AP Environmental Science Unit 2 — Watching Selection Happen: The Moth, the Finch, and the Antibiotic
In eighteen forty-eight, a collector near Manchester caught a moth with black wings, the first one of its kind recorded in that city.
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AP Environmental Science Unit 2 — Watching Selection Happen: The Moth, the Finch, and the Antibiotic
Unit 2 keeps asking the same question in different costumes: when the environment changes, who lives and who doesn't? Half of the answers in this unit come down to one mechanism, and this episode is about the mechanism itself — natural selection — and about how you can tell, from a table of numbers, that it is happening.
It starts with a moth. In 1848 a collector near Manchester caught a black-winged peppered moth, the first of its kind recorded in that city; by 1895, 98 percent of Manchester's peppered moths were black. From there the episode follows the same mechanism through the experiment that caught selection in the act, the mutation that made a black wing possible in the first place, the clean-air reversal that sent the population back the other way, the Galápagos finch drought and its reversals, and antibiotic resistance, where the selective pressure is a drug we prescribe. It closes with five new practice questions worked through out loud, answers and reasoning included.
The short version
Natural selection needs three things at once: individuals that vary, variation that is heritable, and variation that makes a difference to how many offspring an individual leaves. Take away any one of the three and the population does not evolve — variation that is not inherited dies with the individual, and inheritance that changes nothing about offspring numbers leaves nothing to sort.
Fitness is the name for that third condition, and it means reproductive success in one environment, at one time. It never means strength or size. A small-beaked finch in a wet year is the fitter bird, and the same beak is a liability in a drought.
Selection has no aim and no destination. The black form of the peppered moth came from one accidental insertion in the DNA, and its rise and fall followed the colour of the background, not any progress. When the air cleaned up, selection ran in reverse with the same population and the same variation.
The cases behind the numbers
| Case | What was measured |
|---|---|
| Manchester, 1848 to 1895 | The first black peppered moth recorded in the city in 1848; by 1895, 98 percent of Manchester's peppered moths were black. In British towns today the pale form is far more common than the dark one, as it is in the countryside 1 |
| Kettlewell's release and recapture, 1950s | Dark moths came back about twice as often as pale ones in a soot-blackened wood near Birmingham, with the pattern reversed in clean countryside. Only about 36 percent of peppered moths rest on trunks, and the earlier experiments had placed them there at unnaturally high densities 2 |
| Majerus's experiment, Cambridge | 4,864 moths released over six years and allowed to settle where they chose. Birds took the dark form at a rate of roughly 10 percent against it per day — enough on its own to explain the speed of the dark form's later decline 3 |
| The mutation behind the black wing | A transposable element inserted into a gene called cortex, turning the gene up during wing development. Dating the insertion puts it around 1819, about thirty years before the first black moth was caught near Manchester 4 |
| Daphne Major finches, 1977 drought | Survival was not random: larger birds, especially males with deeper beaks, survived best because they could crack the large hard seeds that remained. The selection intensities in that drought are the highest ever recorded in a vertebrate population 5 |
| Daphne Major finches, 1972 to 2001 | Both finch species changed in body size and beak shape several times, in a pattern the authors describe as sometimes unidirectional and sometimes oscillating; the state of the birds at the end could not have been predicted at the start 6 |
| Antibiotic resistance | Bacterial resistance was directly responsible for 1.27 million deaths worldwide in 2019 and associated with close to 5 million, on the global burden estimates the CDC quotes 7 |
Vocabulary you should be able to use in a sentence
- Natural selection — the process in which organisms whose traits suit their environment survive and reproduce at a higher rate, so those traits become more common in the next generation.
- Heritable variation — differences between individuals that travel with the genes rather than with experience. Without it, selection has nothing to sort.
- Fitness — an individual's reproductive success relative to others in the same population, in that environment, at that time. Not strength.
- Selective pressure — the feature of the environment that decides which variants leave more offspring: bird predation in the moth case, hard seeds in the finch case, an antibiotic in the hospital case.
- Directional selection — one end of the range of variation is favoured, so the population mean moves.
- Stabilizing selection — both extremes are penalised, so the mean holds and the range narrows.
- Disruptive selection — both ends do well and the middle does badly, so the distribution can split into two groups.
- Transposable element — a stretch of DNA that can copy itself into a new place in the genome; the one inserted into the cortex gene is what made the black wing possible.
- Mark-release-recapture — catching, marking, releasing and re-catching individuals to estimate survival; in the moth work, the recapture rate of each form is the evidence of which one survives better.
- Antibiotic resistance — resistance genes carried by bacteria, spread as the survivors of antibiotic treatment reproduce. The bacteria evolve; the patient does not.
The five practice questions
1. Reading a recapture table. In a soot-blackened wood in 1953, a researcher releases 500 dark moths and 500 pale moths and traps for a week: 150 dark moths return and 50 pale ones. Which form has the advantage, what is the selective pressure, and what is wrong with saying the moths turned black so the birds would not see them?
The dark form has the advantage — 30 percent recaptured against 10 percent. The pressure is bird predation working through camouflage. The sentence fails twice: no individual moth changed colour, since each one hatched pale or dark; and the birds did not create the colour, they sorted the colours already present.
2. Naming the type of selection. A drought leaves only hard seeds, and mean beak depth rises by half a millimetre while the spread stays about the same. A shorebird's egg size barely moves over forty years, but the largest and smallest eggs stop appearing. A lake holds two kinds of food, and mean body size is unchanged while the spread splits into two humps.
Directional, stabilizing and disruptive. Disruptive selection is the one most likely to end in two species, because two humps that stop meeting in the middle can drift apart as separate populations.
3. The drought graph. Finches before and after a two-year drought: mean beak depth 9.2 mm, then 9.8 mm, with the population falling from about 1,200 birds to about 180. Beak depth is highly heritable. What was the pressure, what happened to the survivors, and why did the mean move less than the survivors' advantage?
The pressure was food: the drought removed the small soft seeds, so only birds that could crack hard seeds fed. Survivors were deeper-beaked than average, and because the trait is heritable their chicks were too, which lifted the population mean. The mean moves less than the survivors do because only part of the difference is passed on, and the deep-beaked survivors bred with shallow-beaked birds.
4. A resistance table. One hospital: 5 percent of E. coli samples resistant to a drug in 2005, 45 percent by 2015. Use of the drug then falls by more than half, and by 2020 resistance is at 30 percent. Across town, where the drug was used without a break, resistance is still above 90 percent. What is the selective agent, who is evolving, and why did resistance not vanish?
The antibiotic is the selective agent and the bacteria are the population that evolves. Resistance fell at the first hospital because resistant strains lost their advantage, but only from 45 percent to 30, because the genes were already widespread, many of them travel between different bacteria, and they cost the cell little. People do not develop resistance to antibiotics; the bacteria infecting them do.
5. Ranking four populations. All four meet the same new predator. A: high heritable variation in shell thickness, 10,000 individuals, one generation a year. B: thick shells, no detectable variation in thickness, same numbers and generation time. C: high variation, 120 individuals descended from twelve survivors of a flood. D: high variation, 10,000 individuals, one generation every twenty years.
A adapts fastest: variation to sort, numbers to keep rare variants from vanishing by chance, and fast generations. B cannot evolve at all, because nothing in it varies — a strong pressure with no heritable variation produces deaths, not adaptation. C is a bottleneck in disguise. D has everything except time.
Four ways this part of the unit gets missed
Fitness is not size or strength. It is relative reproductive success in one environment, and it moves when the environment moves.
Selection has no aim. The black wing came from one accidental insertion around 1819, and nothing about the moth wanted it. A population that "adapted because it needed to" has been given a mind it does not have.
Adaptation needs inheritance. A trait changing inside a single lifetime is not evolution, and a population with no heritable variation has nothing for selection to act on, which is why a bottlenecked population answers slowly.
Read the axis before you read the story. On this unit's graphs the vertical axis is usually a trait — beak depth, or the percentage of dark moths — so a moving mean is a change in the population's appearance, not in its numbers.
References
- 1Butterfly Conservation — Peppered Moth and natural selection
butterfly-conservation.org
- 2Cook and Saccheri, Heredity, 2013
pmc.ncbi.nlm.nih.gov
- 3Cook et al., Biology Letters, 2012
pubmed.ncbi.nlm.nih.gov
- 4van 't Hof et al., Nature, 2016
pubmed.ncbi.nlm.nih.gov
- 5Boag and Grant, Science, 1981
pubmed.ncbi.nlm.nih.gov
- 6Grant and Grant, Science, 2002
pubmed.ncbi.nlm.nih.gov
- 7
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