SAT Subject Biology Practice Question 349: Answer and Explanation
19. The following graphs show the growth of two closely related species of paramecia, both when grown alone (Figure 1) and when grown together (Figure 2). Both species consume bacteria as their food source and reproduce by binary fission as often as several times a day.
The data in Figure 2 indicate that
A. P. aurelia is preying on P. caudata
B. P. aurelia is a better competitor than P. caudata
C. P. aurelia and P. caudata are in a symbiotic relationship
D. P. aurelia is a parasite of P. caudata
E. P. aurelia grew better when combined with P. caudata than it did when grown alone
Correct Answer: B
B Clearly P. aurelia can compete better and get more food that P. caudata; thus it will grow while P. caudata is competed to extinction. Choice A is highly unlikely, because the food source the paramecia prefer is bacteria, not each other. This is not a symbiotic relationship but a competitive one (C and D are eliminated), and the data contradict choice E.