Biological Systematics in the Evo-Devo era

Alessandro Minelli

doi:10.5852/ejt.2015.125

Abstract

Evolutionary developmental biology (evo-devo) suggests a distinction between modular and systemic variation. In the case of modular change, the conservation of the overall structure helps recognizing affinities, while a single, fast evolving module is likely to produce a bonanza for the taxonomist, while systemic changes produce strongly deviating morphologies that cause problems in tracing homologies. Similarly, changes affecting the whole life cycle are more challenging than those limited to one stage. Developmental modularity is a precondition for heterochrony. Analyzing a matrix of morphological data for paedomorphic taxa requires special care. It is, however, possible to extract phylogenetic signal from heterochronic patterns. The taxonomist should pay attention to the intricacies of the genotype→phenotype map. When using genetic data to infer phylogeny, a comparison of gene sequences is just a first step. To bridge the gap between genes and morphology we should consider the spatial and temporal patterns of gene expression, and their regulation. Minor genetic change can have major phenotypic effects, sometimes suggesting saltational evolution. Evo-devo is also relevant in respect to speciation: changes in developmental schedules are often implicated in the divergence between sympatric morphs, and a developmental modulation of ‘temporal phenotypes’ appears to be responsible for many cases of speciation.

Highlights

In the last two centuries, the relationships between biological systematics and developmental biology have widely fluctuated
It is fair to remark: - the abundance of useful diagnostic characters, other than those found in the adults, that are exhibited by non-adult specimens in many groups – exemplary are dragonflies and some groups of Lepidoptera - the taxa of which non-adult specimens are much more accessible than the corresponding adults, and offer adequate diagnostic characters upon which species description and a generic and suprageneric classification can be based; this is the case of many families of mites, the species of which are routinarily described on larvae
The phenotypic effects of heterochrony are usually small, but can occasionally be so conspicuous as to explain why genus-level distinctions have often been introduced in the past to separate groups of species that differ in one obvious trait from their closest relatives, within whose radiation they are deeply nested

Summary

Introduction

In the last two centuries, the relationships between biological systematics and developmental biology have widely fluctuated. A majority of biologists included, tacitly take for granted that development is a sequence of events proceeding from an egg (or a seed, a spore, a bud) towards the adult, with an accompanying increase in complexity, the latter being theoretically difficult to define, but suggested in practice, for example, by the number and diversity of cell types, or differentiated body parts (e.g., Bonner 1988; McShea 2000) This view of development squares well with von Baer’s (1828) ‘law’ that traced a parallelism between the ontogenetic progression throughout embryonic development and the emergence of morphological characters diagnostic of increasingly lower taxa. In order to stimulate research in this field, I will devote here a couple of pages to summarizing evidence demonstrating the phylogenetic signal present in heterochronic patterns in flowering plants

Major phenotypic effects obtained through ‘cheap’ heterochronic changes

Modularity and character independence

Peramorphosis and paedomorphosis in Campanulaceae

Heterostyly and homoplasy

Phylogenetic signal of heterochronic patterns in the Leguminosae