Daphnia Spines After an Invasion
A class monitors a lake before and after an invasive predatory zooplankter is first detected. From 2019 to 2024, mean Daphnia body length falls from 1.4 mm to 0.9 mm, and the frequency of a long-spine morph rises from 10% to 65%. Water temperature also rises 1.8°C, and fish density doubles because a nearby dam was removed. The students collect Daphnia from 2019, from sediment eggs, and from 2024, raise both groups in identical lab conditions for 10 generations, and measure spine length: the 2019 line has mean spine 0.18 mm; the 2024 line has 0.31 mm. A student says the lake population 'grew spines because it needed to avoid being eaten,' a Lamarckian claim. Another says the change is just plasticity. No common-garden test of body size is reported.
1. Using the common-garden result, evaluate the two claims. What does the result support and what does it not establish?
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The common-garden result supports a genetic difference between the 2019 and 2024 populations. When both lines are raised in identical conditions for 10 generations, the 2024 line still has longer spines (0.31 vs 0.18 mm). Plasticity alone would not produce a stable difference after 10 generations in a common environment; the environment is the same, so the remaining difference is attributed to inherited changes. That is evolution, not an organism 'growing' a trait because it needs it. The Lamarckian claim is therefore rejected: selection acts on existing variation, and individuals do not direct their own mutations. However, the experiment does not establish that the invasive predator caused the selection. The field trend also includes warming and doubled fish density, and the common garden shows only that the populations differ genetically. It does not identify the selective agent, measure heritability precisely, or rule out maternal or epigenetic effects. A predator-exposure experiment or selection analysis would be needed for that.
2. Identify a confound in the field trend and explain how it could produce a similar pattern without selection by the invasive predator.
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Fish density doubled after the dam was removed, and fish are visual predators that preferentially eat larger, more conspicuous Daphnia. That could select for smaller body size and, if spines reduce handling, for longer spines, producing the same field pattern without the invasive zooplankter. Warming is another confound: higher temperature can alter development, generation time, and predator activity, and it can change spine expression plastically. The invasion also did not happen in isolation; the lake's fish community, water clarity, and nutrient inputs may all have changed. To attribute the shift to the invasive predator, the study would need to show that the timing of change matches the invasion, that the predator actually selects against the observed phenotypes, and that fish and temperature effects are accounted for. A common-garden experiment with and without predator cues, or a selection experiment, would help separate these agents. The field correlation alone is not enough.
3. What data would you collect to estimate the strength of selection on spine length in this lake? Explain how you would use the data.
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To estimate selection, measure individual Daphnia phenotype and fitness components in the field before and after the predator season. Record spine length and body length for many individuals, then estimate survival by recapture or cohort tracking and fecundity by counting eggs per female. A selection differential is the difference in mean spine length between survivors and the starting population; a selection gradient comes from regressing relative fitness on standardized spine length while controlling for body size. Ideally, also measure predator abundance and exposure, and use a common-garden or reciprocal-transplant design to separate genetic change from plasticity. If long-spined individuals survive better only when the invasive predator is present, that supports predator-driven selection. If selection is similar with fish or temperature, the cause is ambiguous. Repeated sampling across years and sites would strengthen inference, and measuring heritability would show whether the population can respond.