AP Biology Units 7–8 — selection and ecology

The last two units, and the ones that most reward being able to tell two similar-sounding explanations apart.

From natural selection: a Hardy-Weinberg calculation done from raw counts rather than from a given frequency, where the first step — that 125 out of 500 is q squared and not q — is where most of the marks go. Finches blown to an island, which is drift and not selection, because nothing about the island favoured the allele that became common. Homology against analogy, decided by embryonic origin rather than by shared function. A speciation case built to be confusing on purpose, since the populations are separated by mountains and then diverge in flowering time, and the rule that settles it is that speciation is named for what interrupted gene flow first. And moths, where three of the four answers are the three standard ways of getting evolution wrong: individuals changing, populations needing traits, and organisms choosing.

From ecology: energy down a food chain and why ten thousand kilocalories at the bottom leaves ten at the top. Rabbits levelling off at two hundred. Two birds competing for one insect, shown by what happens when one is removed. The nitrogen cycle. And a rocky shore.

That last one is the question worth the most. A predatory starfish is removed from one stretch of coast, and within two years the number of species falls from fifteen to eight. Two explanations fit: the starfish had been holding a mussel in check, or the water warmed and favoured that mussel anyway. Neither can be ruled out from that stretch alone, because you cannot watch a place do what it would otherwise have done. A comparable stretch, same water, starfish left alone, can decide it — and that is both the definition of a control and the reason a keystone species is called one.

Every species, shore and figure here is invented, and no finding is attributed to a real study or place. There are no images; every food web and growth curve is described in words.

Zestly is an independent study tool. It is not affiliated with the College Board, which owns the AP examinations, and it is not an exam centre.

• Carry out a Hardy-Weinberg calculation from counts, starting by identifying which quantity the data give you
• Distinguish genetic drift and the founder effect from selection by what the scenario describes
• Use developmental origin rather than function to establish homology
• Classify a speciation event by what first interrupted gene flow, not by the isolating mechanism that results
• Trace energy through trophic levels and explain what limits the length of a food chain
• Tell logistic from exponential growth and name the carrying capacity, and identify a species interaction from a removal experiment
• Design the comparison that separates a keystone predator's effect from a coincident environmental change

On an invented rocky shore, researchers remove a predatory starfish from one stretch of coast. Within two years the number of species living on that stretch falls from fifteen to eight, and a single mussel covers most of the rock. Two explanations are offered. First: the starfish had been holding the mussel's numbers down, and without it the mussel crowded the others out. Second: the removal happened to coincide with a warming of the water that favoured that mussel anyway. Which observation would best distinguish them?