IB MYP Sciences – Evolution and Interactions Between Organisms Revision Notes

For full explanations and worked examples, start with the study guide. These notes are for the final weeks before your MYP Sciences on-screen examination.

They follow the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014, which lists evolution and interactions between organisms among the topics explored in the on-screen examinations. They suit MYP years 4 and 5. MYP has no prescribed content list – schools design their own units – so these notes cover the standard science behind those two listed topics; your teacher will share the task-specific clarifications for your own assessments.

Course links: MYP Sciences hub · printable checklist · practice questions.

Key definitions

• Variation – differences between individuals of the same species.
• Continuous variation – any value in a range (height, mass). Histogram.
• Discontinuous variation – distinct categories (ABO blood group). Bar chart.
• Mutation – a change in DNA; the source of new alleles.
• Selection pressure – a factor that affects survival, such as predators, disease, competition or climate.
• Natural selection – individuals better suited to the environment survive and reproduce more, so their alleles become more common.
• Evolution – change in heritable characteristics of a population over many generations.
• Adaptation – a structural, behavioural or physiological feature that improves survival and reproduction.
• Homologous structures – same basic plan, different function (pentadactyl limb).
• Trophic level – feeding position in a chain (producer = level 1).
• Intraspecific / interspecific competition – within one species / between different species.
• Symbiosis – close, long-term relationship between two species: mutualism, commensalism or parasitism.
• Quadrat – square frame of known area used to sample plants or slow-moving animals.
• Transect – line across a habitat along which samples are taken at intervals.

Formulas and relationships

Quantity	Relationship	Watch for
Energy transfer efficiency	(energy at next level ÷ energy at previous level) × 100	Put the lower level on the bottom
Population density	mean per quadrat ÷ area of one quadrat	Quadrat area is often 0.25 m², not 1 m²
Population estimate (quadrats)	density × total area	Check area units match
Capture–mark–recapture	N = (marked first × total second) ÷ marked in second	Only the marked recaptures go on the bottom
Percentage change	(new – old) ÷ old × 100	Divide by the original value

These formulas are standard biology. The subject brief does not publish a formula list, so learn them.

Method in steps

Explaining natural selection (5 links)

1. Variation exists, some of it genetic (often from mutation).
2. A selection pressure acts (predation, disease, competition, climate).
3. Individuals with the advantageous variation are more likely to survive and reproduce.
4. They pass on the advantageous allele to offspring.
5. Over many generations, the allele becomes more frequent in the population.

Name the specific variation and the specific pressure from the question’s context. Generic answers lose credit under criterion A.

Estimating a plant population

1. Divide the area into a grid; choose coordinates with random numbers.
2. Place the quadrat; count individuals (or estimate percentage cover).
3. Repeat for many quadrats.
4. Find the mean per quadrat.
5. Divide by quadrat area to get density per m².
6. Multiply by the total area.

Predicting changes in a food web

1. Find every arrow into and out of the changed species.
2. State the direction of change for each linked species.
3. Give the reason (more or less food; more or less predation).
4. Note alternative food sources that could soften the effect.

Describing data on changing populations (criterion C)

1. State the overall trend in words (rises, falls, levels off, cycles).
2. Quote at least two values from the data, with units or percentages.
3. Point out any anomaly, lag or change in rate.
4. Only then explain the pattern with science (selection, predation, competition).
5. Say how confident the data let you be: sample size, number of repeats, other factors not controlled.

Small worked reminders

• 40 marked beetles; next sample 45, of which 6 marked → N = 40 × 45 ÷ 6 = 300.
• Producers 20 000 kJ, primary consumers 1800 kJ → 1800 ÷ 20 000 × 100 = 9.0%.
• Mean 2 plants per 0.25 m² quadrat → 2 ÷ 0.25 = 8 per m².

Must-know distinctions

Pair	Difference
Natural selection vs selective breeding	Environment chooses vs humans choose which individuals breed
Continuous vs discontinuous	Range of values (histogram) vs distinct groups (bar chart)
Genetic vs environmental variation	Only genetic variation is inherited and can be selected
Structural vs behavioural vs physiological adaptation	Body feature vs action vs internal process
Homologous structure	Shared plan from a common ancestor, even if the job differs
Mutualism vs commensalism vs parasitism	Both gain vs one gains, other unaffected vs one gains, host harmed
Predation vs parasitism	Predator kills and eats prey; a parasite usually lives on or in a living host
Intraspecific vs interspecific competition	Same species vs different species
Pyramid of numbers vs biomass vs energy	Numbers can be inverted; biomass usually upright; energy always upright
Random quadrats vs transect	Whole-area estimate vs change along a gradient

Evidence for evolution – four lines

• Fossils: older layers deeper; show change in form over time; record incomplete.
• Comparative anatomy: homologous structures such as the pentadactyl limb.
• Molecular: more similar DNA or protein sequences → more recent common ancestor.
• Observed change: antibiotic resistance in bacteria; pesticide resistance in insects.

Predator–prey cycles in one line

Prey rise → predators rise after a lag → prey fall → predators fall → prey recover. The predator peak comes after the prey peak, and predator numbers are usually lower.

Links to concepts and criteria

The brief names change, relationships and systems as key concepts, and gives energy, movement, transformation and models as example related concepts. This unit maps well onto them:

• Change – evolution of populations over generations.
• Relationships – competition, predation, symbiosis.
• Systems – an ecosystem as interacting parts.
• Energy – transfer through trophic levels.
• Models – food webs and pyramids simplify real feeding relationships.

Criteria use eight achievement levels (1–8) in four bands. Recall questions train criterion A; sampling design trains B; data processing trains C; evaluating uses of science (for example biological pest control, or antibiotic use) trains D.

Quick self-test

1. Give one example of continuous and one of discontinuous variation.
2. What is the original source of new alleles?
3. State the five links in a natural selection explanation, in order.
4. Classify a cactus’s spines and a bird’s migration as types of adaptation.
5. Why do a bat wing and a whale flipper count as evidence for evolution?
6. Which direction do arrows in a food chain point, and what do they show?
7. Producers hold 15 000 kJ; primary consumers 1200 kJ. Calculate the efficiency of transfer.
8. Starting with 50 000 kJ in producers and using a 10% rule, how much energy reaches secondary consumers?
9. A mean of 7.5 plants was found per 1 m² quadrat in a 200 m² plot. Estimate the population.
10. 36 animals are marked. Later, 50 are caught and 9 are marked. Estimate N.
11. Name the relationship: Rhizobium in legume root nodules.
12. Why does the predator peak come after the prey peak?

Answers

1. Continuous: height (or mass). Discontinuous: ABO blood group (or any category feature).
2. Mutation.
3. Variation → selection pressure → better-suited individuals survive and reproduce → pass on alleles → allele more frequent over many generations.
4. Spines: structural. Migration: behavioural.
5. They are homologous – same pentadactyl bone plan adapted to different uses – suggesting a common ancestor.
6. From the organism eaten to the organism that eats it; they show the direction of energy flow.
7. 1200 ÷ 15 000 × 100 = 8.0%.
8. 50 000 × 0.1 × 0.1 = 500 kJ.
9. 7.5 × 200 = 1500 plants.
10. 36 × 50 ÷ 9 = 200.
11. Mutualism.
12. Predators need time to reproduce once food is plentiful, so their numbers rise only after prey numbers have risen.

Where marks are usually lost

• Saying an individual “adapts” or “evolves” during its life instead of describing a population changing over generations.
• Missing the inheritance link: no mention of alleles being passed to offspring.
• Using “survival of the fittest” without explaining what “fit” means in the context given.
• Reversing food chain arrows, or calling decomposers consumers at a trophic level.
• Dividing the wrong way in efficiency calculations, giving answers above 100%.
• Forgetting the quadrat-area step when quadrats are not 1 m².
• Putting the total second-sample count, not the marked recaptures, on the bottom of the capture–mark–recapture formula.
• Listing sampling assumptions without linking one to how it would bias the estimate (lost marks → overestimate).
• Describing a predator–prey graph without mentioning the lag or quoting data values.
• Calling commensalism “mutualism”, or giving the type of symbiosis with no reason.

Next: work through the practice questions, then check the investigation skills exam preparation and the assessment revision notes for the task structure.

Official syllabus

International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. The brief lists evolution and interactions between organisms among the on-screen examination topics; content detail on this page is standard science chosen by Marlbridge.