|
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. |
|||||
|
Previous |
|||||