Wednesday, March 20, 2013

We All Love Stories, Even about Oaks

If you came to this blog seeking more information about the summer evolution road trip, it can be found in the previous entry, and more information will be available in upcoming days.

You could almost define Homo sapiens as the species that loves stories. We evolved that ability around African stone age campfires, where the ability to tell and to understand stories conferred social advantages. We interpret our lives as stories; we could not imagine ourselves as eternal beings. And in science, we love stories also. Stories about how the digestive system works. (The epitome of this approach was the book, published about eighty years ago, entitled Through the Alimentary Canal with Gun and Camera. And apparently Mary Roach, author of Bonk and Stiff, is coming out with a new contribution to the literature of alimentary expeditioning, Gulp.) Stories about how ecosystems work. And stories about evolution. We are (well, most of us) much more interested in the stories of evolution than in the theory. We have to be careful to not read too much into the stories. It is all too easy to think of Tiktaalik crawling out of the water toward life on land, as if he were Odysseus trying to get home to Ithaca after the Trojan War. At least, we have to be careful to keep that kind of thinking out of our science; but it remains part of our folklore.

I sometimes like to imagine the saga of the oaks. Somewhere in Eurasia, maybe 80 million years ago, there was a population of trees that produced acorns: the first population of oaks (genus Quercus). What was the secret of their success? Was it the acorns, or was it something else? These early oaks may have resembled the white oaks (section Quercus), which have spineless leaves and acorns that mature the same year that they are produced, for these oaks are found throughout the Northern Hemisphere.

Then geological and climatic changes occurred throughout the Northern Hemisphere. The Atlantic Ocean formed, and the Rocky Mountains arose, and the arctic climate got colder. This separated the oaks of Europe, Asia, California, and eastern North America from one another. Unique oak species evolved in each of these locations and in others. In some cases, this produced new species of white oaks, such as Q. aliena in eastern Asia; Q. robur in Europe; Q. macrocarpa (the bur oak) in eastern North America; and Q. douglasii in California. In other cases, isolation resulted in whole new sections of the genus. For example, the black and red oaks (section Lobatae) have spines on their leaves and their acorns mature the year after they are produced. They apparently evolved in North America, but then diverged into different species such as Q. rubra (red oak) in eastern North America and Q. kelloggii in California. The ring-cupped oaks (section Cyclobalanopsis) evolved and diversified in Asia. Sections Mesobalanus and Cerris evolved and diversified in Asia and Europe, and section Protobalanus in western North America. What was the story here? Did the divergence between white and black oaks have anything to do with the spines or lack of spines on their leaves? Or with how long it took their acorns to mature? We might never know. In some cases, oak species ended up isolated in unusual places, such as Q. copeyensis in Panama.

Climate changes were also important. The Sierra Nevada and the Rocky Mountains not only separated eastern and western oak populations in North America, but also created rain shadows; the resulting deserts further isolated the oak species. In dry regions (of which there were almost none until about 30 million years ago), many oaks evolved into evergreens, keeping their leaves over the winter. In many places, some of the oaks are evergreen and some are deciduous. All the oaks in eastern North America are deciduous except Q. virginiana (the live oak), and in California they are all evergreen except Q. kelloggii.

The oaks represent just one of many thousands of evolutionary stories: of genealogy, of migration, of response to adversity (such as changing climates). Just as Samuel Butler wrote The Way of All Flesh in the nineteenth century about five generations of the Pontifex family, an oak expert could write The Way of All Leaf.

Pick any group of trees, whether walnuts or sycamores; or any group of wildflowers, such as the phlox; or any group of animals; or any other group of organisms. Each has an evolutionary story, indeed an evolutionary saga.

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