Yellowstone Lake, occupied by S. yellowstonensis, sits in a volcanic caldera, and is heavily influenced by superheated water carrying extraordinarily high levels of dissolved nutrients that diatoms need to grow, levels much higher than typical in other lakes occupied by S. niagarae. The soils of its basin though are composed mostly of obsidian sand (glass!!) which will cause very low levels of other nutrients. Altogether it is a very different lake than those typically having S. niagarae. Why is this important?

a. Such differences might create a different selective environment than those of most lakes occupied by S. niagarae, so that any S. niagarae dispersing into the lake could only survive by adapting and changing its traits.
b. Drift occurs quickly in stressed populations.
c. Both answers are correct.