IB Biology 2 / IHS
Speciation Group Think Problems KEY
1. Identify each of the following types of species-isolating reproductive barriers (see Neil 9e Ch. 24 pp.
490491)
a) Acorns from Scarlet Oaks germinate on wet soil; acorns from Black Oaks prefer dry soil. Prezygotic, habitat (geographic)
b) A horse and a donkey crossbreed, but their offspring (a mule) is sterile. Postzygotic, reduced hybrid fertility
c) Three species of the orchid Dendrobium from the same area of the rain forest flower on different days. Prezygotic,
temporal
d) A male firefly signals to a female of his species by blinking his lights in a characteristic pattern. Prezygotic, behavioral
e) Brown trout breed in the spring; Rainbow trout breed in the Fall. Prezygotic, temporal
For questions 2-4: Identify the probable geographic barrier that existed during allopatric speciation,
and suggest a plausible reproductive barrier that developed while the populations were isolated.
3. Tree squirrel speciation – Probable geographic barrier: the carving out of the Grand Canyon by a river.
4. Pupfish speciation: Probable geographic barrier: the separation of the pools by drying out.
5. Tuolumne snail speciation: Probable geographic barrier: river changing course, carving up the limestone in the Tuolumne
River canyon; the west side of the river happens to have limestone with more deep cracks so a flat-shelled snail could hide
better there.
***
5. Kind of speciation to create O. gigas: sympatric. It couldn’t interbreed with O. lamarckiana because O. gigas’ gametes would
contain 14 chromosomes and those of O. lamarckiana would contain 7. During meiosis 14 chromosomes can’t pair with 7!
6. Why are these marine species on either side of the isthmus of Panama so similar-looking?
1) The environments on either side of the isthmus may be extremely similar, and the fishes,
echinoderms
and crabs might resemble each other as a result of convergent evolution, OR
2) The isthmus geographically isolated fish, echinoderm and crab populations from one another, and over
time they diverged to the point of being called separate species. But they would expect to be genetically very closely
related.
7. For fun: the sterile offspring of a male zebra and a female donkey is a ZONKEY!
Questions 8-11 go with the family tree of horses on pp 530-531 of Neil 9e.
8.
From Hyracotherium to Equus, horses’ bodies became larger and more “leggy,” there was a reduction in the number of toes,
and their grinding teeth (molars) became larger, flatter, and had more ridges.
9.
Since the world of horses was becoming a drier place, the changes in # 8 make sense: less forest, fewer places to hide, more
need to outrun your predators – being bigger, with a hard toe, allows horses to do that; also, big ridged molars enabled
horses to graze on grass when they used to browse on leaves.
10. There is only one Hyracotherium descendant alive today!!! All horse-like critters on Earth today (horse, zebra, donkey, etc.)
belong to genus Equus.
11. An example of a derived character in Equus might be: having just one toe.