What is the fundamental difference between resistance and tolerance?

Resistance is an inherited genetic trait that allows a population to survive a toxicant over generations, whereas tolerance is a non-heritable physiological ability of an individual to survive stress.

How does vertical transmission differ from horizontal transmission in bacterial resistance?

Vertical transmission occurs when resistance genes are passed from a parent cell to daughter cells during binary fission, while horizontal transmission involves the transfer of genes (often via plasmids) between different, often unrelated, bacterial cells.

What is the difference between cross-resistance and multiple resistance?

Cross-resistance is when one genetic mutation provides protection against several similar chemicals, while multiple resistance is when an organism has separate, distinct genes for resisting different types of chemicals.

Why is it a misconception to say that 'bacteria learn to resist antibiotics'?

Evolution is not a conscious or learned process; resistance arises because random mutations that happen to provide protection allow certain individuals to survive and reproduce while others die.

What is the error in claiming that antibiotics cause the mutations that lead to resistance?

Antibiotics do not cause mutations; they act as a filter (selection pressure) that kills susceptible bacteria, allowing the rare, pre-existing resistant mutants to thrive.

Why does failing to finish a prescribed course of antibiotics contribute to resistance?

Stopping early leaves behind the most 'hardy' or partially resistant bacteria, which then have the opportunity to multiply and potentially undergo further mutations to become fully resistant.

Define 'Selection Pressure' in the context of pesticide use.

Selection pressure is the use of a pesticide that kills susceptible insects, thereby determining which individuals (the resistant ones) survive to pass their genes to the next generation.

What is a 'Plasmid' and why is it important for resistance?

A plasmid is a small, circular piece of DNA separate from the main bacterial chromosome that often carries resistance genes and can be easily transferred between bacteria.

What is the 'Refuge Strategy' in resistance management?

It is the practice of leaving a portion of a crop or area untreated so that susceptible individuals can survive and mate with resistant ones, diluting the resistance genes in the population.

How does generation time affect the speed of resistance evolution?

Organisms with shorter generation times (like bacteria) reproduce more frequently, allowing beneficial mutations to spread through the population much faster than in organisms with long life cycles.

Evolution of Resistance | OCR AS-Level Biology

4. Biodiversity, Evolution & Disease

Evolution of Resistance

Summary

The evolution of resistance is a fundamental application of natural selection where populations of organisms, such as bacteria or insects, develop the inherited ability to survive exposure to chemical agents like antibiotics or pesticides. This process is driven by pre-existing genetic variation and intense selection pressure, leading to significant challenges in medicine and agriculture.

1. Definition & Core Concepts

Resistance is the inherited ability of an individual or population to survive a dose of a toxicant (e.g., antibiotic, herbicide, or insecticide) that would normally be lethal to the majority of the species.
It is a population-level phenomenon resulting from changes in allele frequencies over generations, rather than an individual organism 'adapting' during its lifetime.
Selection Pressure refers to the external force (the chemical agent) that determines which individuals are more likely to survive and reproduce based on their genetic traits.
Susceptibility is the baseline state where organisms are effectively controlled or killed by the standard application of a specific chemical agent.

Graph showing the shift in allele frequency over time following the application of selection pressure, where resistant alleles increase and susceptible alleles decrease.

2. Underlying Principles of Selection

Genetic Variation: Within any large population, random mutations occur spontaneously, creating a pool of genetic diversity; some of these mutations may incidentally provide protection against specific chemicals.
Differential Survival: When a chemical agent is applied, individuals lacking the resistance gene die, while those possessing the 'resistant' mutation survive the exposure.
Reproductive Success: The surviving resistant individuals reproduce, passing the resistance genes to their offspring, often with reduced competition for resources like nutrients or space.
Inheritance: In bacteria, resistance can be passed vertically (from parent to offspring via binary fission) or horizontally (between different individuals via plasmids and conjugation).

3. Mechanisms of Resistance

Metabolic Resistance: The organism produces enzymes that break down or detoxify the chemical before it reaches its target site within the cell.
Target-Site Modification: A mutation alters the specific protein or enzyme that the chemical is designed to bind to, rendering the chemical unable to interfere with the organism's biological processes.
Reduced Permeability: The organism's outer membrane or cell wall changes to prevent the chemical from entering the cell in the first place.
Efflux Pumps: Specialized proteins actively pump the toxic chemical out of the cell as soon as it enters, keeping internal concentrations below lethal levels.

4. Human Factors and Selection Pressure

5. Key Distinctions

6. Exam Strategy & Tips