Variation and Natural Selection: Question 8
Syllabus 18.2, 18.3
A species of succulent plant grows in a desert region with very low rainfall. Botanists think it evolved from an ancestral, non-succulent plant species that once grew in a much wetter habitat nearby. The present-day desert species has:
- a thick, swollen stem that stores water
- a waxy, thickened outer layer (cuticle) covering its stem
- very small, spine-like leaves instead of broad, flat leaves
(a) Using the information given, describe two adaptive features of the desert succulent and explain how each one helps it to survive periods of low rainfall. [4]
(b) Explain, using the idea of natural selection, how a population of the wetter-habitat ancestral plant could have evolved into a desert species with these features over many generations. In your answer, refer to variation within the ancestral population, water availability as a factor affecting survival, and the passing on of alleles to offspring. [4]
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Worked solution
Part (a): Two adaptive features of the desert succulent
Thick, swollen stem. By storing water within its tissues, the stem acts as a reserve that the plant can use during long dry spells, allowing it to keep carrying out essential processes such as photosynthesis and growth when no rain is available.
Waxy, thickened cuticle. A cuticle covering the stem’s surface reduces the rate at which water evaporates from the plant. A thicker, waxier cuticle than an ordinary plant would have cuts this water loss further, helping the plant hold on to the water stored in its stem for longer.
Small, spine-like leaves. Broad, flat leaves have a large surface area, which loses a great deal of water by transpiration. Reducing the leaves to small spines dramatically cuts this surface area, so much less water is lost from the plant’s leaves overall.
Any two adaptive features described above, each linked clearly to a survival benefit in low-rainfall conditions, would gain full credit.
Part (b): How natural selection could have produced these features
The ancestral plant population, growing in a wetter habitat, is very unlikely to have had uniform stems, cuticles and leaves; instead it would have shown natural variation, with some individuals already having somewhat thicker stems, waxier cuticles or smaller leaves than others, due to existing genetic differences. As the habitat became drier over time, water availability became a major factor limiting survival: plants with features that reduced water loss and increased water storage were more likely to survive periods of drought, while plants without these features were more likely to die before they could reproduce. The plants that survived went on to reproduce and pass their alleles for thicker stems, waxier cuticles and smaller leaves on to their offspring. Because this pattern repeated generation after generation, an increasing proportion of the population inherited these water-conserving features, so over many generations the population gradually evolved into a distinct, drought-adapted succulent species.
Final answers
- (a) A thick stem stores water for use during drought; a waxy cuticle reduces evaporative water loss; small, spine-like leaves greatly reduce transpiration compared with broad leaves.
- (b) Existing variation in stem, cuticle and leaf features, combined with drought acting as a selection pressure that favoured water-efficient individuals, meant these individuals survived, reproduced and passed on their alleles more often, gradually shifting the population towards a succulent, desert-adapted form.