We can see how natural selection has resulted in the adaptation of plants to different temperature regimes by comparing species in the genus Encelia, which are distributed along a temperature and moisture gradient from the coast of California to Death Valley. James Ehleringer (1980) showed that the leaves of the coastal species, Encelia californica, lack hairs entirely and reflect only about 15% of visible light. He also found that two other species that grow part way between the cool coast regions and Death Valley produce leaves that are somewhat pubescent and reflect about 26% of visible light. The desert species, Encelia farinosa, produces two sets of leaves, one set in the summer and another when it's cooler. The summer leaves are highly pubescent (hairy) and reflect more than 40% of solar radiation. What do you think the cool season leaves are like? If you predict that they are much less pubescent than summer leaves you are correct. Why is that? We know the benefits of leaf pubescence. What might be some costs? (Hint: What do plants do with visible light other than heat up?)
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