Question 1 · 7.1 Introduction to Natural Selection
Biologists studying a population of dune beetles want one measurement that captures an individual's evolutionary fitness. Which measurement should they use?
- A The number of offspring it leaves that survive to breed
- B The total mass it reaches by the end of its second year
- C The number of days it survives after reaching adulthood
- D The number of rivals it displaces from feeding sites
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A — Evolutionary fitness is measured in descendants, because only alleles that reach the next generation change the makeup of a population. Size, lifespan, and dominance matter only to the extent that they translate into surviving offspring. A large, long-lived beetle that breeds once contributes less to the gene pool than a small one that breeds repeatedly, so the offspring count is the direct measure and the others are at best proxies.
Question 2 · 7.2 Natural Selection
A student writes that a cave fish lineage lost functional eyes in order to save the energy that eye tissue would waste in the dark. Which restatement best corrects the reasoning while keeping the same outcome?
- A Individual fish shut down eye development once they sense that the cave is dark
- B The cave environment produced the eye-reducing mutations that the lineage required
- C Variants with reduced eyes were already present and were favored in the dark
- D The lineage anticipated life in the cave and reduced its eye tissue in advance
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C — Natural selection has no goal and no foresight; it can only sort variation that already exists. Mutations degrading eye development occur in surface populations too, where they are removed because sighted fish do better. In a lightless cave that cost disappears and the energy saved becomes a small advantage, so the variants accumulate. The outcome looks purposeful in hindsight, but the causal order runs variation first, then selection, never need first.
Question 3 · 7.3 Artificial Selection
Domestic pigeon breeds differ more in beak shape, plumage and body size than the wild pigeon species of a whole continent do, yet any two domestic breeds interbreed and produce fertile young. What does this comparison illustrate?
- A Selection within a single gene pool can produce strikingly different forms
- B Artificial selection produces new species faster than natural selection does
- C Domestic breeds descend from several wild pigeon species crossed together
- D Physical difference between populations reliably measures reproductive isolation
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A — Every one of these breeds descends from one wild species, and the fertile crosses show that they still share a gene pool. What breeders did was partition the variation already segregating in that species and drive different fragments of it to fixation in different lines. The result is that visible difference can run far ahead of genetic divergence: dramatic changes in form are cheap for selection to produce, while the reproductive isolation that defines a species requires something else. Darwin leaned on exactly this contrast to argue that selection could reshape a lineage.