Natural Selection in Action (OCR A-Level Biology A): Revision Notes

Natural Selection in Action

Evolution is often perceived as a process requiring millions of years, yet observable evolutionary changes can occur within remarkably short timescales. Drug resistance in bacteria and pesticide resistance in insects demonstrate how natural selection can rapidly alter populations when strong selective pressures are applied. These modern examples show evolution happening in real time, with serious implications for medicine and agriculture.

Drug resistance in bacteria

The emergence of antibiotic resistance

Antibiotics became widely available in the late 1940s and revolutionised the treatment of bacterial infections such as tuberculosis. However, within years of their introduction, doctors observed that some antibiotics were becoming less effective. This occurred because a small number of bacteria possessed genes that enabled them to survive exposure to these drugs. When an antibiotic is used to treat an infection, it creates a powerful selective pressure. Bacteria without resistance genes die, while those carrying resistance genes survive, reproduce, and pass these advantageous alleles to their offspring. Over successive generations, the proportion of resistant bacteria in the population increases dramatically.

Origins of antibiotic resistance

Bacteria acquire antibiotic resistance through two main routes. First, a spontaneous mutation may occur in a chromosomal gene, producing a variant that codes for a polypeptide unaffected by the antibiotic. Second, bacteria can obtain a plasmid (a small circular piece of DNA) carrying resistance genes from another bacterium through horizontal gene transfer. This transfer can even occur between different bacterial species, allowing resistance to spread rapidly through diverse bacterial populations.

Mechanisms of antibiotic resistance

Resistance genes operate through several distinct mechanisms:

• Some code for enzymes that chemically break down the antibiotic molecule before it can harm the cell. For example, penicillinase is an enzyme that cleaves penicillin, rendering it ineffective. Penicillin normally kills bacteria by preventing cell wall synthesis, but bacteria producing penicillinase are protected from this effect.
• Other resistance genes produce membrane proteins that actively pump antibiotic molecules out of the cell as quickly as they enter, preventing the drug from reaching concentrations high enough to be lethal.
• Many antibiotics work by binding to and inhibiting key proteins involved in DNA replication, transcription, or translation. Mutations can alter the three-dimensional structure of these target proteins, eliminating the binding site for the antibiotic whilst preserving the protein's normal function.

Bacterial genetics and rapid selection

Bacteria are haploid organisms, possessing only a single circular chromosome and therefore just one copy of each gene. This means that any mutation conferring antibiotic resistance has an immediate effect on the bacterium's phenotype, unlike diploid organisms where recessive mutations may be masked by dominant alleles. When antibiotics are present in the environment, resistant bacteria gain an enormous selective advantage.

Bacteria reproduce asexually through binary fission, where the chromosome is replicated and the cell divides into two identical daughter cells. Under optimal conditions, this process occurs very rapidly. A single resistant bacterium can potentially produce $10,000$ million descendants within $24$ hours. This explosive reproductive capacity means that resistance can spread through a bacterial population with remarkable speed.

Clinical consequences

When a patient takes antibiotics to treat a bacterial infection, susceptible bacteria are killed and the few resistant individuals should be eliminated by the immune system. If the full course of antibiotics is completed correctly, the entire disease-causing bacterial population is usually destroyed. However, if patients stop taking the antibiotic when they start feeling better, some less susceptible bacteria may survive. These survivors form a partially resistant population, and subsequent infections may not respond to the same antibiotic.

The consequences for public health are severe. Medical authorities warn that we are approaching a post-antibiotic era. Despite careful stewardship, strains of bacteria now exist that are untreatable with available antibiotics. Bacteria such as Mycobacterium tuberculosis and methicillin-resistant Staphylococcus aureus (MRSA) have developed resistance to multiple antibiotics. No new class of antibiotic has been discovered since the 1980s, though researchers announced in January 2015 the discovery of teixobactin, a compound isolated from previously unculturable soil bacteria that showed activity against Mycobacterium and Staphylococcus aureus without detectable resistance.

Strategies to reduce antibiotic resistance

Healthcare systems have implemented several strategies to slow the spread of resistance:

• Antibiotics should only be prescribed when absolutely necessary and not for viral infections or as a precautionary measure
• Patients must complete the full course of treatment to ensure all bacteria are killed
• Doctors and hospitals rotate the antibiotics they use, preventing continuous exposure to a single type
• Some antibiotics are reserved as 'last resort' treatments, used only when all other options have failed
• Finally, sustained investment in research to discover new antibiotics is essential, though the economics of antibiotic development present significant challenges

Pesticide resistance in insects

Pesticides and agricultural pests

Pesticides are chemicals designed to kill organisms that damage crops, transmit disease, or compete with desired species. This broad category includes:

• Insecticides (for insects)
• Fungicides (for fungi)
• Herbicides (for weeds)
• Molluscicides (for slugs and snails)
• Rodenticides (for rodents)

Insecticides are used both to control crop pests and to reduce populations of disease vectors such as Anopheles mosquitoes (which transmit malaria) and tsetse flies (which transmit sleeping sickness).

The problem of monocultures

Modern agriculture often involves growing a single crop species over large areas of land, a practice called monoculture. This removes food availability as a limiting factor for specialist pest populations. For example, a field of potatoes provides abundant resources for Colorado potato beetles (Leptinotarsa decemlineata). When other environmental conditions such as temperature are favourable, pest numbers can increase exponentially, causing significant crop damage and reduced yields.

Farmers apply insecticides to control these pest populations. However, if even a small number of individuals possess genetic variants conferring resistance to the pesticide, these insects will survive whilst susceptible individuals die. With reduced competition for food and other resources, resistant insects are more likely to survive and reproduce successfully.

Selection for resistance

The selection process in insect populations mirrors that in bacteria. Within a population of Colorado beetles, a few individuals may be homozygous for an allele providing resistance to a particular insecticide. When farmers spray the field, susceptible beetles are killed whilst resistant individuals survive. These resistant beetles face little competition and successfully reproduce, passing the resistance allele to their offspring. Repeated applications of the same insecticide progressively increase the frequency of the resistance allele. Eventually, the entire population may consist of resistant individuals, rendering the insecticide ineffective.

In this example, the insecticide acts as a selective agent – an environmental factor that influences survival and reproduction, thereby driving natural selection. The same principle applies to antibiotics acting on bacterial populations.

Managing pesticide resistance

Pesticide manufacturers and agricultural advisors have developed strategies to delay the evolution of resistance:

• Farmers are advised to limit applications of any single pesticide to a specified number of times before switching to a different chemical with a different mode of action. This rotation strategy prevents continuous selection for resistance to one pesticide type.
• Mixing two or more pesticides with different mechanisms before application means an individual insect would need resistance to multiple chemicals simultaneously to survive, substantially reducing the probability of resistance evolving.
• Farmers are discouraged from applying insecticides as a preventative measure or 'insurance policy' against potential pest invasions. Instead, they should monitor their crops carefully and only apply chemicals when pest populations reach levels that pose a genuine threat to yield.

Integrated pest management combines multiple control strategies to reduce reliance on chemical pesticides:

• Biological control involves introducing or encouraging natural predators and parasites of pest species
• Plant breeders use selective breeding to develop crop varieties with natural resistance to pests
• Genetic modification can introduce genes enabling crops to produce toxins lethal to specific pests
• These approaches, combined with judicious use of chemical pesticides only when necessary, help maintain effective pest control whilst minimising the evolution of resistance

Selective agents in evolution

A selective agent is any environmental factor that influences which individuals within a population survive and reproduce. In the examples discussed, antibiotics and insecticides serve as powerful selective agents, creating strong directional selection favouring resistant individuals. Other selective agents include predators, availability of food or nesting sites, disease, temperature extremes, and competition for mates. Understanding how selective agents operate is essential for predicting and managing evolutionary change in populations.