American toads and Fowler's toads breed in the same ponds. American toads call and lay eggs in early spring, while Fowler's toads call and lay eggs in late spring and early summer. Which reproductive barrier separates them?

Eastern and western meadowlarks share grasslands in the central United States and look almost identical. A female of each kind responds only to the song of a male of her own kind. Which reproductive barrier separates them?

In two land snail populations, one has shells that coil to the right and the other has shells that coil to the left. A pair with opposite coiling cannot line their bodies up to transfer sperm. Which reproductive barrier separates them?

Two sea urchin species release eggs and sperm into the same water at the same time. A protein on the sperm surface attaches only to eggs of its own species, so sperm from the other species never fertilizes an egg. Which barrier is this?

Closely related snapping shrimp live on opposite coasts of Central America, one group in the Caribbean and the other in the Pacific. The Isthmus of Panama rose between them about three million years ago. Which barrier separated them?

Two garter snakes live in the same California county. One spends nearly all its time in and beside streams, and the other stays on dry ground, so adults of the two almost never meet. Which reproductive barrier separates them?

The Colorado River cut the Grand Canyon and left one antelope squirrel population on the north rim and another on the south rim. The two rims now hold different species. How did this split begin?

In the 1860s some apple maggot flies in New York began laying eggs in introduced apples instead of native hawthorns. Apples ripen earlier, so the two groups now mate at different times in the same orchards. How did this split begin?

In eastern Washington a new goatsbeard plant was first found in 1949. It formed when two introduced goatsbeard species crossed and the hybrid doubled its chromosome number. It breeds with itself but not with either parent. How did it arise?

Ground finches reached separate islands in the Galapagos, and each island population came to have a different beak and a different diet. Birds from different islands now rarely breed together. How did these species arise?

In one bay of Lake Victoria two closely related cichlids live side by side with nothing dividing the water. Females of each choose mates by the male's breeding color, red or blue, and the two kinds rarely cross. How did this split begin?

A horse and a donkey are bred together. The cross produces a healthy mule, but mules themselves almost never produce offspring. Under the biological species concept, what do these results show?

The Kaibab squirrel lives only on the North Rim of the Grand Canyon and Abert's squirrel only on the South Rim, and their coat colors differ. Suppose the two were brought together and produced fertile young. What would that show?

Two damselfly populations breed along the same stream. Over five years biologists identified 2,000 adults and found no hybrids, and pairings set up in the laboratory never produced eggs. What do these data indicate?

Biologists cross two salamander populations in the laboratory. The eggs are fertilized and the embryos begin to develop, but every hybrid embryo dies before hatching. What does this result indicate?

Enos Lake held two stickleback forms that bred separately, one feeding in open water and one on the bottom. After a crayfish was introduced and the water clouded, the two interbred and only mixed fish remain. What has happened?

Nutrient pollution has clouded parts of Lake Victoria so that females can no longer tell a red male from a blue male. Cichlids that once bred separately by color now interbreed freely. What is happening to these populations?

Two beetle populations were separated by a glacier for 40,000 years and built up different alleles. The glacier melted, the beetles met again, and they now interbreed freely and produce fertile offspring. What has happened?

Two fly populations that were separated for a long time meet again. They can still mate, but their hybrid offspring survive poorly. Over the next several thousand generations, what is most likely to happen to their courtship differences?

A canyon has divided one lizard population into two groups that no longer exchange genes. What must happen before biologists would call the two groups separate species?