The Rising Star: Altitude Adaptation in a Plant Species
A species of alpine plant, Silene acaulis, grows from 1,000 m to 3,000 m elevation in a mountain range. At lower elevations, plants are tall (30 cm) with large leaves; at higher elevations, they are compact (5 cm) with small, thick leaves. A common garden experiment shows that when seeds from low and high elevations are grown together at 1,500 m, low-elevation plants grow taller and produce more seeds, while high-elevation plants remain compact and produce fewer seeds. At 2,500 m, the pattern reverses: high-elevation plants produce more seeds. Leaf thickness increases with elevation, and stomatal density decreases. The plant is pollinated by a bee species that is less active at high elevations. A warming climate is causing the low-elevation plants to encroach upward. The ecologist asks: (1) Is the variation in plant height due to phenotypic plasticity or local adaptation? (2) How might climate change affect the interaction between low- and high-elevation populations?
1. Determine whether the differences in plant height are due to phenotypic plasticity or local adaptation, using the common garden results.
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The common garden experiment shows that when grown in a common environment, plants from different elevations retain their height differences: low-elevation plants grow taller and produce more seeds at 1,500 m, while high-elevation plants remain compact and produce more seeds at 2,500 m. This indicates a genetic basis for the trait differences, i.e., local adaptation. Phenotypic plasticity would be evident if plants from both elevations grew similarly in the same environment, but here the differences persist. However, plasticity may also play a role: the high-elevation plants might be able to adjust leaf thickness or stomatal density in response to the environment, but the height difference itself appears to be genetically based. The fact that each population performs best at its home elevation (home advantage) is a classic sign of local adaptation, likely driven by natural selection for different optima at different elevations.
2. Predict how climate change might affect the interaction between low- and high-elevation populations, considering the pollinator and the potential for competition or hybridization.
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As the climate warms, low-elevation plants may encroach upward, competing with high-elevation plants for space, nutrients, and pollinators. The high-elevation plants are adapted to a short growing season and may be outcompeted by the faster-growing low-elevation plants. However, the low-elevation plants may be less tolerant of the harsh conditions at high elevations, so their upward migration might be limited. The pollinator bee is less active at high elevations; if low-elevation plants attract more bees, they could reduce pollination services for high-elevation plants. Alternatively, if the two populations come into contact, they might hybridize, potentially swamping the locally adapted high-elevation genotypes. The outcome depends on the degree of genetic differentiation and the strength of reproductive barriers. Conservation strategies might include assisted migration or protecting high-elevation habitats to allow the high-elevation plants to persist.