| Date | May 2022 | Marks available | 2 | Reference code | 22M.2.SL.TZ0.2 |
| Level | Standard Level | Paper | Paper 2 | Time zone | Time zone 0 |
| Command term | Describe | Question number | 2 | Adapted from | N/A |
Question
Figure 2: Distribution of continents and diversity of species over time
[Source: Biodiversity (in thousands of genera) since the Cambrian explosion of evolution, https://commons.wikimedia.org/wiki/File:Phanerozoic_Biodiversity.svg. This file is licensed under the Creative Commons Attribution-Share Alike
3.0 Unported license. (CC BY-SA 3.0) https://creativecommons.org/licenses/by-sa/3.0/deed.en.
USGS (United States Geological Survey) maps at https://pubs.usgs.gov/gip/dynamic/historical.html
From This Dynamic Earth: The Story of Plate Tectonics by W. Jacquelyne Kious and Robert I. Tilling
Public Domain.]
With reference to Figure 2, identify when the diversity of species was lowest in the past 400 million years.
Describe what may have caused the deviation from the trend line at point X in Figure 2.
Identify the relationship between the number of continents and the diversity of species during the past 250 million years.
Describe two reasons why there is a relationship between the number of continents and the diversity of species.
Outline the role of natural selection in increasing the diversity of species.
Markscheme
around 250 million years ago/Ma/Mya;
Note: Accept 240–255 Mya.
Do not credit if units (million years/Mya) are not given.
mass extinction/extinction event / mass extinction of dinosaurs (65 Mya);
due to meteor/asteroid impact / volcanic activity / climatic change / ice age;
the greater the number of continents, the greater the diversity / positive correlation (OWTTE);
separation of continents isolates populations leading to (allopatric) speciation;
continents are moved into different climatic regions causing evolution of different species;
continental movement creates new habitats, e.g. mountain ranges/islands;
natural selection (resulting from changing environment) acts on existing genetic variation (that has arisen through mutations over the eons);
natural selection increases the survival of fittest genetic variations/traits;
these traits are heritable/passed onto offspring / selected individuals will have higher reproduction potential / so species will adapt to different environments;
the proportion of these traits may increase in some populations/in subsequent generations;
which can lead to speciation/increasing species diversity if there is reproductive isolation;
Examiners report
Most were able to identify the lowest point on the species diversity curve. There were some careless responses that failed to acknowledge the figures represented millions of years.
Most were able to recognise a mass extinction and offer a reasonable cause.
The great majority could identify the simple positive correlation. Those that missed it got caught up in complex attempts at explanation (required in the next question) and failed to clearly identify the key relationship.
A good number were able to identify either geographical isolation or climate diversity as a key reason ... a few were able to offer both appropriate reasons.
Many candidates struggled to write accurately about natural selection often referring to species mating with other species and similar confusions.
