Observed Selection: Peppered Moths and Finches
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You should understand the basic mechanism of natural selection as differential survival and reproduction of heritable variation, and be familiar with the concept of heritability (\(h^2\)) as the proportion of phenotypic variance attributable to additive genetic variance.
Why this matters
Observational evidence transforms natural selection from a theoretical framework into a documented biological process.
While the logical structure of natural selection was established in the nineteenth century, its acceptance in the scientific community required empirical demonstration that trait frequencies change in response to environmental pressures. The cases of the peppered moth and Darwin’s finches provide the foundational observational data that link specific environmental changes to shifts in allele frequencies. These examples are not merely historical anecdotes; they are among the best-known applications of the quantitative genetic framework to wild populations, demonstrating that adaptation is not a static state but a dynamic response to selection pressures. By examining these systems, we move from the abstract logic of natural selection to the concrete reality of evolutionary change in real time.
The Logic of Observable Change
Imagine a population of moths resting on tree trunks. In a pre-industrial environment, the trunks are covered in light-colored lichens. Most moths are light-colored, blending in with the background, while a few are dark-colored due to genetic variation. Predators, such as birds, hunt by sight. The dark moths are more visible and are eaten more frequently. The light moths survive to reproduce. Because the light colour is heritable, the next generation has more light moths. This is not random; it is a directional shift caused by the environment. Now, imagine industrial soot darkens the trunks. The light moths are now visible, and the dark moths are camouflaged. The predators eat the light moths. The dark moths survive and reproduce. The population shifts towards dark moths. When the air is cleaned and the lichens return, the shift reverses, and this reversal is key evidence. If the change were due to random drift or non-heritable plasticity, it would not track the environmental change so precisely. The plain language here is that the environment acts as a filter. It does not create the variation; it selects which existing variations are passed on. The "struggle for existence" is not a battle between individuals, but a competition for survival against a specific, changing background. The result is a change in the average trait of the population. This is evolution. It is observable because we can count the moths before and after the environmental shift. The logic is simple: variation exists, selection acts, and the population changes. This is the core of natural selection, stripped of equations and placed in a biological context.
Case Study: The Peppered Moth
The peppered moth (Biston betularia) is the classic example of industrial melanism. The wild-type form is light-colored with dark speckles, while the melanic form is almost entirely black. In the mid-nineteenth century, the melanic form was rare, constituting less than 1% of the population in many areas. As industrialization progressed, soot from coal-fired factories killed the light-colored lichens and darkened the tree bark. This changed the visual landscape. Predators, primarily day-feeding birds, could more easily detect the light moths against the dark background. Consequently, the frequency of the melanic allele increased dramatically. In some urban areas, the melanic form reached about 98% around Manchester by the 1890s. This is a direct observation of natural selection in action. The change was not due to the moths "deciding" to change colour, but due to the differential survival of the two forms. The heritability of the trait was confirmed by breeding experiments, showing that the melanic form is controlled by a single gene with a dominant allele. The selection coefficient (\(s\)) can be estimated from the change in allele frequency over time. If the melanic allele frequency increased from 0.01 to 0.90 over 50 years, the selection pressure was strong. This case demonstrates that adaptation is a response to environmental change, not a pre-programmed outcome. When clean-air laws (in Britain, the Clean Air Act of 1956) were introduced, the lichens returned, and the light moths became camouflaged again. The frequency of the melanic form declined back to low levels. This reversibility is crucial evidence against alternative explanations, such as genetic drift or non-heritable plasticity. The population tracked the environmental change with high fidelity, confirming that selection was the driving force.
Case Study: Darwin’s Finches
Darwin’s finches in the Galápagos Islands provide a second, independent line of evidence. These birds exhibit significant variation in beak size and shape, which is strongly heritable. The size of the beak determines the types of seeds the birds can eat. In years of abundant small seeds, birds with smaller beaks are more efficient and have higher survival rates. In years of drought, when small seeds are scarce, birds with larger beaks can crack the larger, harder seeds that remain. This creates a fluctuating selection pressure. Peter and Rosemary Grant’s long-term studies on Daphne Major Island documented these shifts. During the severe drought of 1977, the population of medium ground finches (Geospiza fortis) fell by about 85%. The survivors were those with larger beaks. The average beak depth in the population increased by 0.5 mm in a single generation. This is a measurable evolutionary change. The heritability of beak depth is approximately 0.8, meaning that most of the variation is genetic. The response to selection (\(R\)) can be calculated using the breeder’s equation, \(R = h^2 S\), where \(S\) is the selection differential. In this case, the selection differential was large because the survival advantage of large beaks was significant. The result was a rapid shift in the population mean. This case demonstrates that selection can act quickly and strongly, even in wild populations. It also shows that the direction of selection can change depending on the environment. In wet years, the selection pressure may favour smaller beaks, leading to a reversal of the trend. This dynamic nature of selection is a key feature of natural selection in the wild.
Quantifying Selection
To move from qualitative observation to quantitative understanding, we use the framework of fitness and adaptation. Fitness is defined as the reproductive success of an individual relative to others in the population. In the peppered moth case, fitness can be measured as the number of offspring produced per adult. If the melanic moths have a fitness of 1.0 and the light moths have a fitness of 0.5, the selection coefficient \(s\) is 0.5. This means that the light moths have a 50% lower reproductive success. When the favoured allele is dominant, only the recessive homozygotes (fitness \(1-s\)) are selected against, and the frequency \(q\) of the recessive allele changes as \(q_{t+1}=\frac{q_t(1-s\,q_t)}{1-s\,q_t^2}\). In the finch case, the selection differential \(S\) is the difference between the mean trait value of the survivors and the mean trait value of the entire population. With illustrative round numbers: if the survivors\' mean beak depth was 0.6 mm greater than that of the whole population, then \(S = 0.6\) mm, and with \(h^2 = 0.8\) the predicted response is \(R = 0.8 \times 0.6 \approx 0.5\) mm, the size of shift seen in the next generation. The breeder’s equation provides a powerful tool for predicting evolutionary change. It links the genetic architecture of the trait (heritability) to the environmental pressure (selection differential) to predict the outcome (response). This quantitative approach allows us to test hypotheses about the strength and direction of selection. It also allows us to compare different populations and different traits. By applying this framework to the peppered moth and the finches, we can confirm that natural selection is the primary driver of the observed changes.
The concepts
Natural selection
Differential survival and reproduction of heritable variation.
Fitness and adaptation
Reproductive success shapes the traits of a population.
A common misconception is that natural selection always leads to the "best" possible adaptation. In reality, selection is constrained by the available genetic variation, the trade-offs between traits, and the changing environment. The peppered moth did not evolve perfect camouflage; it evolved a shift in frequency that reduced predation. The finches did not evolve the "perfect" beak size; they evolved a shift in mean beak size that improved survival in a specific year. Selection is a process, not a goal. It acts on the variation present in the population, not on the ideal variation that might be possible. This distinction is crucial for understanding the limits of adaptation and the role of contingency in evolution. C
Worked example
Problem. In a peppered moth population the melanic allele \(M\) (dominant) has frequency \(p_0 = 0.10\), so the light allele \(m\) has \(q_0 = 0.90\). Melanic moths (\(MM\) and \(Mm\)) have fitness 1.0 and light moths (\(mm\)) have fitness 0.8. Assuming random mating, find the melanic allele frequency after one and two generations.
- Selection coefficient against \(mm\): \(s = 1 - 0.8 = 0.2\). Only \(mm\) is selected against, so use \(q' = \dfrac{q(1 - sq)}{1 - sq^2}\).
- Generation 1: \(q_1 = \dfrac{0.90(1 - 0.2 \times 0.90)}{1 - 0.2 \times 0.81} = \dfrac{0.90 \times 0.82}{0.838} = \dfrac{0.738}{0.838} = 0.8807\).
- So \(p_1 = 1 - 0.8807 = 0.1193\).
- Generation 2: \(q_2 = \dfrac{0.8807(1 - 0.2 \times 0.8807)}{1 - 0.2 \times 0.8807^2} = \dfrac{0.7256}{0.8449} = 0.8588\), so \(p_2 = 0.1412\).
- The melanic allele rises by about two percentage points per generation at first. The rise speeds up as \(M\) becomes commoner, then slows again when \(m\) is rare and hides in heterozygotes.
Units check. Allele frequencies are ratios of allele counts, so they have no units, and \(p + q = 1\) at every step.
Three quick tests
- If the favoured allele is dominant, \(s = 0.1\) against the recessive homozygote and \(p_0 = 0.5\), what is \(p\) after one generation?
Answer
\(q_1 = \frac{0.5(1 - 0.05)}{1 - 0.1 \times 0.25} = \frac{0.475}{0.975} = 0.487\), so \(p_1 \approx 0.513\), a change of about 0.013. - Why is the reversibility of the peppered moth colour change important evidence for natural selection?
Answer
Reversibility shows that the change is a response to the environment, not a random drift or a one-time mutation. When the environment changed back, the population changed back, confirming that selection was the driving force. - In the finch example, if the heritability \(h^2\) is 0.8 and the selection differential \(S\) is 1.0 mm, what is the expected response to selection \(R\)?
Answer
Using \(R = h^2 S\), \(R = 0.8 \times 1.0 = 0.8\) mm. This is the expected change in the population mean beak depth.
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Lecture notes checked line by line by the editor; every number in the worked example was recomputed.
Concepts cited: T-017 · T-018 · Threads: evolution