Constructive neutral evolution

Constructive neutral evolution (CNE) is a theory that seeks to explain how complex systems can evolve through neutral transitions and spread through a population by chance fixation (genetic drift).^[1] Constructive neutral evolution is a competitor for both adaptationist explanations for the emergence of complex traits and hypotheses positing that a complex trait emerged as a response to a deleterious development in an organism.^[2] Constructive neutral evolution often leads to irreversible or "irremediable" complexity^[3] and produces systems which, instead of being finely adapted for performing a task, represent an excess complexity that has been described with terms such as "runaway bureaucracy" or even a "Rube Goldberg machine".^[4]

The groundworks for the concept of CNE were laid by two papers in the 1990s, although first explicitly proposed by Arlin Stoltzfus in 1999.^[1][2][5] The first proposals for the role CNE was in the evolutionary origins of complex macromolecular machines such as the spliceosome, RNA editing machinery, supernumerary ribosomal proteins, chaperones, and more.^[4][6][7] Since then and as an emerging trend of studies in molecular evolution,^[8] CNE has been applied to broader features of biology and evolutionary history including some models of eukaryogenesis, the emergence of complex interdependence in microbial communities, and de novo formation of functional elements from non-functional transcripts of junk DNA.^[9][10] Several approaches propose a combination of neutral and adaptive contributions in the evolutionary origins of various traits.^[11]

Many evolutionary biologists posit that CNE must be the null hypothesis when explaining the emergence of complex systems to avoid assuming that a trait arose for an adaptive benefit. A trait may have arisen neutrally, even if later co-opted for another function. This approach stresses the need for rigorous demonstrations of adaptive explanations when describing the emergence of traits. This avoids the "adaptationist fallacy" which assumes that all traits emerge because they are adaptively favoured by natural selection.^[9][12]