What is Hierarchy Theory of evolution?
Biological systems are arranged hierarchically, with smaller units forming the components of larger systems: trees in a forest, cells in a body, organisms in a population, trophic groups in ecosystems, genes within chromosomes, within cells, within organs, within organisms, within populations, within species. Hierarchy Theory of evolution is a theory of how biological hierarchies are organized, how they function, and how evolution takes place through them over time.
Proposed by eminent paleontologist and evolutionist Niles Eldredge since the 1980’s, the Hierarchy Theory provides a unifying approach to represent the complex multi-level structure of the organic world and an explanatory framework for a wide range of natural phenomena. This theory identifies two different hierarchies: one is named ‘economic’ or ‘ecological’, and it is based on matter-energy transfers and ecological processes. The other is named ‘genealogical’ or ‘evolutionary’, and it is based on information and replication. The two hierarchies interact in complex ways, and it is their interaction that brings about the patterns of evolution at all scales. According to the Niles Eldredge’s fundamental intuition, i.e. the so-called Sloshing Bucket model, there is a direct correlation between the magnitude of disturbance and its evolutionary consequences: the higher the level of perturbation in the economic hierarchy, the higher the level of the genealogical hierarchy at which the evolutionary pattern of change in diversity and disparity will be recorded.
Eldredge’s theory has given rise to a network of scholars informally known as “The Hierarchy Group”, which comprises scientists, philosophers of science and historians of science (see section People) focused on its development and extension. Hierarchy Theory is now at a crucial point. On the one hand, it needs to be systematized, because evolutionary thinking has not been assimilating its crucial intuitions that would solve or at least reframe many long-standing theoretical problems. Think, for example, to the endless debate on multi-level selection. On the other hand, much work is still to be done on clarifying the two hierarchies: their nature, their general organization principles, the levels which compose them, all the processes that take place in their interaction, and their sufficiency in accounting for evolution (see section Aims). According to Niles Eldredge himself, future directions towards a more advanced Hierarchy Theory should be, firstly, information theory and biological networks modelling and, secondly, evolutionary biology from an empirical and interdisciplinary point of view (e.g. macro-evolutionary patterns, integrated phylogenies, eco-evo-devo, niche construction).
In the wake of these new perspectives and developments, future research will also focus on the several applications made possible by the hierarchy approach to different evolutionary patterns and processes, from the research in genetic evolutionary fields, up to the macro-evolutionary patterns and processes, including human evolution field. Hierarchy Theory is a non-reductionist way to see evolutionary changes at different levels.
As evolutionary biology advances, work on Hierarchy Theory must become a priority: while the theory needs constant integration as biology proceeds, biology needs the theory — the elephant in the room — the 30-years old Hierarchy Theory of evolution.
Eldredge’s Sloshing Bucket model of evolutionary stability and change