Discussion
Muscle performance is positively correlated with Darwinian fitness. The graph below describes the relationship between mean environmental temperature and muscle performance for two closely related lizard species, one living in temperate areas (solid line) and another living in tropical areas (dashed line). Current mean environmental temperatures are marked with vertical dotted lines at (i) for the temperate species and (ii) for the tropical species. If mean environmental temperatures at both locations increase at a steady rate over time, which one of the following is most likely to occur in the near future?
Answer: B
The likely outcome is that the temperate species may be positively impacted but the tropical species will be negatively impacted.
The temperate lizard's current temperature, mark (i), lies well below the peak of its broad performance curve. As temperatures rise it moves up the curve toward its optimum, so its muscle performance, and with it its fitness, may increase.
The tropical lizard's current temperature, mark (ii), is already at the peak of its narrow curve, and just beyond it the curve drops steeply. Any warming pushes it down that slope, so its performance falls sharply.
The two species therefore do not suffer to the same extent, the effects are not reversed, and saying neither is affected ignores the curves. This is the general point that tropical ectotherms, living near their thermal maximum, are the more vulnerable to warming.
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