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 pond is colonized by a single water flea that reproduces asexually, so every flea in the resulting population carries the same genotype. Over the next three summers, a parasite kills about 40 percent of the fleas each summer, and no new mutations affecting parasite resistance arise. Which statement best explains why the population's susceptibility to the parasite stays the same across the three summers?
- A Selection needs many more summers of parasite attack before any response can appear
- B A parasite that kills 40 percent of the fleas is too weak to act as a selective pressure
- C Asexual organisms lie outside the reach of evolution by natural selection altogether
- D Genetically identical fleas give selection no heritable differences to sort among
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D — Natural selection changes a population only by letting some heritable variants reproduce more than others. Here every flea has the same genotype, so the fleas that die and the fleas that survive differ only by chance, and the survivors' offspring are genetically identical to the population they came from. Killing 40 percent of a population each summer is an intense pressure, and selection can act within a single generation when variation exists, so neither weak pressure nor too little time explains the result. Asexual populations do evolve by selection whenever they carry genetic variation; this one simply has none to sort.
Question 3 · 7.3 Artificial Selection
Three groups of small fish live in fast mountain streams on three continents. DNA places each group in a different order of fish, and the three orders last shared an ancestor about 100 million years ago. Which explanation best accounts for the pattern shown?
- A The flat body and disc came down to all three from one stream-dwelling ancestor
- B The flat body and disc formed in each fish during life as currents pressed on it
- C The flat body and disc show that P, Q and R are one another's closest relatives
- D The flat body and disc arose separately in each lineage under swift water
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D — Each group's closest relatives are deep-bodied still-water fish without a disc, so the common ancestor of the three almost certainly lacked these features. The simplest history is that the flat body and sucker disc evolved independently three times, because swift current favored fish able to cling to rocks. That is convergent evolution, and the traits are analogous. The tempting reading, that one stream-dwelling ancestor passed the traits to all three, would require the traits to have been lost in every one of the three relative groups. Similar appearance is misleading here: the DNA places the groups in different orders.