Typology now: homology and developmental constraints explain evolvability

Abstract

By linking the concepts of homology and morphological organization to evolvability, this paper attempts to 1) bridge the gap between developmental and phylogenetic approaches to homology and to 2) show that developmental constraints and natural selection are compatible and in fact complementary. I conceive of a homologue as a unit of morphological evolvability, i.e., as a part of an organism that can exhibit heritable phenotypic variation independently of the organism's other homologues. An account of homology therefore consists in explaining how an organism's developmental constitution results in different homologues/characters as units that can evolve independently of each other. The explanans of an account of homology is developmental, yet the very explanandum is an evolutionary phenomenon: evolvability in a character-by-character fashion, which manifests itself in phylogenetic patterns as recognized by phylogenetic approaches to homology. While developmental constraints and selection have often been viewed as antagonistic forces, I argue that both are complementary as they concern different parts of the evolutionary process. Developmental constraints, conceived of as the presence of the same set of homologues across phenotypic change, pertain to how heritable variation can be generated in the first place (evolvability), while natural selection operates subsequently on the produced variation. TYPOLOGY NOW3

Neural correlates without reduction: the case of the critical period

“…5 But this principle, which has been endorsed by some philosophers, is not widely accepted in the sciences, especially the special sciences where individuation according to relational or extrinsic properties, including historical or etiological properties, is commonplace. Phylogenetic taxonomy is largely etiological and biological categories are often characterized by "homology thinking," which is based on historical descent or etiology (Ereshefsky 2007;Brigandt 2007). Moreover, there are other bona fide scientific kinds outside biology that are distinguished, at least in part, on the basis of relational or etiological properties (e.g.…”

The ontology of organisms: Mechanistic modules or patterned processes?

“…These modular sub-systems might be considered central (in the aforementioned sense) to an evo-devo ontology on account of their being ontogenically explanatory with respect to the most contemporarily important "units" of evolutionhomologues, discrete morphological features whose broad phenotypic similarity among their various instances is underwritten by shared structural-cum-causal developmental mechanisms which exhibit a traceable phylogenetic lineage. 2 For if the evolutionary process can be conceived as the successive propagation and progression of homologue variation and canalisation, then these highly integrated genetic regulatory networks (GRNs) embedded in the "bottleneck" of ontogenesis (Galis & Metz 2001;Kalinka et al 2010), in virtue of their exerting downstream spatial and temporal regulatory control via the production of transcription factors whose patterns of expression causally specify the particularised developmental pathways of those morphological structures, are surely prime candidates for being the "real players" in an adequate evo-devo ontology (Brandon 1999;Brigandt 2007;McCune & Schimenti 2012;Wagner 2014).…”

Evolutionary Developmental Biology and the Limits of Philosophical Accounts of Mechanistic Explanation

“…The reverse situation is that some traits are uncorrelated and thus one can be modified by natural selection without impacting other traits and diminishing their fitness contribution. Even traits on different levels of organismal organization, such as developmental processes and morphological structures, can evolve independently of each other (Brigandt 2007). It is the particular mechanism of development that explains how among organisms of a species functional morphological variation can be generated, how complex traits can change in an integrated fashion, and how some traits can vary and evolve independently of each other.…”

“…Despite the presence of such close functional and developmental connections among levels, features on different levels can evolve independently of each other. There are many instances in which a gene is involved in different developmental pathways and the formation of different morphological structures in different species, and, conversely, where the same, homologous structure develops by means of different developmental processes, from different tissues, or by the involvement of different genes in different species (Brigandt 2007;Brigandt and Griffiths 2007;Wagner and Misof 1993). A case in point is digit identity in the bird forelimb.…”