Biology · Unit 17

Evolution and Ecology

Natural selection and ecosystems

Natural selection is a short argument with enormous consequences: more offspring are produced than survive, they vary, some of that variation is heritable, and so the population changes. Nothing in the mechanism requires foresight or effort.

Ecology is the same story at the level of whole systems - food webs, energy flow, nutrient cycles, and what happens to all of it under human pressure. The two belong in one unit because selection only makes sense against the environment doing the selecting.

This unit breaks down into 15 short steps and 90 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 15
Questions: 90
Difficulty: 1-3

What this unit covers

Ecosystems and Habitats
Evidence for Evolution
Nutrient Cycles and Human Impact
Food Webs and Energy Flow
Natural Selection and Adaptation

Where this fits

Best after DNA and Genetics. It also connects across to the Animals and Nature track.

Where people slip

Organisms do not adapt during their lives in the evolutionary sense. Individuals do not change to suit the environment; the proportions of individuals in the population change.

How the unit is structured

Evolution and Ecology runs as 15 short steps that unlock in order. 11 are practice rounds and 4 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 15 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 90 in the unit altogether.

Ecosystems and Habitats

Picture questionLevel 1
1. 🌳 This organism makes its own food using sunlight. What is its role in an ecosystem?
- Producercorrect
- Decomposer
- Predator
- Scavenger
A tree is a producer because it makes its own food from sunlight through photosynthesis.
Fill the blankLevel 2
2. A relationship in which one species benefits while the other is neither helped nor harmed is called ____.
- commensalismcorrect
- mutualism
- parasitism
- competition
In commensalism one partner gains an advantage while the other is unaffected, such as a bird nesting in a tree.
Tap the pairsLevel 2
3. Tap each predator to match it with the prey it hunts.
Answer: Lion = Zebra; Owl = Mouse; Frog = Fly; Shark = Seal
Predators hunt and eat prey, and these predator-prey relationships help control population sizes in ecosystems.
Multiple choiceLevel 3
4. What best describes a keystone species in an ecosystem?
- A species whose removal would dramatically change the whole ecosystemcorrect
- The species with the largest number of individuals
- Any producer at the base of a food chain
- A species that has recently gone extinct
A keystone species has an outsized effect on its ecosystem, so removing it can cause many other species to decline or disappear.
Put in orderLevel 3
5. Put these stages of ecological succession on bare rock in order, from first to last.
Answer: Bare rock -> Lichens and mosses grow -> Small grasses and plants take root -> Shrubs and small trees appear -> Mature forest climax community
Pioneer lichens break rock into soil, letting larger plants and eventually a mature forest develop.
Sort into groupsLevel 3
6. Sort each relationship into the correct type of symbiosis.
Answer: A bee and a flower both benefit = Mutualism; A tapeworm harms the animal it lives in = Parasitism; A barnacle rides a whale, which is unaffected = Commensalism; A tick feeds on a dog's blood = Parasitism; A clownfish and an anemone both gain protection = Mutualism
In mutualism both gain, in parasitism one is harmed, and in commensalism one benefits while the other is unaffected.

Evidence for Evolution

Fact or fibLevel 2
7. Fossils provide evidence that living things have changed over long periods of time.
Answer: True
Fossils preserve organisms from the past, revealing how species have gradually changed across millions of years.
Match the pairsLevel 2
8. Match each type of evidence for evolution to its description.
Answer: Fossils = Preserved remains in rock; Homologous structures = Similar bones in different limbs; Extinction = A species dying out completely; Speciation = One species splitting into two
Fossils, homologous structures, extinction, and speciation are all lines of evidence that support the theory of evolution.
Picture questionLevel 2
9. 🧍 The human appendix and tailbone no longer do their original jobs. What are such leftover body parts called?
- Vestigial structurescorrect
- Homologous limbs
- Pioneer organs
- Analogous wings
Vestigial structures are reduced remains of features that were useful in an ancestor, giving evidence of evolution.
Fill the blankLevel 3
10. A group that contains an ancestor together with all of its descendants is called a ____.
- cladecorrect
- niche
- cohort
- biome
A clade (a monophyletic group) includes a common ancestor along with all of its descendants.
Multiple choiceLevel 3
11. Sharks are fish and dolphins are mammals, yet both have streamlined bodies and fins. What does this similarity show?
- Convergent evolution, where unrelated species evolve similar features in similar environments
- That sharks and dolphins are closely related cousins
- That one copied the body shape of the other on purpose
- That both have vestigial fins with no use
Convergent evolution produces similar traits in distantly related species because they face the same environmental challenges.
Odd one outLevel 3
12. Three of these structures are homologous (inherited from a shared ancestor). Tap the odd one out.
- An insect's wingcorrect
- A bird's wing
- A whale's flipper
- A human arm
A bird wing, whale flipper and human arm are homologous vertebrate forelimbs, whereas an insect wing has a completely different origin and is analogous.

Food Webs and Energy Flow

Multiple choiceLevel 1
13. In a food chain, which group makes energy available by producing their own food?
- Producerscorrect
- Consumers
- Decomposers
- Predators
Producers, such as plants, capture sunlight and form the base of every food chain.
Choose all that applyLevel 2
14. Only about 10% of energy passes from one trophic level to the next. Where does the rest of the energy go? Select all that apply.
- Lost as heat from respirationcorrect
- Used for movement and life processescorrect
- Passed out in waste and undigested foodcorrect
- Destroyed so that it no longer exists
Most energy is lost as heat, movement, and waste, so only around a tenth is stored and passed on.
Guess the numberLevel 2
15. About what percentage of energy passes from one trophic level to the next in a food chain?
Answer: 10 %
Roughly 10 percent of energy is passed to the next trophic level, while the rest is lost mainly as heat and through life processes.
Odd one outLevel 2
16. Which one is NOT a producer?
- Rabbitcorrect
- Grass
- Oak tree
- Algae
A rabbit is a consumer that eats plants, while grass, oak trees, and algae are producers that make their own food.
Put in orderLevel 2
17. Put this food chain in order, starting with the organism that makes its own food.
Answer: Grass -> Grasshopper -> Frog -> Snake
Energy flows from grass (a producer) to the grasshopper, then the frog, then the snake, with each eating the one before it.
Sequence recallLevel 3
18. Memorize and repeat the trophic levels in order from the base of the food chain upward.
Answer: Producer -> Primary consumer -> Secondary consumer -> Tertiary consumer
Energy moves up the food chain from producers to primary, secondary, and then tertiary consumers.

Natural Selection and Adaptation

Fill the blankLevel 1
19. The natural differences between members of the same species are called ____.
- variationcorrect
- extinction
- migration
- hibernation
Variation describes the differences among individuals, and it is the raw material that natural selection acts on.
Build the sentenceLevel 2
20. Build the sentence about how natural selection changes a population.
Answer: Helpful traits become more common
Over many generations, traits that improve survival and reproduction become more common in a population.
Choose all that applyLevel 2
21. Which conditions are needed for natural selection to occur? (Select all that apply)
- Variation among individualscorrect
- Traits can be inheritedcorrect
- More offspring are produced than can survivecorrect
- Every individual survives equally
Natural selection needs heritable variation and competition to survive, so some traits become more common over generations.
Fact or fibLevel 2
22. The rise of antibiotic-resistant bacteria is an example of natural selection happening over just a few years.
Answer: True
Resistant bacteria survive treatment and reproduce, so resistance spreads quickly - natural selection acting fast.
Guess the numberLevel 3
23. In a population at Hardy-Weinberg equilibrium, 16% of individuals show the recessive phenotype, so q^2 = 0.16. What is the frequency of the recessive allele, q?
Answer: 0.4 allele frequency
The recessive allele frequency q is the square root of q^2, so q = sqrt(0.16) = 0.4.
Match the pairsLevel 3
24. Match each reproductive isolation mechanism to its description.
Answer: Temporal isolation = Species breed at different times of the year; Behavioural isolation = Differences in courtship displays prevent mating; Mechanical isolation = Reproductive structures do not physically fit together; Hybrid sterility = Offspring such as mules cannot reproduce
Isolation mechanisms prevent gene flow either before mating (prezygotic) or after (postzygotic), keeping species distinct.

Nutrient Cycles and Human Impact

Build the sentenceLevel 2
25. Build the sentence about how eutrophication harms a lake.
Answer: Fertilizer runoff feeds algae that later use up the oxygen
Fertilizer washing into water feeds algae, and when the algae die, decomposers use up the oxygen fish need.
Fill the blankLevel 2
26. Bacteria that turn nitrogen gas from the air into a form plants can use are part of the ____ cycle.
- nitrogencorrect
- water
- rock
- oxygen
In the nitrogen cycle, nitrogen-fixing bacteria convert nitrogen gas into compounds that plants can absorb and use.
Guess the numberLevel 2
27. Roughly what percentage of Earth's atmosphere is nitrogen gas?
Answer: 78 %
About 78% of the air is nitrogen gas, but most living things cannot use it until bacteria fix it.
Multiple choiceLevel 2
28. What do denitrifying bacteria do in the nitrogen cycle?
- Convert nitrates in the soil back into nitrogen gascorrect
- Turn nitrogen gas from the air into ammonia
- Change ammonia into nitrates
- Break down sugars to release energy
Denitrifying bacteria break down nitrates in the soil and release nitrogen gas back to the atmosphere.
Choose all that applyLevel 3
29. Which processes return carbon dioxide to the atmosphere? Select all that apply.
- Respiration by living thingscorrect
- Combustion of fuelscorrect
- Decomposition of dead matter by microbescorrect
- Photosynthesis in green plants
Respiration, combustion, and decomposition release carbon dioxide, while photosynthesis removes it.
Match the pairsLevel 3
30. Match each type of bacteria to its role in the nitrogen cycle.
Answer: Nitrogen-fixing bacteria = Turn nitrogen gas into compounds in the soil; Nitrifying bacteria = Change ammonia into nitrates; Denitrifying bacteria = Return nitrates to nitrogen gas; Decomposer bacteria = Release ammonia from dead organisms
Different bacteria fix, nitrify, denitrify, and decompose, keeping nitrogen moving through the cycle.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Evolution and Ecology

90 questions across 15 steps. Start with step one and crawl at your own pace.

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Read about Evolution and Ecology

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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