Adaptive feature — the two-element definition
Adaptive feature = an inherited feature that helps an organism survive and reproduce in its environment. Both halves carry marks. Inherited means genetically determined and passed from parent to offspring — not learned, and not acquired during one lifetime. A feature is adaptive only relative to the environment named in the question: thick fur is an advantage in the cold but causes overheating in a hot desert.
Hydrophyte (Extended) — gas exchange, not water loss
Hydrophyte = a plant adapted to living in water or waterlogged soil. Water is abundant, so the challenges are gas exchange to submerged tissue and support. Large air spaces (aerenchyma) store oxygen and carbon dioxide, let them diffuse to submerged tissue, and give buoyancy. A thin or absent cuticle with stomata on the upper leaf surface is safe because water loss is no threat, and little woody tissue is needed because the water holds the plant up.
Xerophyte (Extended) — three routes to conserving water
Xerophyte = a plant adapted to a dry environment, where the challenge is conserving water. A thick waxy cuticle blocks evaporation; small, spine-like or rolled leaves cut the surface area for transpiration; sunken or fewer stomata trap still, humid air; extensive or deep roots reach more water; succulent tissue stores it. Each works by one of three routes — less surface area, a smaller water-potential gradient, or a physical barrier.
Drawn from real examiner reports.
Dropping "inherited" from the definition
"A feature that helps an organism survive" is only half the definition. Inherited is the element that separates an adaptive feature from a learned skill or a change acquired during a lifetime, and mark schemes credit it as its own point. State both halves — inherited, and helps survival and reproduction in that environment.
Definition-precision pattern — digest Topic 18: population written loosely as "all organisms in an area" instead of the precise wording (s22 P11/12/13 Q38; w22 P11 Q38)
Feature named, mechanism missing
"The cactus has spines, which helps it survive in the desert" names a feature and a habitat but stops short of the marks. A full answer links three parts: the named feature, the specific challenge, and the mechanism joining them — "leaves reduced to spines cut the surface area available for transpiration, and water is scarce in a desert."
Water potential, not concentration
When water vapour leaves a leaf, the mark scheme wants the water-potential gradient between the leaf air spaces and the outside air. Writing "reduces the concentration gradient" loses the precision mark even when the rest of the reasoning is right. Use "water potential" for every xerophyte feature that works by trapping humid air.
Flagged digest Topic 3: "concentration gradient" written where "water potential" was required (s23 P41 Q2a ii; w22 P41 Q3c; s22 P31 Q1)
Humid air means LESS transpiration
Sunken stomata, rolled leaves and leaf hairs all work by making the air next to the stoma more humid. Humid air has a higher water potential, so the gradient out of the leaf shrinks and the transpiration rate falls. Writing that trapped humidity speeds water loss reverses the mechanism and scores zero.
Flagged digest Topic 8: candidates stated high humidity increases transpiration rate — it decreases it (s23 P22 Q15; s22 P12 Q18; w22 P41 Q3c)
Hydrophyte and xerophyte features swapped
The two habitats pose opposite problems, so their features never cross over. A thick cuticle or sunken stomata on a pond plant is wrong — it has no water-loss problem, and a thick cuticle would restrict the gas exchange it does need. Air spaces for buoyancy on a desert plant are equally wrong. Read the habitat first, then choose the features.
Organisms do not adapt within a lifetime
An adaptive feature is the inherited structure or behaviour itself (this sub-topic). Adaptation is the process by which a population becomes better suited to its environment over many generations by natural selection (18.3). Writing that a cactus "grew spines because it needed to survive the desert" describes a purposeful lifetime change and is wrong on both counts.
Judging the plant by leaf size alone
Small leaves do not automatically mean xerophyte, and broad leaves do not mean hydrophyte — a floating pond leaf is broad and aquatic. Cite the feature that is diagnostic of the challenge instead: air spaces and upper-surface stomata for a hydrophyte, thick waxy cuticle and reduced or rolled leaves for a xerophyte.
Feature, challenge, mechanism
Build every adaptive-feature answer in this fixed order: (1) Feature — name it precisely ("leaves reduced to spines", not "small leaves"); (2) Challenge — the specific problem the habitat poses; (3) Mechanism — exactly how the feature reduces that problem.
Describe versus explain
Describe = name the feature and link it to the environment (steps 1-2). Explain = feature, challenge and mechanism (all three). "Explain" always demands the mechanism, so if your sentence stops at "which helps it survive" you have answered a describe question.
Unfamiliar species: work from the habitat
The syllabus tests a method, not memorised species. Read the habitat stated or shown, list the challenges it creates (water scarcity, strong current, cold, predation), then take each feature in the image or text and match it to one challenge.
Adaptive feature: an inherited feature that helps an organism survive and reproduce in its environment. A feature is only adaptive relative to a specific environment - what is an advantage in one habitat may be useless or harmful in another.
Interpreting an unfamiliar species (Core skill): given an image or written information about a species you have not studied before, identify each named feature, then state the specific environmental challenge it overcomes.
(Extended) Hydrophyte: a plant adapted to living in water or waterlogged soil, where water is abundant but gas exchange to submerged tissue and physical support are the main challenges.
(Extended) Xerophyte: a plant adapted to living in a dry environment, where the main challenge is conserving water and minimising transpiration.
Define an adaptive feature.
(a) Define an adaptive feature.
(b) A photograph shows a mountain hare with a white winter coat, living in a snow-covered habitat. Identify one adaptive feature shown, and state the environmental challenge it helps the hare overcome.
