1Why some organisms evolve so fast

The speed at which a population can evolve depends on how much new variation appears and how many generations selection gets to act on.

• Generation time. A bacterium can divide every half-hour or so in good conditions; an elephant breeds every few years. Fifty years is perhaps fifteen elephant generations and hundreds of thousands of bacterial ones.
• Population size. A single infection can contain billions of bacteria or viruses. Even a rare mutation, present in one cell in a hundred million, is then likely to exist somewhere in the population.
• Mutation rate. Some viruses, especially those with RNA genomes, copy their genetic material with many errors, producing a great deal of variation.

Resistance is not caused by the drug: the drug kills the susceptible individuals, and resistant ones that were already present survive and multiply.

2Genomic change over time

Because DNA can be sequenced cheaply, scientists can now watch evolution in the genome directly.

In a long-running laboratory experiment begun in 1988, twelve populations of the bacterium E. coli have been grown in a simple medium, with samples frozen at regular intervals. Over tens of thousands of generations, the populations accumulated mutations, grew larger cells and became faster-growing in that medium. In one population, after more than 30,000 generations, bacteria appeared that could use citrate, a nutrient in the medium that E. coli normally cannot use when oxygen is present. Because ancestors were frozen, the researchers could compare genomes before and after and trace the changes. The same approach is used on real populations: sequencing a virus from patients month by month shows which mutations are spreading.

3Continuous change in the fossil record

Where rock layers are well preserved over long periods, a lineage can be followed through successive layers. The fossils change: in size, in shape of teeth or shells, in the number of particular structures. The changes are rarely sudden or neat, but they are continuous, and different lineages change at different rates (topic 7.10 deals with fast and slow patterns). What the fossil record does not show is a species that appears and then stays exactly the same for all time while its environment changes around it.

4Resistance: antibiotics, pesticides, herbicides, chemotherapy

The four cases follow one pattern. Learn the pattern once.

1. Variation exists. By random mutation, a few individuals already carry an allele that reduces the effect of the chemical: an enzyme that breaks it down, a pump that removes it, or a target protein the chemical no longer binds.
2. The chemical is applied. It kills or stops the reproduction of susceptible individuals.
3. Resistant individuals survive and reproduce. Their offspring inherit the resistance allele.
4. Repeated use makes resistant individuals the majority. The chemical stops working.

Antibiotics. Bacteria such as Staphylococcus aureus have evolved resistance to one antibiotic after another; strains resistant to methicillin (MRSA) are a serious problem in hospitals. Bacteria can also pass resistance genes to each other on plasmids, which speeds the spread. Using antibiotics where they are not needed (for example, against viral infections, which they cannot treat) applies selection with no benefit.

Pesticides. Many insect pests, including mosquitoes that carry malaria, are resistant to insecticides that once controlled them (topic 7.2, DDT).

Herbicides. Weeds such as Palmer amaranth in the United States have evolved resistance to glyphosate, a weedkiller sprayed widely on crops engineered to tolerate it. Spraying the same herbicide year after year is strong, consistent selection.

Chemotherapy. A tumour is a population of dividing cells, and mutations make those cells genetically varied. A drug may kill most of them, shrinking the tumour, but a few cells with a resistance mutation survive and divide. The tumour regrows, now made of resistant cells. This is natural selection within one patient's body, and it is why doctors often combine several drugs at once: a cell would need resistance to all of them to survive.

5Pathogens and emerging diseases

An emerging disease is one that has newly appeared in a population, or whose cases or range are rising quickly. Many emerging diseases are caused by pathogens that evolved.

• New hosts. Viruses that infect animals sometimes acquire mutations that let them infect humans and spread between people. Several recent human diseases, including COVID-19, are thought to have started this way.
• Escaping immunity. Influenza viruses change their surface proteins year by year, so antibodies made against last year's strains recognise this year's less well. That is why flu vaccines are updated every year. SARS-CoV-2 likewise produced a series of variants that spread faster or escaped existing immunity.
• Within one host. HIV mutates so rapidly that a single infected person carries a varied population of viruses, and drug-resistant variants can be selected within months unless several drugs are combined.

6Evaluating evidence and proposing an investigation

The skill for this topic is proposing a new investigation from an evaluation of a design or evidence. Practise on an invented study.

An invented study. A farmer sprayed the same herbicide on a field every year for six years and counted the percentage of a weed species surviving one week after spraying.

Evaluate. The data show survival rising from 2% to 66%, consistent with selection for resistance. But the design has weaknesses: there is one field and no unsprayed comparison, so other changes (weather, a change in the herbicide batch, spraying technique) could explain some of the rise; and survival was measured in the field, so it is not shown that resistance is inherited.

Propose. A strong follow-up:

• Question: is the increase in survival caused by inherited resistance?
• Design: collect seeds from surviving weeds in year 6 and from an unsprayed field nearby. Grow both groups side by side in a greenhouse under identical conditions (soil, water, light), then spray both with the same dose of herbicide.
• Variables: independent variable, seed source; dependent variable, percentage surviving; controlled variables, dose, growing conditions, plant age; several trays of each group as replicates.
• Prediction: if resistance is inherited, plants from the sprayed field will survive at a much higher rate than plants from the unsprayed field.

Other follow-ups that a reader would credit: sequence the gene for the herbicide's target protein in both groups and look for a mutation; or grow the two groups without herbicide to test whether resistance has a cost in growth or seed production.