Darwin and Wallace (1858
; Darwin 1859
) proposed that natural selection acts on random variation in plants and animals to shape new species. Natural selection encompasses the environmental constraints acting on the organism, one of the most important being energy availability.
Life exists in a nonequilibrium thermodynamic state requiring the constant flow of energy to sustain its complex structures and to permit the accumulation and transmission of biological information. In the absence of energy flow, complex systems decay. Therefore, life exists through the interplay among structure, energy, and information.
The source of most biological energy is the Sun. The high-energy photons collected by photosynthetic cyanobacteria and their chloroplast descendants are used to split water into hydrogen and oxygen. The oxygen is released into the atmosphere and the hydrogen is used to reduce CO2 to generate glucose. Plant glucose is consumed by herbivores and the energy therein sequentially passed through the animal and fungal food chains. Ultimately, the degraded energy is dissipated as infrared radiation into space.
Animal populations grow and multiply until energy becomes limiting. Animals can adapt to available energy resources at three levels: interspecific exploitation of different energy reservoirs that define the species’ niche, intraspecific exploitation of differences in regional energy resources, and individual responses to oscillating environmental energy resources ().
Figure 1 Three hypothesized levels of animal eukaryotic cell adaptation to varying energy resources. The original eukaryotic symbiosis brought together the glycolytic nucleus-cytosol with the oxidative mitochondrion. Most of the mitochondrial genome was then transferred (more ...)
Interspecific energy resource adaptation involves exploitation of distinct energy reservoirs through anatomical variation. This helps to delineate species and genera. Energy reservoirs are the distinct source of calories used by a species throughout its existence, perhaps over hundreds of thousands of years. Factors that can define an energy reservoir include being an herbivore versus a carnivore. For herbivores, these could involve eating fruits versus nuts or leaves versus sap. For carnivores, they could encompass being a predator versus a scavenger or consuming blood versus bugs, etc. To exploit alternative energy reservoirs, selection acts on nDNA genetic variation to alter animal anatomical traits, permitting access to novel energy reservoirs. This interaction between the species’ anatomy and the energy reservoir helps to define the species’ niche ().
Intraspecific energy resource adaptation involves the exploitation of regional energy environments through changes in physiology. Within a species’ niche, regional energy resources can vary in energy type (carbohydrate, fat, protein), amount, and demand (activity level, stress, etc.). These regional differences can be stable over thousands of years and lead to development of distinct geographically constrained populations within the species. To exploit different regional energy environments, genetic variants in mtDNA and nDNA bioenergetic genes are selected that adjust the bioenergetic metabolism to be in balance with local energy resources ().
Figure 2 Bioenergetic interface with the environment explains the importance of mtDNA and epigenomic variation of intraspecific animal adaptation. Energy availability and demand are the central factors in an animal’s environment, the energy environment. (more ...)
Individual energy resource adaptation involves adjustments within the individual to short-term oscillations in energy availability and demand. These changes can occur within days to years and include seasonal changes and inflammation. Adjustment to these changes is accomplished through epigenomic modulation of nDNA and mtDNA bioenergetic gene expression ().
These three levels of energy adaptation strategy result from the fact that nDNA has a low mutation rate and encodes all of the genes for determining anatomy. Hence, changes in nDNA genes will define the species’ structure and thus the energy reservoir that it can exploit. In contrast, the mtDNA has a very high mutation rate and encodes the central mitochondrial bioenergetic genes, augmented by hundreds of nDNA bioenergetic genes. Hence, mtDNA plus nDNA variation in bioenergetic genes has a dominant role in adapting to regional energy environments. Finally, the bioenergetic genes of mtDNA and nDNA must be coordinately regulated in response to environmental energetic oscillations. This is accomplished by modification of the epigenome, mediated by high-energy intermediates generated by the flow of calories through cellular bioenergetic systems ( and ).
Interspecific variation leading to species, genera, families, orders, and classes is therefore the product of genetic variation in anatomical genes of nDNA. Alternatively, intraspecific variation resulting in population and individual adaptation to energy environments is dominated by mutations in bioenergetic genes, particularly those of the mtDNA, plus alterations in bioenergetic gene regulation through modifications of the epigenome ().
Since Darwin and Wallace proposed natural selection, considerable attention has been focused on changes in the genes and processes that dictate animal structures. However, little consideration has been given to the role of energy flow through the biosphere. Yet it is the flow of energy that drives the generation of complexity, the accumulation of information, and animal radiation. Indeed, it is precisely the intraspecific adaptation to regional energy resources that determines whether a population will survive, reproduce, and radiate.
The role of intraspecific energy adaptation is particularly pertinent for medicine, which is concerned with the health and well being of a single species, Homo sapiens. Therefore, genetic and epigenetic adaptations affecting energy metabolism are more likely to mediate environmental interactions than are changes in anatomy. Hence, the strong anatomical and nDNA focus of Western medicine may account for the reason that it has been so difficult to understand genetics and environmental inter actions associated with common “complex diseases” that have a bioenergetic component to their etiology ().