Saturday, October 6, 2012

Oklahoma Evolution Workshop, Part Four

The evolution workshop for Oklahoma teachers continued after lunch. I had the first session. I began by advertising my own website, where you can find essays and photographs, as well as copies of scholarly articles  I have written. It has links to my YouTube channel and to this evolution blog.

Then I began with the story of the Ghosts of Evolution. (I have a recent YouTube video where Darwin discusses this subject.) I held up a big, sticky, green fruit of Maclura pomifera, known as bois d'arc, Osage orange, or horse-apple. The native range of this tree includes Oklahoma. I modeled the kind of questions that they can ask their students. Why do some plants produce juicy sweet fruits? Animals eat them, swallowing the seeds and transporting them to a new location. This is useful to the plant only if the animal swallows undamaged seeds, as we do with strawberry or kiwi seeds, or as raccoons do with persimmon seeds. With their finicky fingers, some raccoons could pick out the persimmon seeds, but this would waste valuable time, during which other raccoons would eat up all the fruits. The concept that students might grasp is coevolution. In this case, juicy fruits evolved in response to animals that eat them and disperse their seeds.

But what eats big, sticky bois d'arc fruits? In Oklahoma, someone will always say, horses. In other parts of the country, nobody may have any idea. But modern horses are not native to North America. There were native horses in North America at the end of the last ice age, but they have become extinct. Perhaps at the end of the last ice age, there were large animals (horses, mammoths, mastodons, gomphotheres, etc.) that ate bois d'arc fruits, but today, there are no native animals that eat them. Thousands of years ago, there was a coevolutionary waltz between bois d'arcs and gomphotheres; today, the bois d'arc is doing a waltz with no partner.

We went outside, into the cold wind. The temperature has dropped 50 degrees in two days. If you want to get kids outside and still teach them about evolution, just take them to an oak tree and ask them what they know about oak trees. The wood is strong. The trees live a long time. Acorns are large, as are the seedlings that emerge from them. Now, think about what kind of environment would benefit trees that have these characteristics. In an old, stable forest, large seeds and large seedlings would be better able to compete with the dense populations of plants that are already there. They invest for the long term in strong wood. Then take the kids to a cottonwood tree. Cottonwoods produce cheap wood and do not live very long. Their seeds are numerous and small. These characteristics make sense for trees that live near rivers, where they have a lot of water but also face a great risk of being destroyed in a flood. It would make no sense for a cottonwood to invest in wood that will last for centuries, since a cottonwood tree would probably get killed in a century or less. This may help students to understand that evolution does not always produce the same "superior" set of adaptations; the "superior" traits for a tree in an old, stable forest are different from those in a young, frequently-disturbed, floodplain forest.

Then Dr. Cecil Lewis had a second session, this time devoted to his specialty: the evolutionary and medical genetics of humans. He explained how population genetics works. A large, old population that has been large for a long time may have a great deal of genetic variation, including a lot of rare variants. Large populations can lose genetic variation if they become small--that is, they go through a population bottleneck. Small populations can lose genetic variation just by chance. If the population then grows rapidly, it retains the genetic makeup that it had while it was small. That is, a newly-large population will have no more genetic variability than it did when it was small. If you find a population that is large but has low genetic variability, this probably means that there was a recent genetic bottleneck. This appears to have happened in the human species. The entire human species, all seven billion of us, has the same amount of genetic variation that one would expect from a mammal population with only twenty thousand individuals. This suggests that (perhaps 70,000 years ago) our species had a brush with extinction--there may have been, at that time, only about twenty thousand of us.

You can make a closer examination of the genetics of human groups. The greatest genetic variation is in Africa. The genetic variation in the Middle East is a subset of African genes. The genetic variation in Europe is a subset of Middle Eastern genes; Asians are a separate subset. North American native genetic variation is a subset of Asian genes; South American native genetic variation is a subset of North American native genes. Each time a migration occurs, only a subset of people leave home, therefore some genes get left behind. Human genes, therefore, are an invisible record of the pattern of human migrations during the last 200,000 years.

Some human genetic patterns have resulted from natural selection. Lewis gave an example: the ALDH2-2 gene, the gene that codes for alcohol dehydrogenase. One variant of this gene is much more common in Asians than in anyone else. The Asian variant is the reason that Asians generally have a lower tolerance of alcohol than other people. However, since refiend alcohol is a recent invention, the Asian variant of the gene was probably selected by exposure to pathogens.

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