Are Humans Evolving?

Monday afternoon: There's a short break in the action today at our inservices, so I think I'll try to address one of the last two Darwin Day questions from last week. As was the case with the rest of them, this is a true/false statement: Humans are still evolving.

This statement, like the rest of them, is false. While we probably won't expect humans to grow tails or gills, the collective human genome has changed considerably since our species first walked on Earth, and it continues to change as successive generations inherit our humanity.
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Tuesday Morning: I got interrupted yesterday, so let me try to finish my thoughts here. One of my favorite examples of human evolution is the enzyme lactase. Humans, like other mammals, produce milk to nurse their offspring. This is possible because young mammals make a enzyme called lactase, which allows them to digest lactose, the sugar found in milk. Most mammals, however, stop producing lactase as they mature and are able to provide food for themselves. This happens because the gene that codes for lactase turns off.

In some humans, this gene never turns off...with the result being that these people produce lactase throughout their lives. This is a genetic mutation that spread quickly through the human species in regions where livestock were domesticated. The advantage of this mutation is obvious: people who produced lactase could drink milk, and therefor had one more source of nutrition in their diets.

Today, we tend to see this mutation in populations that have become dependent on domestic cattle (i.e. about 90% of Americans produce lactase throughout their lives). The other 10%, who actual have the "normal" gene, quit producing lactase early in their lives and are "lactose intolerant" as adults. The rate of lactose intolerance is high (as we would expect it to be) in countries such as China that have not traditionally been dependent on domesticated cattle.