7.8 Continuing Evolution

Mutations keep arising, populations keep facing selection and drift, and environments keep changing, so evolution is ongoing in every living species, including humans. It's easiest to see in organisms with short generation times and huge populations, such as bacteria, viruses and insects, because many generations pass quickly.

• Genomic changes over time: comparing DNA from the same population at different times shows allele frequencies shifting. In a long-running laboratory experiment begun in 1988, populations of E. coli have been tracked for tens of thousands of generations, and one population evolved the new ability to use citrate as food when oxygen is present after more than 30,000 generations.
• Continuous change in the fossil record: sequences of fossils through rock layers show gradual or stepwise changes in body form within lineages over time.
• Resistance: populations evolve resistance to chemicals people use to kill them, including antibiotics (bacteria), pesticides (insects), herbicides (weeds such as glyphosate-resistant pigweed) and chemotherapy drugs (cancer cells within a patient's body).
• Emerging diseases: pathogens evolve, and some gain the ability to infect new hosts or spread better. COVID-19, caused by the SARS-CoV-2 coronavirus, appeared in humans in late 2019, and new variants with mutations that spread more easily kept arising. Flu viruses change every year, which is why the flu vaccine is updated.

In a large bacterial population, a few cells may already carry a mutation or a plasmid gene that lets them survive an antibiotic. When the antibiotic is used, susceptible cells die and resistant cells survive and divide. Within a short time, most of the population is resistant. Bacteria can also pass resistance genes to other bacteria, even of different species, through horizontal gene transfer such as conjugation (6.7), which speeds the spread.

This is why doctors try to prescribe antibiotics only when they're truly needed, and why overusing antibiotics, including in livestock, is a public health concern. Methicillin-resistant Staphylococcus aureus (MRSA) is a well-known resistant bacterium.

Because evolution in microbes is fast, it can be studied directly. A strong experiment includes a control population grown without the selective agent, several replicate populations, and a measured change in a trait (such as survival at a given drug dose) or in allele frequency over generations. Questions on this topic often ask you to propose a follow-up experiment, such as testing whether resistance persists when the drug is removed or whether it spreads to a second species.