Abstract
Several mutually incompatible theories exist about how and why endothermy evolved in mammals and birds. Some take the primary function to have been thermoregulation, selected for one adaptive purpose or another. Others take the high aerobic metabolic rate to have been primary. None of these theories is incontrovertibly supported by evidence, either from the fossil record of the synapsid amniotes or from observations and experiments on modern organisms. Furthermore, all are underpinned by the tacit assumption that endothermy must have evolved in a stepwise pattern, with an initial adaptive function followed only later by the addition of further functions. It is argued that this assumption is unrealistic and that the evolution of endothermy can be explained by the correlated progression model. Each structure and function associated with endothermy evolved a small increment at a time, in loose linkage with all the others evolving similarly. The result is that the sequence of organisms maintained functional integration throughout, and no one of the functions of endothermy was ever paramount over the others. The correlated progression model is tested by the nature of the integration between the parts as seen in living mammals, by computer simulations of the evolution of complex, multifunctional, multifactorial biological systems, and by reference to the synapsid fossil record, which is fully compatible with the model. There are several potentially important implications to be drawn from this example concerning the study of the evolution of complex structure and the new higher taxa that manifest it.
Highlights
Nothing is more fundamental for understanding the biological nature of birds and mammals than their endothermic temperature physiology: for organisms to pay the price of some ten-fold increase in their daily food requirements it must be balanced by a very substantial benefit to their ultimate reproductive potential
Typical endothermic temperature physiology of mammals is characterized by four measurable aspects: 1. The basal or resting metabolic rate (BMR) is high
Over the course of the evolution of new complex structure, including the broad biological reorganization associated with the origin of a new higher taxon, all the structures and associated functions must evolve by respective sequences of small steps in loose correlation with each other in order to maintain continuous functional integration
Summary
Nothing is more fundamental for understanding the biological nature of birds and mammals than their endothermic temperature physiology: for organisms to pay the price of some ten-fold increase in their daily food requirements it must be balanced by a very substantial benefit to their ultimate reproductive potential. There is relatively little fossil evidence available that bears helpfully on the details of the origin of the avian version of endothermy (Ruben, 1995; Schweitzer & Marshall, 2001), beyond. Lee & Doughty, 1997; Budd, 1998), but largely ignored in practice when discussing major evolutionary transitions to new higher taxa. Before taking up this issue, the biological nature of endothermy must be clarified, and the menu of current theories purporting to explain its origin in mammals briefly reviewed
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