In a new video, Darwin looks at leaves—and leaflets and pinnae. He finds an evolutionary puzzle here.
We all think we know what leaves are. They are the (usually) thin green structures on plants that allow rapid photosynthesis. I, like many other botanists, have studied the plasticity of leaf thickness and size in response to sun and shade. Leaves on individual plants grown in the shade are thinner than those grown in the sun; that is, they have fewer layers of photosynthetic cells. Each cell, however, has more chlorophyll. Therefore, even though a sun leaf and a shade leaf may have nearly equal amounts of chlorophyll per square centimeter, and appear equally green to you, the shade leaves have much more chlorophyll per gram than the sun leaves. Shade leaves tend to be bigger, also. Plant species that grow down in the shade tend to have bigger leaves than plant species that usually grow out in the sun. I studied this pattern in the velvetleaf plant Abutilon theophrasti (Rice, S. A. and F. A. Bazzaz 1989. Plasticity to light conditions in Abutilon theophrasti: comparing phenotypes at a common weight. Oecologia 78: 502-507).
But this argument applies equally well to other photosynthetic surfaces. In many plants, each leaf consists of three or more leaflets. In the video, Darwin explores the leaflets of nandina, a common garden shrub. Its leaflets are divided into even smaller surfaces, known as pinnae. Leaflets can be larger or smaller, thicker or thinner, depending on their growth environment.
Natural selection favors a flexibility of photosynthetic surfaces. But, it seems, it does not matter whether these surfaces are leaves, leaflets, or pinnae. Natural selection works on whatever variation happens to be available. Oaks do not have compound leaves; natural selection favors flexibility of leaf area in oaks. Plants in the legume family frequently have leaves that consist of leaflets or leaflets that consist of pinnae. In this case, natural selection favors flexibility of leaflet or pinnae area. This is an example of what has been called the phylogenetic effect: natural selection works within the limits of the variation that is available to it, which might be limited by the evolutionary ancestors of the population.
Why do oaks have leaves but nandinas have pinnae? Maybe it is just because of who their ancestors were.