Natural Selection Case Studies
INstructions: Read the following two stories below. Then complete the table on page two to explain the natural
selection event that has occurred for each of these species.
The Rock Pocket Mouse
The rock pocket mouse, Chaetodipus intermedius, a small, nocturnal animal, is found in the deserts of the
southwestern United States. Most of these mice have a sandy, light-colored coat that enables them to blend in
with the light-colored desert rocks and sand on which they live. However, populations of primarily dark-colored
rock pocket mice have been found living in areas where a dark rock called basalt covers the ground. The basalt
formed from cooling lava flows thousands of years ago. Before the dark basalt appeared, it is believed that
nearly all rock pocket mice were light in color. Predators in this region are mostly visual hunters, and pocket
mice rely on their ability to blend into their surroundings to survive. While populations of rock pocket mice
living on the light colored sands are still mostly light, those found in areas of basalt are mostly dark.
African Elephants in Mozambique
A survey of African savanna elephants revealed that populations declined by 30%
between 2007 and 2014. As of 2014, about 350,000 savanna elephants were living in
Africa. Their current rate of decline is 8% per year, primarily due to poaching, which
is when an animal is killed illegally. A big question scientists have is how this
poaching is affecting elephant populations.
Most African elephants have tusks, but about 2 to 6% of females, and even
fewer males, never grow them. Elephant tusks are important for obtaining food and water, and are essential to
male elephants for competing for mates. Usually, there is a strong natural selection for having tusks, but more
recently the number of elephants without tusks has increased in some populations.
So why might this be happening? Elephant tusks are made from ivory, which is a hard white substance
that is similar to what teeth are made up of. In some countries, ivory is used and sold to make art pieces and
decorations. However, to obtain the ivory, poachers must remove it from an elephant, often killing the animal in
the process. Despite ivory trade being illegal in many countries, poachers continue to hunt and kill elephants for
their ivory tusks. This has resulted in the natural selection for elephants without tusks.
Using our Natural Selection Model to Explain Change
Natural Selection
Model
Variation exists
within a species.
Selective
Pressure:
Environmental
changes can cause
species to struggle
to survive.
Some variations
are better at
surviving and
reproducing than
others. This
advantageous trait
is now known as an
adaptation.
Fitness: Surviving
organisms are able
to reproduce.
Eventually the
majority of the
population will
have the more
advantageous trait
(adaptation) and
fewer will have the
disadvantageous
trait
Rock Pocket Mouse
African Elephants