Directions: Use the information provided and your knowledge of Life Science to answer the following questions. Show all work where necessary.
Directions: Use the information provided and your knowledge of Life Science to answer the following questions. Show all work where necessary.
Diagram 1.
Source: https://www.sciencehistory.org/stories/magazine/a-study-in-scarlet/
Warfarin, a common rodenticide used since the 1950s, works by interfering with blood clotting. When rats ingest it, even small injuries can cause fatal bleeding. For many years, warfarin was extremely effective at reducing rat populations in cities. However, over time, researchers noticed that some rat populations were no longer affected by the toxic compound.
Initial studies showed that resistance was controlled by mutations in the VKORC1 gene, which encodes a protein involved in vitamin K recycling, crucial for blood clotting. Rats with certain VKORC1 mutations could survive doses of warfarin that would kill non-resistant individuals. Because this resistance is heritable, offspring of resistant rats inherit the advantageous trait.
Urban environments create conditions where rats reproduce quickly - females may have 6 - 12 pups per litter and multiple litters per year. This high reproductive potential means traits can spread rapidly through populations. When warfarin is used repeatedly, resistant rats survive at much higher rates than susceptible rats. Over several generations, resistant individuals make up a larger portion of the population, an example of differential survival and reproduction driven by selection pressure.
Competition intensifies this shift. As susceptible rats die, the remaining individuals must compete for limited food and nesting sites. Rats with warfarin-resistant alleles not only survive the poison but also gain a reproductive advantage by facing fewer competitors. Over time, the proportion of resistant rats can rise sharply.
Scientists use statistics and probability to analyze how quickly resistance spreads. Frequency data show clear increases in resistant alleles across urban populations. Survival-rate studies reveal dramatic differences between resistant and susceptible rats when exposed to warfarin. Together, these patterns illustrate how selection pressure, heritable variation, and population growth interact to drive evolutionary change.
The spread of warfarin resistance also demonstrates that evolution can be observed over short timescales - sometimes within a few decades or even years. Many cities have now documented rat populations where more than half carry resistance-associated alleles. This real-world example demonstrates how traits that increase survival and reproductive success become more common, precisely as predicted by evolutionary theory.
Diagram 2.
Source: https://www.slideserve.com/adina/edexcel-gcse-science
Table 1.
Year | Warfarin-Resistant Rats ( | Non-Resistant Rats ( |
|---|---|---|
2000 | 5 | 95 |
2005 | 12 | 88 |
2010 | 25 | 75 |
2015 | 38 | 62 |
2020 | 52 | 48 |
2025 | 67 | 33 |
Graph of Information - Figure 1.
Table 2.
Rat Type | Survival Rate ( |
|---|---|
Non-Resistant | 8 |
Partially Resistant | 42 |
Highly Resistant | 79 |
Graph of Information - Figure 2.
Using the reading, explain how warfarin acts as a selective pressure on rat populations in urban environments.
Using Table 1 and Figure 1, describe how the frequencies of resistant and non-resistant rats changed from 2000 to 2025.
Include at least one numerical example.
Based on the reading, which statement BEST explains why resistance to warfarin can be passed from parent rats to their offspring?
Using Table 2 and Figure 2, which rat type shows the lowest survival rate when exposed to warfarin?
Using Table 1, Table 2, and Figures 1–2, explain how differences in survival among rat types support evolutionary change in urban populations.
Construct a CER explaining how environmental pressure from warfarin use causes shifts in rat population traits over time.
Respond using the structure: Claim, Evidence, and Reasoning.