MINELLI, A.; Univ. of Padova: Conserved vs. innovative features in animal body organization

A search for the origin of structural innovations is better placed within evolutionary developmental biology rather than in the study of phylogeny or adaptation. Cladistics provides methods for distinguishing plesiomorphic from apomorphic features, but the apomorphies most reliably used in cladistic reconstruction rarely deserve the title of significant innovative features. In addition, extensive convergence can affect the origin of structurally important body features, especially to the extent that these originate from generic physicochemical properties of the living matter rather than from specific genetic changes. As for adaptation, this provides plausible accounts of the spread of traits with which the organism is already provided. Evolutionary developmental biology shows that larval or juvenile stages are often much more plastic and innovative than the corresponding adults. Larval and juvenile novelties are thus more productively considered with respect to the stage where they occur than as preparatory to the adult. Lineages typically acquire fundamental changes through the divergence of duplicates (paramorphism), or through the functional replacement of organ A by organ B under suitable selective regimes. Individual body features may be subject to functional as well as developmental constraints. Function helps understanding why no insect flies with more than four wings, development should eventually explain why no adult centipede walks with an even number of legs. There seems to be an upper limit to the degree of complexity an animal may develop along any of its body axes, but evolutionary bursts of morphological innovation may accompany the origin of new morphological axes by paramorphism, as with the vertebrate and arthropod appendages.