Natural selection and antibiotic resistance

Last time

Before we start: last time, in Xerophytes and hydrophytes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A superbug is born

A hospital patient has an infection, and the antibiotics stop working. The bacteria have changed, not the patient. Nobody planned it: it happened by natural selection. Today: how natural selection works, what adaptation means, and why antibiotic resistance is a warning about it.

Adaptive features

An adaptive feature is an inherited feature that helps an organism survive and reproduce in its environment. A polar bear has thick fur and a layer of fat to keep warm, and white fur to hide in the snow. Adaptation is the process by which a population becomes better suited to its environment over generations.

Variation comes first

Individuals in a population are not identical. They vary because of their alleles, and because of mutation. Some variants survive and reproduce better in the environment than others. Without variation, every individual would be the same, and there would be nothing for selection to act on.

Natural selection

Step one: there is genetic variation in the population. Step two: organisms produce more offspring than survive. So there is a struggle for survival: competition for food, mates and space, and predators and disease. Step four: the better adapted individuals survive and reproduce, and pass on the alleles that helped them.

Over many generations

Next generation, more individuals carry the helpful allele, because its owners left more offspring. This repeats every generation. Over a long time, the whole population becomes better adapted. Evolution is this slow change in the characteristics of a population.

Antibiotic resistance

An antibiotic kills bacteria. But a mutation can make one bacterium resistant, so it survives the antibiotic. The antibiotic kills the non-resistant ones. That removes the competition, so the resistant one has room to multiply. It passes the resistance gene to its offspring, and the next infection is not killed by the same antibiotic.

Slowing resistance down

Use antibiotics only when essential, and never for viral infections such as colds, because antibiotics do not work on viruses. Finish the whole course. Stopping early leaves the toughest bacteria alive to multiply. Fewer antibiotics used means less selection pressure, so resistance spreads more slowly.

Natural or artificial?

In natural selection, the environment does the selecting: predators, disease, climate and food. In artificial selection, humans choose which individuals breed. Both need genetic variation and reproduction. Both work through alleles passed to the next generation.