The Grand Canyon lies in Arizona, USA. It is thought to have been formed more than five million years ago, when the Colorado River started cutting a deep channel (canyon) into the surrounding rocks.
Before the canyon came into being, an ancestral species of antelope squirrel lived in the area. An antelope squirrel is a rodent and a member of the squirrel family, Sciuridae.
It is thought that about 3.6 million years ago, this ancestral species split into the two species found today:
• Harris’s antelope squirrel, $A. harrisii$, has a habitat range that extends from the south rim of the canyon.
• The white-tailed antelope squirrel, $A. leucurus$, has a habitat range that extends from the north rim of the canyon.
Fig. 4.1 shows where the Colorado River is in the Grand Canyon and where these antelope squirrel species are found.
(a)[4]
Suggest and explain the manner in which $A. harrisii$ and $A. leucurus$ evolved from an ancestral species.
(b)[3]
Scientists studied the evolutionary relationships of the squirrel family, Sciuridae. The scientists sampled present-day species in the family and carried out DNA sequencing and morphological analysis.
To compare modern species with those from the past:
• museum specimens were used to supply tissue for DNA sequencing
• teeth and skulls from fossils were compared as part of the morphological analysis.
Describe the advantages of using DNA sequencing rather than morphological analysis to learn more about the evolutionary relationships of the squirrel family.
(c)[1]
The estimated time at which an ancestral species split into $A. harrisii$ and $A. leucurus$ is 3.58 million years ago. This estimate has a large uncertainty.
DNA sequencing, including DNA taken from fossils, was used to estimate this divergence date.
Suggest a reason why the uncertainty around this date estimate is so large.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Isolation caused by geography” …