A flash flood sweeps 31 of the 40 tree frogs in an isolated pond downstream. The frogs lost were of every size and call type; the 9 survivors happened to be under one log. The new generation's allele frequencies differ sharply. Which mechanism did this?

Two wildflower patches of about 200 plants each grow 300 meters apart. Bees now carry pollen between them. A white-petal allele that existed only in the first patch begins appearing in seedlings of the second patch. Which mechanism moved it?

A culture holds about 5 million bacteria. In one cell a single base is copied incorrectly during DNA replication, and that cell's descendants carry a form of the gene found nowhere else in the culture. What produced this new allele?

In a herd of 250 goats, sequencing shows every allele for coat length and horn shape was already present in the parents. Yet many kids carry pairings of those alleles that neither parent had. Which mechanism supplied this variation?

An island shrew population holds 25 adults. In one poor year only 6 of them happen to find mates, for reasons unrelated to size, color or health. In the next generation several alleles are far more common and one is gone. Which mechanism is this?

Ten wolves cross winter ice from the mainland and join an island pack of 60 wolves, where they breed for several generations. The island pack's allele frequencies shift steadily toward those of the mainland population. What best explains the shift?

A storm blows 8 finches from a mainland flock of thousands onto an uninhabited island, where their descendants now number 4,000. The island birds carry only a fraction of the mainland flock's alleles. Which term names what happened?

Northern elephant seals were hunted in the 1890s until fewer than about 100 animals were left. Protection allowed the species to recover to over 100,000, but the modern population shows extremely low genetic variation. Which term describes this history?

What is the difference between a bottleneck and a founder effect?

Which situation describes a founder effect rather than a bottleneck?

A lab colony of 300 fruit flies loses all but 6 flies when an incubator fails overnight. The colony is bred back up to 300 within a year. Which term names what the crash did to the colony?

Two beetle populations have been separated for centuries. Population A carries a dark-color allele at 85 percent and population B at 12 percent. A cleared corridor joins the habitats and the beetles now interbreed. What happens over time?

A disease reduces a population of 4,000 prairie chickens to 12 birds before the population slowly recovers. What is the most likely effect on the gene pool?

Two populations of one lizard live on neighboring islands, one of 20 lizards and one of 2,000. Both start with a striped-tail allele at 50 percent, and it affects survival in neither place. Where will the frequency swing more over 10 generations?

A new allele appears by mutation in one individual of a population of 10,000 moths. The allele neither helps nor harms the moth. Just after it appears, what is true of the gene pool?

A pond population of 100 fish has been isolated for decades and its allele frequencies vary little. A flood connects the pond to a large river, and a few river fish arrive and breed there each year. What happens to the pond population's genetic diversity?

In a population of 4,000 beetles, spot pattern has never been linked to survival or mating success at the site. The frequency of spotted beetles has wandered between 30 and 38 percent over 20 generations with no consistent direction. What explains this?

On a dark lava flow, birds eat pale beetles far more often than dark ones. Over 15 generations the dark allele rises from 25 percent to 70 percent in a population of 3,000 beetles. Which mechanism is at work?

In a small population, an allele can become more common over several generations purely by chance, even when it gives its carriers no survival or reproductive advantage.

Why do biologists treat genetic drift as something other than natural selection?