The background selection of metabolism explains allometric outliers like shrews and bowhead whales
The number of generations through which a given lineage is selected is an essential factor that affects the current evolutionary stage of the species. This is maybe best illustrated by species that evolve a small or large body mass at an early stage relative to the other species in a clade. These species will evolve over a larger or smaller number of generations that the main clade and, thus, they will have a tendency to evolve a higher or lower metabolic rate.
This evolution may explain shrews (
These outliers from typical Kleiber scaling do not reflect adaptations in a traditional sense. Instead, they reflect differences in natural selection time; where each lineage has its own time-scale that is defined by the number of generations it evolves.
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