This mini-lesson covers variation and selection from CCEA GCSE Biology (Unit 2): genetic vs environmental variation, continuous vs discontinuous variation, how natural selection drives evolution (and extinction), the evidence for evolution, selective breeding and genetic engineering.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Variation · genetic vs environmental
Genetic and environmental variation
Individuals of a species differ from one another — this is variation. It has two causes:
Genetic variation — differences caused by the alleles you inherit. It comes from mutations (random changes to a gene or chromosome number) and from sexual reproduction mixing alleles.
Environmental variation — differences caused by your surroundings and lifestyle (e.g. a scar, a tan, a plant grown in the shade).
Many features (like human height) are affected by both genes and environment.
Examples to remember: eye colour and blood group are purely genetic; a language you speak or a scar is purely environmental; height and weight are a combination of both.
Variation · displaying data
Continuous & discontinuous variation
Continuous variation — a range of values with no gaps (e.g. height, length, mass). Shown on a histogram.
Discontinuous variation — distinct categories with no in-betweens (e.g. tongue rolling, hand dominance, blood group). Shown on a bar chart.
Continuous data has bars that touch (histogram); discontinuous data has separated bars (bar chart).
Quick test: if you could measure "half-way" between two values it's continuous; if the answer is one of a few fixed groups it's discontinuous.
Sort it
What kind of variation?
Tap a feature, then tap the group it belongs to.
🧬 Genetic only
🌦️ Environmental only
🔀 Both
Quick check
Which chart?
?A student measures the hand span of everyone in the class. This is continuous variation. Which graph should they use to display the data?
Selection · natural selection
Natural selection & evolution
Variation and natural selection can lead to evolution. The steps are:
There is variation in the phenotypes of a population.
Organisms compete for resources; the best-adapted phenotypes are more likely to survive.
Survivors are more likely to reproduce and pass on their genes to the next generation.
Over many generations this causes gradual change, which may form a new species.
Example:antibiotic resistance — a few bacteria have an allele giving resistance. When antibiotics are used, the non-resistant ones die but the resistant ones survive, reproduce and spread.
Watch the wording: organisms do not "choose" to adapt or "try" to change. Useful variations already exist by chance; selection simply favours them.
Quick check
Why do resistant bacteria spread?
?After a course of antibiotics, the surviving bacteria are all resistant. What best explains why?
Selection · evidence & extinction
Evidence for evolution & extinction
Fossils are the preserved remains (or traces) of organisms from long ago. They provide evidence for evolution by showing how organisms have changed over time.
Extinction is when the last member of a species dies out. It happens when a species fails to adapt to environmental change (e.g. climate change, a new predator or disease, loss of habitat).
Fossils in deeper rock layers are older; comparing them shows gradual change over time.
Link it up: extinction is the opposite outcome of successful natural selection — the variation needed to survive the change simply wasn't there.
Selection · selective breeding
Selective breeding
Selective breeding (artificial selection) is when humans choose which organisms breed:
Choose parents with the desired characteristic (e.g. more milk, disease resistance, better appearance).
Breed them together.
Choose the best offspring and breed again.
Repeat over many generations until all offspring show the desired feature.
Natural vs artificial: in natural selection the environment "chooses" the survivors; in selective breeding humans choose. Both work on existing variation over many generations.
Quick check
Who does the choosing?
?A farmer keeps breeding only their highest-milk-yield cows together, generation after generation. What is this process called?
Selection · genetic engineering
Genetic engineering
Genetic engineering modifies an organism's genome by transferring a gene to give it a desired characteristic. The classic CCEA example is making human insulin:
The human insulin gene is cut out using restriction enzymes (leaving "sticky ends").
The gene is inserted into a plasmid of a bacterium → a genetically modified (GM) bacterium.
The GM bacteria are grown in a fermenter, where they produce human insulin.
The insulin is extracted and purified (down-streaming) so it can treat diabetes.
Advantage: the insulin is a genuine human protein, so it works well and does not trigger the allergic reactions that older animal insulin sometimes caused — and it can be made in huge amounts.
Calculate
Your turn — scaling up insulin
1A fermenter makes 3 grams of insulin per litre of culture. How many grams of insulin would a 500 litre fermenter produce?
g
Hint: grams = grams per litre × litres = 3 × 500.
Match it
Match each term to its meaning
Tap a term on the left, then its matching meaning on the right.
Term
Meaning
Quick check
Where does new genetic variation come from?
?Which of these is the source of brand-new genetic variation in a population?
Recap
The big ideas to know
Variation: genetic (alleles, from mutation & sexual reproduction) vs environmental (surroundings); many features are both
Types: continuous (range → histogram) vs discontinuous (categories → bar chart)