Alex Gunderson asked the question: What ecological axes are involved in ecological divergence during adaptive radiation, and what phenotypic traits occur along them?
Here are the specific questions he investigated:
Alex pointed out that we usually talk about the ecomorphs that have diverged to use different parts of the structural habitat (e.g., twigs, canopy, grass, etc.), but less attenion is paid to divergence along the thermal niche axis, yet since Ruibal’s work in the early 1960’s, we’ve known such divergence occurs. Morphological divergence allows species to coexist by using different structural microhabitats; does divergence in thermal physiology have the same effect?
Most of the research involved the Puerto Rican cristatellus group, in which there are four pairs of sister species that differ in thermal environment, one more in the sun, one more a shade species. Some data also included Jamaican anoles. The study focused on two aspects of physiology related to thermal niche use, the critical thermal maximum temperature (CtMax) and the optimal temperature for sprint performance (Topt).
Results: In 3 of 4 sister taxa, the species in the warmer environment had a higher CtMax. In 2 of 4, the species had a higher optimal temperature (in both cases, in the other comparisons, the species did not differ statistically).
Q2: What are the performance consequences of physiological divergence?
Alex measured temperatures in shaded and open habitats and asked what the risk was of a species overheating in each habitat. In shaded habitats, no species were at risk of overheating, but in open habitats, for three pairs of sister taxa, the species from the cooler environment was at greater risk of lethal overheating.
Q3: Does physiological divergence promote species co-occurrence?
In cases where morphologically similar species co-occur (same ecomorph), do they diverge in physiology? The answer: Invariably yes in Puerto Rico and Jamaica. When morphologically similar species co-occur, they always differ in thermal physiology. Thus, thermal physiological differentiation seems to be important for increasing local species richness.
Q4: How quickly does physiology evolve relative to morphology?
Surprisingly (at least to me), physiology evolves considerably more slowly than morphlogy.
Summarizing across this work, Alex concluded that physiological differentiation may be an important component of adaptive radiation. In many cases, workers studying adaptive radiation focus on morphology for a number of reasons, not the least of which that it is much easier to measure. But, by doing so, they may be missing an important part of the puzzle.
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How do CTmax and Topt relate to each other? Are there tradeoffs between being able to perform maximally at the average (or modal) environmental temperature and survive the maximum temperatures?