Forbidden phenotypes and the limits of evolution

Abstract

Evolution has produced an astonishing array of organisms, but does it have limits and, if so, how are these overcome and how have they changed over the course of time? Here, I review models for describing and explaining existing diversity, and then explore parts of the evolutionary tree that remain empty. In an analysis of 32 forbidden states among eukaryotes, identified in major clades and in the three great habitat realms of water, land and air, I argue that no phenotypic constraint is absolute, that most constraints reflect a limited time–energy budget available to individual organisms, that natural selection is ultimately responsible for both imposing and overcoming constraints, including those normally ascribed to developmental patterns of construction and phylogenetic conservatism, and that increases in adaptive versatility in major clades together with accompanying new ecological opportunities have eliminated many constraints. Phenotypes that were inaccessible during the Early Palaeozoic era have evolved during later periods while very few adaptive states have disappeared. The filling of phenotypic space has proceeded cumulatively in three overlapping phases characterized by diversification at the biochemical, morphological and cultural levels.

Three‐Legged Locomotion and the Constraints on Limb Number: Why Tripeds Don’t Have a Leg to Stand On

“…[5] Despite tripedalism being relatively common and the potential for tripeds to be stable and energy-efficient, three-legged animals do not exist and they do not appear to have ever evolved in the past. Various constraints (e.g., energy, phylogenetic, functional, etc.)…”

“…A version of it adorns the coat of arms for the Isle of Man where it is represented by three legs, flexed at the knees and conjoined at the thighs, clad in medieval armor. [4,5] Such "forbidden" phenotypes may be due to various types of constraints (e.g., energy, phylogenetic, etc. In his groundbreaking work of science fiction literature "War of the Worlds," H. G. Wells described Martian invaders attacking Earth with massive, three-legged war machines called tripods that they had modeled after their own three-legged body plan.…”

“…In Chinese mythology, the Jin Chan (money frog) and Sanzuwu (sun crow) are three-legged versions of common fourlegged creatures. [5] The natural tendency is to focus our attention on those animals that we can observe in nature and ask questions about the features they possess. While human imagination is certainly able to conjure up the concept of three-legged creatures, there is a curious lack of any such animal in nature today or in the fossil record.…”

Hunting Darwin's Snark: which maps shall we use?

“…These contributions, along with those by Vermeij [15] and McGhee [16], will help to provide an important framework for future discussion. In defence of the general importance of evolutionary convergence, it may also be argued that the many dazzling examples, be they as different as the multiple inventions of carbonic anhydrase [17] or ant myrmechory [18], do more than provide an impressive catalogue raisonné and in at least two different ways contribute to the question of the limits of evolution.…”

Shell features associated with the sand-burying habit in gastropods

“…This method purportedly controls for history and clade membership as explanations for morphological similarities, but it suffers from the unwarranted assumption that clades are inherently constrained by historical legacies (Vermeij, 2015). In any case, the choice of phylogenetically independent clades with sandburying species obviates the necessity of resorting to this commonplace but questionable practice.…”