In Yellowstone National Park, there are dozens of spectacular thermal pools filled with very hot water that rises from deep underground. In each thermal pool, there is a community of microbes that create the spectacular colors we can see in them. These microbes are very temperature sensitive. Between 1990 and 1997, the temperature in this pool was 89 degrees C. In 1997, a small, cool surface stream changed course and flowed into the pool, lowering the temperature by 7 degrees C. This significantly lowered the allele frequency of the heat tolerant allele. After 2 years, the stream changed course again and the temperature in the pool returned to its original 89 degrees, returning the allele frequency of the heat tolerant allele to its pre-1990 level. This story illustrate

The type of natural selection that favors one of the extreme phenotypes over the intermediate and another extreme phenotype is called directional selection. Here, the extreme phenotype exhibits better survival and reproductive fitness over the other phenotypic ranges.

In the given example, the frequency of the heat-tolerant allele in microbes is shown to increase under the conditions of higher temperatures of the water of the springs in which they live.

When the temperature of spring water is increased, the frequency of the heat-tolerant allele is reduced and is increased again upon an increase in water temperature.

Since the natural selection favors the extreme phenotype (microbes with heat tolerance) when the spring water exhibits higher temperatures, it is directional selection.