Geoscience Reference
In-Depth Information
These ideas, while evidently reasonable, have spurred a massive amount
of debate, as well as a growing body of experimental studies. Nick But-
terfield (we met Nick in
chapter 10)
, forever the contrarian, has taken a
shot at the dragonflies. He feels that ecological considerations have
been a much stronger drive toward gigantism, with any assistance from
higher oxygen levels taking a secondary role. One cannot overstate the
importance of ecology in driving evolution, but Nick does not discuss
contemporaneous gigantism among flightless insect groups. These would
presumably be subject to different ecological constraints than the drag-
onflies. Therefore, is it simply fortuitous that a different set of ecological
interactions should also drive gigantism in the flightless insect groups,
or was another driver, like oxygen, in play?
There have also been a series of direct experiments on various insect
groups, and these have led to equivocal results. Much of this work has
been done by Jon Harrison and his group at Arizona State University.
hen Jon's group grew a number of different insect types under lower
levels of oxygen, they usually observed a reduction in body size, a reduc-
tion in growth rates, and lower rates of survival (we will take up this topic
again below). However, when grown at oxygen levels higher than those
of today, mixed results were obtained. In some cases, including ground-
dwelling beetles, fruit flies, and also dragonflies, body size increased at
high oxygen levels. For the giant mealworm,
Zophobus morio
, body size in-
creased at moderately elevated oxygen levels, but began to decrease again
as oxygen increased further. In many other species there was no effect.
To summarize, while the results were mixed, some of experiments
clearly showed increased body size at elevated oxygen levels. This result
was likely a direct physiological response to higher oxygen concentra-
tions, although larger organisms could also have been selected for in
experiments running multiple generations. Nevertheless, the time scales
of the experiment are probably too short to allow for the slow evolu-
tionary changes which likely resulted in true gigantism in the fossil re-
cord. Therefore, the jury is still out, but a causal relationship between
elevated oxygen levels and insect gigantism in the Carboniferous and
early Permian periods remains a plausible explanation.
Back to the models. Since our publication of the history of Pha-
nerozoic oxygen levels from rock abundance data, Bob has returned to
