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Courses in Complexity


Universal Scaling Laws in Biology from Genomes to Ecosystems; Towards a Quantitative Theory of Biological Structure and Organization
By Geoffrey West
23rd January 2007

Abstract: Life is the most complex physical phenomenon in the Universe manifesting an extraordinary diversity of form and function over an enormous scale from the largest animals and plants to the smallest microbes and sub-cellular units. Yet, many of its most fundamental and complex phenomena scale with size in a surprisingly simple fashion. For example, metabolic rate scales as the 3/4-power of mass over 27 orders of magnitude from molecular and intra-cellular levels up to the largest organisms. Similarly, time-scales (such as lifespans and growth-rates) and sizes (such as bacterial genome lengths, tree heights and mitochondrial densities) scale with exponents which are typically simple powers of 1/4.

The universality and simplicity of these relationships suggest that fundamental universal principles underly much of the coarse-grained generic structure and organisation of living systems. We have proposed a set of principles based on the observation that almost all life is sustained by hierarchical branching networks, which are assumed to be space-filling and optimised by natural selection. It will be shown how these explain quarter power scaling and lead to a general quantitative, predictive theory that captures many of the essential features of many diverse biological systems.

Examples will include animal circulatory systems, plant vascular systems, growth, cancer, aging and mortality, sleep, cell size and genome lengths, mitochondrial densities, DNA nucleotide substitution rates and the concept of a universal molecular clock. Thermodynamic considerations, dimensionality and the role of invariants will be discussed.