Antibiotic & DDT Resistance

Analyse asks you to identify the components of the process and relate them. The components are the pre-existing variation, the spraying, the reproductive advantage and the spread, and the analytical work is showing how the features of the human pressure set the speed of the outcome.

5 marks. Establish the variation, then the sorting, then the rate, then what made this pressure so effective, then the consequence.

Mosquito populations are genetically variable, and before DDT was used a small proportion of individuals already carried alleles conferring some tolerance, arising by random mutation. Two mechanisms are well documented: altered forms of the target site in the insect nervous system, so that DDT binds less effectively, and raised activity of enzymes that break the insecticide down before it reaches that target. These alleles were present at low frequency because they conferred no particular advantage, and may have carried a small cost, in the absence of the chemical.

Spraying killed the susceptible majority of the population and left the tolerant minority alive. Those survivors then bred, with greatly reduced competition for breeding sites and resources, and passed their resistance alleles to their offspring. The proportion of resistant individuals in the next generation was therefore far higher than in the generation before, and repeated spraying repeated the sorting.

The speed of the outcome follows from specific features of the situation. Mosquitoes have very short generation times, often measured in one to two weeks, and produce large numbers of offspring, so many rounds of selection occur in a single season. The spraying was applied over wide areas and repeatedly, so a large proportion of the population was exposed each time, and at a dose intended to kill every susceptible individual, which produces the strongest possible selection differential. Resistance appeared in some sprayed populations within a few years.

The selective agent is a human decision rather than a feature of the natural environment. People chose the chemical, the concentration, the area covered and how often to repeat it, and each of those choices directly set how intense the selection was. A pressure of this strength and consistency would be unusual in nature, which is why human-induced pressures can produce evolutionary change at a rate that is observable within a human lifetime.

The consequence was a loss of control in affected areas, with DDT becoming progressively less effective against malaria-transmitting mosquitoes, contributing alongside environmental concerns to changes in vector-control strategy. Throughout, the chemical did not create resistance and no individual mosquito became resistant: the alleles existed beforehand and the spraying changed only their frequency in the population. Describing mosquitoes as "becoming resistant" is a statement about the population, not about any insect in it.

Assess asks for a judgement of value or quality. Here you are judging a source, so you need criteria, evidence against each criterion, and a conclusion that follows from them.

5 marks. Set out the criteria you will use, apply them, distinguish reliability from validity, then give a judgement with a stated condition.

A secondary source is assessed on its origin, the evidence behind it, and how it presents that evidence. The questions to ask are: who produced it and what expertise or interest do they have; does it cite primary data such as a published study or a surveillance report; is it current; is it consistent with other independent sources; and does it distinguish what was measured from what is being inferred.

Check whether the article is from a health department, a peer-reviewed journal, a hospital's own reporting, or a news outlet summarising one of these, since each is a different distance from the data. Check whether it cites a specific study or surveillance dataset that can be located and read, because an article that names no source cannot be verified and its claims cannot be traced. An author or organisation with a commercial interest in a particular treatment, or a strong campaigning position, is a reason for additional caution rather than automatic rejection.

Check the date, since resistance patterns change and a figure from a decade ago may no longer describe the situation. Then check whether independent sources report the same trend, because agreement between sources that did not copy one another is the strongest available support, while a claim appearing in many outlets that all trace back to one press release has been repeated rather than corroborated.

Reliability concerns whether the reported figures are dependable and repeatable. Validity concerns whether they support the conclusion being drawn. A source may report accurate figures and still not support the claim made from them: a rise in recorded resistant infections could reflect a genuine rise, or increased testing, or a change in how cases are defined or reported, or the hospital taking referrals of more serious cases. Unless the article addresses those alternatives, the figures can be reliable while the conclusion drawn from them is not valid.

My judgement is that a single article of this kind is usable as a starting point but not as evidence for a conclusion on its own. It becomes usable as evidence if it cites a traceable primary source, is recent, is corroborated by an independent body such as a national surveillance programme, and reports the denominator, meaning the number of infections tested, alongside the number found resistant, so that a change in testing can be distinguished from a change in resistance. Without the denominator in particular, a reported rise cannot be interpreted at all.