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Evolution of warfarin resistance
The size of a rat population can increase quickly. One female rat and one male rat can produce 20 offspring every 2 months. Warfarin is a poison that has been used to control rat populations. Rat populations can become resistant to warfarin by the process of evolution. A population of rats was given warfarin for 4 months. Table 1 shows information about resistance to warfarin in the rat population for a year after warfarin was first given.
| Number of months since warfarin was first given | Was warfarin being given? | Percentage (%) of rat population resistant to warfarin |
|---|---|---|
| 0 | Yes | 1 |
| 2 | Yes | 24 |
| 4 | Yes | 81 |
| 6 | No | 99 |
| 8 | No | 92 |
| 10 | No | 85 |
| 12 | No | 70 |
Explain the trends in the data when warfarin was being given and when warfarin was no longer given. Use Table 1.
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Marking points
- 1 Accurately describes the increase, delayed peak and subsequent slower decrease shown in Table 1.
- 2 Explains the increase by mutation, survival under warfarin, reproduction and inheritance of the resistance allele.
- 3 Explains the decrease after warfarin stops through the competitive disadvantage of smaller resistant rats and greater reproduction by non-resistant rats.
- 4 Logically links relevant trends and causes into a clear account suitable for Level 3.
Full-mark answer
The resistant proportion rose from 1% at month 0 to 81% at month 4 and continued to 99% at month 6, so the change did not reverse immediately when warfarin stopped. Random mutations had produced resistance alleles. While warfarin was used, resistant rats survived more often, reproduced and passed the allele to many offspring over several generations. After warfarin stopped, resistance fell more slowly, from 99% at month 6 to 70% at month 12. Resistant rats were smaller, so non-resistant rats outcompeted them for resources and mates and reproduced more successfully, reducing the proportion resistant.
Why this answer loses marks
The rats got used to the poison, and resistance fell as soon as it was removed.
Resistance is inherited variation, not acquired tolerance; link survival and reproduction under treatment, then competitive disadvantage after the pressure is removed.