Figure 4.

Three hypothesized levels of animal adaptation to energy resources and demands. The primary contributor to the biological environment is the flux of energy through the biosphere. The dichotomy between structure and energy in eukaryotes results from the symbiotic origin of the eukaryotic cell involving the proto-mitochondrion and the proto-nucleus–cytosol. The mitochondrion is specialized in energy production and retains the core genes for controlling energy production within the mtDNA. The nucleus–cytosol is specialized in encoding structure. Because growth and reproduction must be coordinated with the availability of energy, the status of the energetic flux through cellular bioenergetic systems, particularly the mitochondrion, is communicated to the nucleus–cytosol by alterations in nDNA chromatin, the epigenome, and cytosolic signal transduction systems, based on the production and availability of high-energy intermediates, reducing equivalents, and ROS produced primarily by the mitochondrion. As a consequence, biological systems interface with the energy environment at three levels: the species level, at which nDNA gene variation alters anatomical forms to exploit different environmental energy reservoirs; the species subpopulation level, at which primarily mtDNA bioenergetic genetic variation permits adaptation to long-term regional differences in the energetic environment of the species' niche; and the individual level, at which the epigenome and signal transduction pathways are modulated by the availability of high-energy intermediates in response to cyclic fluctuations in the individual's energy environment (Wallace 2009, 2010). (Reprinted, with permission, from Cold Spring Harbor Laboratory and Proceedings of the National Academy of Sciences.)