature by feature, never "the whole organism".
Worked Example: Two Features, One PlantC18.1
Problem Tomato plants in one bed differ in fruit mass and in whether their stems are hairy or smooth. What type of variation does each feature show?
Step 1 Fruit mass: any value across a range, so it's continuous.
Step 2 Stem type: hairy or smooth, nothing in between, so it's discontinuous.
Step 3 Both features show variation, and one plant can absolutely show a continuous feature and a discontinuous feature at the same time. Classify feature by feature.
Random Glow-UpC18.1
A mutation is a genetic change: a change in the genetic material of a cell. Here's the line worth memorising word for word: mutation is the way in which new alleles are formed. No mutation, no new alleles, ever, full stop. Mutations happen naturally at a low background rate, but certain things speed that rate up, called mutagens. Two are on your syllabus: ionising radiation (UV light, X-rays, radiation from radioactive substances) and certain chemicals (some of what's in tobacco smoke, for one). More mutagen exposure = more mutations = more new alleles appearing in a population.
> 🔑 Key idea: mutation = genetic change = the way new alleles are formed. Ionising radiation and certain chemicals raise the mutation rate.
Worked Example: Radiation and Rare New TraitsC18.1
Problem A group of plants growing near a source of ionising radiation shows more new inherited differences than a similar group growing far away. Why?
Step 1 Ionising radiation is a named mutagen, it increases the rate of mutation.
Step 2 More mutation means more new alleles appearing.
Step 3 So the extra inherited differences come from a higher mutation rate caused by the radiation, giving more genetic variation. Not the soil, not the plants "deciding" to mutate, just plain chance mutation happening more often.
Built For ItC18.2
An adaptive feature is an inherited feature that helps an organism survive and reproduce in its environment. Two boxes have to tick: it has to be inherited (from genes, not learned during life), and it has to actually help, in that specific habitat. A rabbit's genetically long ears that help it hear predators? Adaptive. A rabbit that learned to freeze when a shadow passes over? Not adaptive, that's a learned behaviour that dies with that one rabbit and can never be passed on. Exam questions love handing you an unfamiliar organism and asking you to pick out its adaptive feature from a description, so always check both boxes before you answer.
> 🔑 Key idea: adaptive feature = inherited + actually helps survival/reproduction here. Learned or acquired traits never count, no matter how useful they look.
Worked Example: Ears vs. FreezingC18.2
Problem A rabbit is born with long ears that help it hear predators. A different rabbit learned to freeze whenever a shadow passes over it. Which one has an adaptive feature?
Step 1 Check box one: is it inherited? The long ears are, "born with" means from the genes.
Step 2 The freezing was learned during that rabbit's own life, so it isn't inherited and it can't be passed to offspring.
Step 3 So only the rabbit with the long ears has an adaptive feature. Being useful isn't enough on its own, it has to be inherited too.
Survival of the Best-FitC18.3
Natural selection is the five-step process that drives evolution. Learn it as a sequence, not a vibe: 1. Variation already exists in a population (some individuals already differ, thanks to mutation). 2. Competition — more offspring are born than the environment can support, so there's a fight for limited resources. 3. Survival of the better-adapted — individuals whose features suit them survive that fight more often. 4. Reproduction — the survivors breed and pass their helpful alleles on. 5. Change over time — those alleles become more common in the population, generation after generation. The single biggest trap in this whole chapter: organisms never change themselves on purpose. A giraffe didn't stretch its neck longer, giraffes born with a longer neck already just happened to survive better where food grows high up.
> 🔑 Key idea: variation → competition → survival of the better-adapted → reproduction → alleles more common over time. Variation always comes first, by chance, selection just decides what survives.
Worked Example: Bacteria vs. AntibioticC18.3
Problem A patient takes an antibiotic. Most of the bacteria are killed, but a few carrying a resistance allele survive and multiply. Why does the population become mostly resistant?
Step 1 The resistance allele was already there, in a few bacteria, before the antibiotic was ever taken, pure chance mutation.
Step 2 The antibiotic kills the non-resistant bacteria, that's the selection pressure. The resistant ones survive.
Step 3 The survivors multiply fast, passing the resistance allele on, so the population becomes mostly resistant over time. The antibiotic didn't cause the resistance, it just selected for bacteria that already had it.
Humans Playing MatchmakerC18.3
Selective breeding, also called artificial selection, is the exact same filtering logic as natural selection, except now a human does the choosing instead of the environment. The cycle: select the individuals with the best form of a feature, breed them together, select again from their offspring, and repeat over many generations. That's how we've shaped crop plants (higher yield, disease resistance, drought tolerance) and domesticated animals (more meat or milk, faster growth) into what they are today.
> 🔑 Key idea: selective breeding = select, breed, repeat over many generations, with a human choosing instead of the environment. Drop "repeat over many generations" and you drop the mark.
Worked Example: Drought-Proof BeansC18.3
Problem A farmer wants fava bean plants that tolerate drought. Step 1 is choosing the parents with the highest drought tolerance. What are steps 2 and 3?
Step 1 Step 2 is to breed the chosen plants together.
Step 2 Step 3 is to choose the offspring with the highest drought tolerance.
Step 3 Then repeat this over many generations, so drought-tolerance alleles keep piling up. Just giving the plants more water, or picking the tallest ones, isn't selective breeding at all, that doesn't change which alleles get passed on.
Key Points to Remember
- Variation = differences between individuals of the same species. Genetic (inherited) or environmental (not inherited).
- Continuous variation: full range, all intermediates possible; many genes + environment; smooth curve.
- Discontinuous variation: fixed separate categories, no intermediates; one or few genes; bar chart.
- Investigate variation with a large sample; classify feature by feature, not organism by organism.
- Mutation is a genetic change and the way new alleles are formed. Ionising radiation and certain chemicals raise the mutation rate.
- Adaptive feature = inherited feature that genuinely helps survival/reproduction in that habitat, being inherited alone isn't enough.
- Natural selection: variation → competition → survival of the better-adapted → reproduction → advantageous alleles become more common over generations.
- Selective breeding: humans select, breed, and repeat over many generations, to improve crops and animals.