Variation & Natural Selection

Learn it by playing

Answer these questions to earn energy, then fish and explore. No account needed.

For teachers: ready-to-use lesson slides, revision notes, diagrams for Variation & Natural Selection (Biology, CIE) — use them in your lesson, or run the topic as a live class game.

Notes

Variation in Biology

**Variation** = differences between individuals of the same species.
**Phenotypic variation** = differences in features; caused by genes (genetic variation) and/or environment.
**Continuous variation** = many small degrees of difference, measured on a scale (e.g. height, mass). Graph gives a smooth bell curve.
**Discontinuous variation** = distinct categories with no intermediates (e.g. blood group, tongue rolling). Graph is step-shaped.
Discontinuous variation is usually caused by **genetic variation alone**.
Continuous variation often results from **both genetic and environmental factors** (e.g. height: genes + diet).
**Mutation** = a change in a gene (DNA base sequence). Most have no effect; rarely give a survival advantage or cause harm (e.g. sickle cell anaemia).
Mutation rate increased by **ionising radiation** (gamma, X-rays), **UV light**, and **certain chemicals** (e.g. tar).

Adaptive Features

**Adaptive features** = inherited characteristics that help an organism survive and reproduce in its environment.
Organisms with better adaptations are more likely to survive and pass on their alleles.
**Hydrophytes** (e.g. water lily) are adapted to wet conditions: large air spaces for flotation, small roots, stomata on upper epidermis, stomata open all the time.
**Xerophytes** (e.g. cactus) are adapted to dry conditions: thick waxy cuticle, sunken stomata, rolled leaves with hairs, small leaves, extensive shallow roots, water-storage tissues.
Adaptations can be identified from images or data (e.g. gerbils produce very concentrated urine to conserve water in deserts).

Natural Selection

**Natural selection** (Darwin's theory) = individuals with the best adaptive features survive and reproduce more.
Steps: (1) Variation exists in a population. (2) Overproduction of offspring leads to competition. (3) Individuals with advantageous traits survive ('survival of the fittest'). (4) They reproduce and pass on beneficial alleles. (5) Over generations, the advantageous trait becomes more common.
Example: Snail shell colour – a mutation for white shell gives better camouflage, so white snails survive longer, reproduce more, and the white allele spreads.
Another example: **Peppered moths** – industrial melanism; dark moths became more common in polluted areas.
**Evolution** = change in adaptive features of a population over time due to natural selection.
If the environment changes, different traits may be favoured, leading to evolution.
**Antibiotic resistance in bacteria** is a key example: resistant bacteria survive antibiotic treatment, reproduce, and the resistant strain becomes dominant.

Artificial Selection

**Artificial selection (selective breeding)** = humans select individuals with desirable characteristics and breed them together.
Process repeated over many generations until the desired trait is reliably inherited.
Examples: dog breeds (coat colour), crop plants (disease resistance, high yield, drought tolerance, better taste).
Wild brassica has been selectively bred to produce cauliflower, cabbage, broccoli, Brussels sprouts, kale, and kohlrabi.
Unlike natural selection, artificial selection is driven by **human choice**, not environmental pressures.

Monohybrid cross showing continuous variation? No, this is a discontinuous trait. But the template illustrates inheritance of alleles.

Practice questions

Free preview — 8 of 40 questions. Sign up to see them all.

1.What is a mutation?
Easy
- AA change in a gene
- BA type of continuous variation
- CA process used in genetic engineering
- DA condition caused by a dominant allele
2.Which of the following characteristics shows discontinuous variation?
Easy
- ATongue rolling
- BHeight
- CWeight
- DLength of foot
3.Which of these is not a feature of a hydrophyte?
Easy
- ALeaves rolled up and covered with hairs
- BRoots and xylem reduced
- CLarge air spaces in the tissues
- DLeaves with stomata on the upper surface
4.Which of the following is not a feature of natural selection?
Easy
- ASelection by humans
- BProduction of many offspring
- CCompetition for resources
- DVariation in the population
5.Which of these phenotypes is affected by both environmental and genetic factors?
Easy
- AWeight
- BTongue rolling
- CBlood group
- DGender
6.Over the last 30 years some antibiotics have become less effective in treating bacterial infections. What is the reason for this?
Easy
- ANatural selection
- BArtificial selection
- CAsexual reproduction
- DMore effective new antibiotics
7.Which statement about selective breeding is correct?
Easy
- AIt involves parents that possess desirable features
- BIt does not involve humans
- CIt involves a struggle for survival
- DIt always involves only one parent
8.Which adaptation is shown in the leaves of hydrophytes?
Easy
- AContain large air spaces
- BHairs present on the upper and lower surfaces
- CStomata present on the lower surface only
- DThick cuticle present on the upper and lower surfaces

Slides

Variation in Biology

Variation means differences between individuals of the same species.
Phenotypic variation is the difference in features between individuals.
It can be caused by genetic differences (genetic variation) or the environment.

What is variation in biology?

Differences between individuals of different species
Differences between individuals of the same species
Differences within an individual over time
Differences caused only by the environment

Continuous vs Discontinuous Variation

Continuous variation: many small degrees of difference, can be measured on a scale (e.g. height, mass).
Discontinuous variation: distinct categories, no intermediates (e.g. blood group, tongue rolling).
Continuous variation gives a bell-shaped curve; discontinuous gives a step-shaped graph.

Which of the following shows discontinuous variation?

Height
Weight
Tongue rolling
Finger length

Causes of Phenotypic Variation

Genetic variation: controlled entirely by genes (e.g. blood group, eye colour, gender).
Environmental variation: caused by factors like climate, diet, accidents (e.g. scarring, weight, language).
Discontinuous variation is usually genetic; continuous variation often involves both genes and environment.

Which of these are true about variation?

Height is an example of continuous variation.
Blood group is an example of continuous variation.
Environmental factors can affect phenotypic variation.
Discontinuous variation has many intermediates.

Mutation

Mutations are genetic changes (changes in DNA base sequence).
Most mutations have no effect on phenotype; some create new alleles.
Rarely, a mutation gives a survival advantage (e.g. new feather colour).
Mutations can be harmful (e.g. sickle cell anaemia).
Mutation rate increases with ionising radiation, UV, and certain chemicals.

What is a mutation?

A change in a gene
A type of continuous variation
A process used in genetic engineering
A condition caused by a dominant allele

Adaptive Features

Adaptive features are inherited characteristics that help an organism survive and reproduce in its environment.
Examples: fennec fox has large ears for heat loss, pale fur for camouflage, furry feet for insulation.
Gerbils produce very concentrated urine to conserve water in deserts.

Adaptive features are ____ features that help an organism survive and reproduce in its environment.

Hydrophytes and Xerophytes (Extended)

Hydrophytes: plants adapted to wet conditions. Features: large air spaces for flotation, small roots, stomata mainly on upper surface.
Xerophytes: plants adapted to dry conditions. Features: thick waxy cuticle, sunken stomata, rolled leaves with hairs, small leaves, extensive roots, water storage.

Which is NOT a feature of a hydrophyte?

Large air spaces in leaves
Stomata on upper surface
Thick waxy cuticle
Small roots

Natural Selection

Individuals in a species show variation due to genetic differences.
Overproduction: more offspring are produced than the environment can support.
This leads to competition and a 'struggle for survival'.
Individuals with advantageous characteristics survive longer and reproduce more.
Their alleles become more common in the next generation (survival of the fittest).
Theory proposed by Charles Darwin.

Which of the following is NOT a feature of natural selection?

Production of many offspring
Selection by humans
Competition for resources
Variation in the population

Example of Natural Selection: Snail Shell Colour

Snails show variation in shell colour (black, grey, white).
A mutation produces a white shell, which is better camouflaged on certain backgrounds.
White snails are less likely to be eaten by predators, so they survive longer and reproduce more.
Over generations, the white allele becomes more common in the population.

Which statements about natural selection are correct?

It results in evolution over time.
It involves humans selecting desirable traits.
It requires variation in the population.
It always leads to larger organisms.

Artificial Selection (Selective Breeding)

Humans select individuals with desirable characteristics and breed them together.
Offspring showing the desired traits are selected and bred over many generations.
Examples: dog breeds (coat colour), crop plants (disease resistance, high yield).
Wild brassica gave rise to cauliflower, cabbage, broccoli, etc.

Which statement about selective breeding is correct?

It does not involve humans.
It involves a struggle for survival.
It always involves only one parent.
It involves parents that possess desirable features.

Selective breeding is also known as ____ selection.

Summary

Variation: continuous (measurable) or discontinuous (distinct categories).
Adaptive features: inherited traits that help survival and reproduction.
Natural selection: differential survival and reproduction due to variation, overproduction, and competition.
Artificial selection: humans breed organisms for desired traits.

Unlock all 40 questions, slides & more

Create a free account to see every question, the slides, flashcards and revision notes for this topic.

Past papers

Past-paper practice for this topic is coming soon.

🗂️ Coming soon