Coastal shrub: adaptation or acclimatisation?
Two populations of the same shrub species grow 800 m apart on a windswept coastal headland and in a sheltered valley. Headland plants average 40 cm tall with leaves 2 mm thick, waxy and densely hairy. Valley plants average 120 cm tall with leaves 0.5 mm thick, glossy and hairless. A researcher collected seeds from both sites and grew them in a common garden with uniform soil, water and shelter. After 18 months, headland-origin plants were still short (45 cm) with thick hairy leaves, while valley-origin plants were taller (110 cm) with thin glossy leaves. When valley-origin plants were transplanted onto the headland, they grew to only 60 cm and produced thicker leaves than in the valley, but did not become hairy.
1. Explain how two of the headland leaf traits could adapt the shrub to its environment.
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A thick, waxy cuticle reduces water loss by evaporation from the leaf surface. On a windswept headland, fast-moving air increases transpiration, so a thicker wax layer helps the plant conserve water and keep stomata functioning without excessive dehydration. Dense hairs on the leaf surface can trap a thin layer of still air next to the leaf, reducing the drying effect of wind, and can also reflect some sunlight and deter small herbivores. Both traits would be favoured if they increase survival and reproduction in the exposed habitat. The smaller leaf size and compact growth habit also reduce the area exposed to wind, lowering the risk of physical damage and water loss. These explanations are adaptation arguments only if the traits are heritable and improve fitness on the headland; the common-garden result supports a genetic basis because headland-origin plants retained short, thick, hairy leaves even when grown in uniform conditions.
2. Evaluate the evidence that the differences between populations are genetic adaptations rather than acclimatisation.
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The common-garden result is strong evidence for a genetic difference. When both populations were grown in the same soil, water and shelter, headland-origin plants remained short with thick hairy leaves and valley-origin plants remained tall with thin glossy leaves. Acclimatisation, or phenotypic plasticity, would be expected to reduce the differences in a common environment. However, the transplant result shows that some traits are also plastic: valley-origin plants moved to the headland became shorter and produced thicker leaves, though they did not become hairy. This suggests height and leaf thickness have a plastic component, while hairiness is more strongly genetic. To decide whether these differences are adaptations, we need evidence that the traits increase survival or reproduction in each habitat, not just that they differ. A reciprocal transplant measuring survival, growth and seed production over more than one season, with multiple sites and families, would be needed. Maternal effects and small sample sizes are also possible limitations, so the conclusion should be that there is good evidence for genetic differentiation, but adaptation requires fitness evidence.