IB MYP Sciences – Evolution and Interactions Between Organisms Study Guide

This study guide is for IB MYP Sciences and is aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. It covers two topics the brief lists for the sciences on-screen examinations: evolution and interactions between organisms. It suits MYP years 4 and 5, when most schools prepare for eAssessment.

MYP has no prescribed content list – schools design their own units – so this page teaches the standard biology behind two topics the IB’s brief names. Your own school’s unit may use different examples or go further in places. For anything about how a particular task is marked, your teacher will share the task-specific clarifications.

Use this guide with the revision notes and the practice questions. The MYP Sciences course hub and the printable checklist show where this unit sits in the course.

What this unit covers

Area	What you should be able to do	Criteria it feeds
Variation	Tell continuous from discontinuous variation; give genetic and environmental causes	A
Natural selection and adaptation	Explain selection in ordered steps; classify adaptations	A, D
Evidence for evolution	Describe fossil, anatomical and molecular evidence	A
Food chains and webs	Assign trophic levels; calculate energy transfer efficiency; predict knock-on effects	A, C
Competition, predation, symbiosis	Compare the types of relationship with examples; read predator–prey data	A, C
Population sampling	Use quadrats, transects and capture–mark–recapture; state assumptions	B, C

The brief sets out four criteria: A Knowing and understanding, B Inquiring and designing, C Processing and evaluating, and D Reflecting on the impacts of science. The on-screen examination has three tasks: Knowing and understanding (criterion A, 25 marks), Investigation skills (criteria B and C, 50 marks) and Applying science (criterion D, 25 marks). The sampling and data sections below matter most for the 50-mark task. For the task structure itself, see the investigation skills exam preparation and the assessment revision notes.

Variation

Individuals of the same species differ. This is variation.

• Continuous variation takes any value in a range: height, mass, leaf length. Plot it as a histogram; it often gives a bell-shaped distribution.
• Discontinuous variation falls into distinct groups with no in-between: ABO blood group, whether a pea seed is round or wrinkled. Plot it as a bar chart.

Causes:

• Genetic – new alleles arise by mutation. Sexual reproduction reshuffles existing alleles: meiosis produces varied gametes, and fertilisation joins gametes at random.
• Environmental – diet, light, temperature, soil minerals. A plant grown in shade may be taller but paler than a genetically identical plant in full sun.

Most continuous features are affected by both. Only genetic variation can be passed to offspring, so only genetic variation matters for evolution.

Natural selection

Evolution is a change in the heritable characteristics of a population over many generations. Charles Darwin and Alfred Russel Wallace both proposed natural selection as its mechanism; Darwin’s On the Origin of Species was published in 1859.

Write the explanation as a chain. Examiners look for each link:

1. There is variation within the population, and some of it is genetic.
2. More offspring are produced than the environment can support, so there is competition (for food, mates, space) or pressure from predators or disease – a selection pressure.
3. Individuals with a variation that suits the environment are more likely to survive and reproduce.
4. They pass on the alleles for that variation to their offspring.
5. Over many generations, the advantageous allele becomes more common in the population.

Worked example – camouflage in a fictional beetle

A beetle species on a fictional island has pale and dark forms. After a volcanic eruption, ash darkens the tree bark.

• Variation: the beetles are pale or dark, and colour is inherited.
• Selection pressure: birds hunt by sight.
• Dark beetles are harder to see on dark bark, so fewer are eaten. More survive to breed.
• Dark beetles pass on the allele for dark colour.
• Over many generations the proportion of dark beetles rises.

Notice what is not said: the beetles did not “decide” to change, and an individual beetle did not turn darker. The population changed because survival differed.

Real cases follow the same chain. Bacteria exposed to an antibiotic are selected for resistance: any bacterium with a resistance mutation survives, divides, and passes the allele on. The peppered moth in industrial Britain is a classic example of the darker form becoming more common where soot darkened tree bark.

Adaptations

An adaptation is a feature that increases the chance of survival and reproduction in a particular environment.

• Structural – body features: thick fur, a cactus’s spines, the long neck of a giraffe.
• Behavioural – actions: migration, hibernation, nocturnal hunting.
• Physiological – internal processes: producing concentrated urine in a desert mammal, venom production.

Selective breeding is a useful comparison. Humans choose which individuals breed, so the “selection pressure” is human choice rather than the environment. The steps are otherwise the same.

Evidence for evolution

• Fossils – preserved remains or traces in rock. Deeper rock layers are usually older, so fossils can show changes in form over time. The record has gaps because fossils form only under certain conditions (rapid burial, few decomposers).
• Comparative anatomy – homologous structures have the same basic plan but different uses. The pentadactyl (five-digit) limb appears in a human arm, a bat wing and a whale flipper, which suggests a common ancestor.
• Molecular evidence – comparing DNA base sequences or protein amino acid sequences. The more similar the sequences, the more recently two species shared a common ancestor.
• Observed change – antibiotic resistance in bacteria and pesticide resistance in insects can be watched happening within a human lifetime.

Food chains and food webs

A food chain shows one feeding pathway. Arrows point in the direction energy flows – from the organism eaten to the organism that eats it.

grass → rabbit → fox

• Producers (plants, algae) make their own food by photosynthesis. They are trophic level 1.
• Primary consumers eat producers (trophic level 2); secondary consumers eat primary consumers (level 3), and so on.
• Decomposers (many bacteria and fungi) break down dead material and waste.

A food web links many chains. It is a model – one of the related concepts the brief lists for MYP sciences. Like any model, it simplifies: it rarely shows every species, it hides how much each organism eats, and diets change with season and age.

Energy transfer

Only part of the energy at one trophic level reaches the next. Energy is lost as heat from respiration, in movement, and in parts that are not eaten or not digested (bones, fur, faeces). A figure of about 10% passing to the next level is often used as a rough guide, but the real value varies.

Efficiency of energy transfer (%) = (energy passed to next level ÷ energy in previous level) × 100

Worked example – energy efficiency

A fictional grassland: producers 24 000 kJ m⁻² year⁻¹, primary consumers 2160 kJ m⁻² year⁻¹, secondary consumers 216 kJ m⁻² year⁻¹.

Producers → primary:   2160 ÷ 24 000 × 100 = 9.0%
Primary → secondary:   216 ÷ 2160 × 100   = 10%
Producers → secondary: 216 ÷ 24 000 × 100 = 0.9%

This loss explains why food chains rarely have more than four or five levels, and why pyramids of energy are always pyramid-shaped. Pyramids of numbers can be inverted (one oak tree supports thousands of caterpillars); pyramids of biomass are usually upright.

Knock-on effects in a web

If one species is removed, trace every arrow into and out of it. Its prey may increase (less predation). Its predators may decrease, or switch to other prey, which then decreases. Always give the reason as well as the direction of change.

Competition, predation and symbiosis

Competition

• Intraspecific – between members of the same species, for food, water, light, territory or mates. It usually limits population size.
• Interspecific – between different species for the same resource. If two species need exactly the same resources in the same place, one usually out-competes the other.

Plants compete for light, water and minerals; animals for food, territory and mates.

Predation

In a predator–prey relationship the populations often rise and fall in cycles. Prey numbers rise; with more food, predator numbers rise after a lag; more predation makes prey fall; with less food, predators fall; prey recover. The predator peak comes after the prey peak. Records of lynx and snowshoe hare pelts traded in Canada are a well-known example of such cycles. Real populations are also affected by disease, weather and other food sources, so cycles are rarely perfectly regular.

Symbiosis

Symbiosis is a close, long-term relationship between two species.

Type	Effect on each partner	Example
Mutualism	Both benefit	Rhizobium bacteria in legume root nodules fix nitrogen; the plant supplies sugars
Commensalism	One benefits, the other is unaffected	Barnacles attached to a whale gain transport and feeding opportunities
Parasitism	One benefits, the host is harmed	A tapeworm absorbs digested food in its host’s gut

Population sampling

You cannot count every organism in a habitat, so you sample and scale up.

Quadrats

A quadrat is a square frame of known area. Place quadrats at random (for example, using random-number coordinates on a grid) to avoid bias. Count individuals, or estimate percentage cover for plants that are hard to count.

Estimated population = mean number per quadrat ÷ area of one quadrat × total area

Worked example – quadrat estimate

Ten 0.5 m × 0.5 m quadrats were placed at random in a 40 m × 25 m field. Daisy counts: 3, 0, 5, 2, 4, 1, 6, 2, 3, 4.

Total = 30         Mean = 30 ÷ 10 = 3.0 per quadrat
Quadrat area = 0.5 × 0.5 = 0.25 m²
Density = 3.0 ÷ 0.25 = 12 daisies per m²
Field area = 40 × 25 = 1000 m²
Estimate = 12 × 1000 = 12 000 daisies

More quadrats give a more reliable mean.

Transects

Where conditions change across a habitat – from shade to open ground, or up a seashore – lay a tape as a transect and place quadrats at regular intervals along it. This shows how distribution changes with a factor such as light intensity, which you should measure at each point.

Capture–mark–recapture

For mobile animals, capture a sample, mark them harmlessly, release them, then capture a second sample later.

Estimated population N = (number marked in first sample × total in second sample) ÷ number marked in second sample

Worked example – Lincoln index

48 woodlice were caught and marked. A week later 60 were caught, of which 12 were marked.

N = (48 × 60) ÷ 12 = 2880 ÷ 12 = 240 woodlice

The method assumes: marks do not rub off or harm the animal; marked animals mix fully back into the population; there are no significant births, deaths or migration between samples.

Common errors

• Writing that organisms “adapt because they need to”. Selection acts on variation that already exists.
• Leaving out “passes on alleles to offspring” – without inheritance there is no evolution.
• Drawing food chain arrows from predator to prey. Arrows show energy flow: prey → predator.
• Saying energy is “used up” or “destroyed”. Energy is transferred, mostly as heat.
• Forgetting to divide by quadrat area when the quadrat is not 1 m².
• Stating that a predator peak happens at the same time as the prey peak. It lags.
• Calling any close relationship “symbiosis” without saying which type, or calling commensalism mutualism.

Next steps

Condense this with the revision notes, then test yourself with the practice questions. For how the criteria are applied to real work, see criteria in practice and the MYP Sciences subject guide. Related units: cells and organisms and cycles.

Official syllabus

International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. The brief lists evolution and interactions between organisms among the topics explored in MYP sciences on-screen examinations; it does not set a detailed content list for them.