Natural Selection
It is not uncommon for a species of plant to grow widely in the foothills of mountain ranges, and in the valleys between ranges. It is also not uncommon for the environment to change--we might consider global warming as an example. What happens to these plants as the temperature increases?
 
Over the years, seeds in the upper range of the plants germinate and grow well, but those in the lower range do poorly because it’s too hot. Eventually, we have separate populations of plants on each mountain range, separated by valleys that are too warm for them.
 
Now what?
 
The Sierra Nevada and the White Mountains of California provide an interesting example of mountains in which different outcomes might be expected. The Sierras push up the moist air that blows in from the Pacific. The water condenses, and precipitates, mostly on the western slopes.
 
The White Mountains are in the rain shadow of the Sierras, and are quite dry.
 
At elevations where the Sierras would have live oaks, giving way to ponderosa, scrub oak, manzanita, and incense cedar, the Whites would have sagebrush, giving way to an open piñon/juniper woodland. At higher elevations, the Sierras have spruce and fir; the Whites have bristlecone pines.
 
The following pages imagine how leaf shape might be selected in these two different environments, starting with the same initial genetic diversity. Here, it is presented as full pages of leaf pictures, over several generations. In the classroom, we might ask groups of students to draw the distributions of leaf shapes that they would expect, for one generation, then the next, then the next. For selection pressure, a moist environment might favor broad leaves that can capture the most sunlight. A dry environment might favor narrow leaves that lose less moisture to transpiration.
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