Martin Wikelski and L. Michael Romero (Body size, performance and fitness in Galápagos marine iguanas, Integrative and Comparative Biology 43 [2003]:376-86) measured the snout-to-vent (anus) length of Galápagos marine iguanas and observed the percent survival of different-sized animals, all of the same age. The graph shows the log snout-vent length (SVL, a measure of overall body size) plotted against the percent survival of these different size classes for males and females.
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Examine the figure. What type of selection for body size appears to be occurring in these marine iguanas?
A) directional selection
B) stabilising selection
C) disruptive selection
D) You cannot determine the type of selection from the above information.

Question

Martin Wikelski and L. Michael Romero (Body size, performance and fitness in Galápagos marine iguanas, Integrative and Comparative Biology 43 [2003]:376-86) measured the snout-to-vent (anus) length of Galápagos marine iguanas and observed the percent survival of different-sized animals, all of the same age. The graph shows the log snout-vent length (SVL, a measure of overall body size) plotted against the percent survival of these different size classes for males and females. 
-
Examine the figure. What type of selection for body size appears to be occurring in these marine iguanas?
A) directional selection 
B) stabilising selection 
C) disruptive selection 
D) You cannot determine the type of selection from the above information.

Quizplus · Accepted Answer

The graph shows a curve that is relatively flat in the middle and steep on each end, indicating that intermediate body sizes have the highest survival rates while very large and very small individuals have lower survival rates. This is characteristic of stabilizing selection, which favors intermediate phenotypes and reduces variation in the population.