Variation and Selection

Variation is the raw material; selection is what acts on it. Whether selection is done by the environment (natural selection) or by humans (selective breeding), the underlying mechanism is the same — and questions reward you for setting it out as a sequence.

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1. Variation

Variation is the differences between individuals of the same species.

Continuous vs discontinuous

	Continuous variation	Discontinuous variation
Range of values?	Yes — a continuous range	No — limited, distinct categories
Intermediates?	Yes	No intermediates
Caused by	Genes and environment	Genes only
Graph	Bell-shaped curve (histogram)	Bar chart with separate categories
Examples	Height, mass, leaf length	Blood group, sex, seed shape or colour

The two phrases examiners want are “range of values” for continuous and “limited number of phenotypes with no intermediates” for discontinuous. Vaguer wording usually doesn’t score.

A trap worth expecting: anything you count — the number of leaves, the number of petals — is a range of values, so it behaves as continuous. If you’re asked for an example of discontinuous variation in plants, give shape or colour, not number.

Causes of variation

• Genetic variation arises from meiosis (new combinations of alleles in gametes), random fertilisation, and mutation
• Environmental variation arises from surroundings — diet, light, temperature. A plant’s height depends on its genes and on how much light and water it received

Only genetic variation can be inherited. A characteristic caused entirely by the environment — a scar, or stunted growth from poor soil — is not passed to offspring.

Mutation

A mutation is a genetic change.

Mutations are the source of new alleles, and therefore the ultimate source of all genetic variation.

Factors that increase the rate of mutation: ionising radiation (X-rays, nuclear radiation, gamma rays) and certain chemicals.

Most mutations are neutral or harmful, but occasionally one produces an allele that gives an advantage — and that is where natural selection begins.

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2. Adaptive features

An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment.

Xerophytes — plants adapted to dry conditions

Adaptation	How it helps
Thick waxy cuticle	Barrier to evaporation; also reflects heat
Sunken stomata (in pits)	Water vapour is trapped in the pit, reducing the concentration gradient and slowing diffusion out
Small, needle-shaped leaves	Reduced surface area for transpiration
Rolled leaves with hairs	Trap moist air, reducing the concentration gradient
Few stomata, often only on the lower surface	Less water lost
Thick, fleshy stem	Stores water
Shallow, widespread roots	Absorb rainfall quickly over a large area

Hydrophytes — plants adapted to very wet conditions

Adaptation	How it helps
Large, flat leaves floating on the surface	Maximum light absorption; float on the water
Stomata on the upper surface	The lower surface is in water, so gases could not be exchanged there
Large air spaces in tissues	Provide buoyancy and store gases
Small or reduced roots	Water and ions are absorbed over the whole surface; no need to search for water

The stomata position is the classic distinguishing question. Most land plants have stomata on the lower surface to reduce water loss. A floating hydrophyte has them on the upper surface, because the lower one is underwater.

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3. Natural selection

The full sequence — write it in this order and each step is a mark:

1. There is genetic variation within a population, caused by mutation
2. There is competition for limited resources, and a struggle for survival
3. Individuals with advantageous features are more likely to survive
4. Survivors reproduce and pass on the alleles for those features
5. Over many generations, the frequency of the advantageous allele increases in the population

The advantageous variation must already exist before the pressure is applied. Organisms do not develop a useful feature because they need it. Writing that bacteria “become resistant because antibiotics are used” describes the wrong mechanism — resistance arises by random mutation first, and the antibiotic then selects for it.

Say “increases the frequency of the allele”, not “the species changes.” Selection acts on individuals; what changes is the population.

Antibiotic resistance — the standard example

1. A random mutation in a bacterium produces an allele for antibiotic resistance
2. The antibiotic is used, killing the non-resistant bacteria
3. Resistant bacteria survive and have less competition
4. They reproduce rapidly, passing on the resistance allele
5. The proportion of resistant bacteria increases in the population

Reducing the problem: only prescribe antibiotics when genuinely needed, and always complete the full course so that no partially-resistant bacteria survive.

Antibiotics do not work against viruses. Prescribing them for a viral infection cannot help and contributes to resistance. This appears in questions constantly.

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4. Selective breeding (artificial selection)

Selective breeding is when humans select individuals with desired features and breed them together over many generations.

The process:

1. Select individuals showing the desired characteristic
2. Breed them together
3. Select the offspring showing the characteristic most strongly
4. Repeat over many generations

Uses: crops with improved yield or disease resistance; cattle producing more milk or meat; domestic dogs.

Natural vs artificial selection

	Natural selection	Selective breeding
Who selects	The environment	Humans
Selected for	Features aiding survival and reproduction	Features useful to humans
Speed	Usually slow	Faster
Effect on variation	Maintains variation	Reduces genetic variation

Selective breeding reduces genetic variation, because only a few individuals are chosen as parents. That makes the population more vulnerable to a new disease or environmental change — the same argument as monoculture in the ecosystems topic.

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5. Mistakes that cost marks

Saying a characteristic develops because it is needed. Variation arises first, by random mutation; selection acts afterwards.

Saying “the species adapts.” Individuals don’t adapt — the frequency of alleles in the population changes.

Giving a number as an example of discontinuous variation.

Saying environmental variation can be inherited.

Saying antibiotics kill viruses.

Saying stomata are always on the lower surface. Floating hydrophytes have them on the upper surface.

Describing xerophyte adaptations without linking them to reducing water loss.

Omitting “over many generations” from a natural selection answer.

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Frequently asked questions

What is the difference between continuous and discontinuous variation? Continuous variation has a range of values with intermediates and is influenced by genes and environment. Discontinuous has a limited number of categories with no intermediates and is caused by genes only.

What is a mutation? A genetic change — the source of new alleles and therefore of all genetic variation.

How does natural selection work? Genetic variation exists; competition means those with advantageous features survive and reproduce; they pass on the alleles; over many generations the frequency of those alleles increases.

How do bacteria become resistant to antibiotics? A random mutation produces a resistance allele. The antibiotic kills the non-resistant bacteria, so resistant ones survive, reproduce and increase in proportion.

Why should you finish a course of antibiotics? So that all the bacteria are killed, including partially resistant ones that would otherwise survive and multiply.

How is selective breeding different from natural selection? Humans do the selecting, for features useful to humans, and it works faster — but it reduces genetic variation.

Why are sunken stomata useful in a desert? Water vapour is trapped in the pit, reducing the concentration gradient so less water diffuses out.

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Quick revision checklist

• I can define variation and distinguish continuous from discontinuous, with the right phrases
• I can give examples of each, and avoid using number for discontinuous
• I know only genetic variation is inherited
• I can define a mutation and name factors increasing its rate
• I can define an adaptive feature
• I can give xerophyte adaptations with reasons
• I can give hydrophyte adaptations, including stomata position
• I can write out natural selection as a five-step sequence
• I know variation arises before the selection pressure
• I can explain antibiotic resistance as an example of natural selection
• I know antibiotics do not work on viruses
• I can describe selective breeding and compare it with natural selection
• I know selective breeding reduces genetic variation

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These notes cover topic 18 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.