Tetrapods have two distinct nasal chemosensory systems, the main olfactory system and the vomeronasal system (VNS). Defined by certain morphological components, the main olfactory system is present in all groups of vertebrates, while the VNS is found only in tetrapods. Previous attempts to identify a VNS precursor in teleost fish were limited by functional and morphological characters that could not clearly distinguish between homologous and analogous systems. In the past decade, several genes that specifically function in the VNS have been (...) discovered. Here we first describe recent evolutionary studies of mammalian VNS‐specific genes. We then review evidence showing the presence and tissue‐specific expression of the VNS‐specific genes in teleosts, as well as co‐expression patterns of these genes in specific regions of the teleost olfactory epithelium. We propose that a VNS precursor exists in teleosts and that its evolutionary origin predated the separation between teleosts and tetrapods. BioEssays 28: 709–718, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

Advances in developmental genetics and evo-devo in the last several decades have enabled the growth of novel developmental approaches to the classic theme of homology. These approaches depart from the more standard phylogenetic view by contending that homology between morphological characters depends on developmental-genetic individuation and explanation. This article provides a systematic re-examination of the relationship between developmental and phylogenetic homology in light of current evidence from developmental and evolutionary genetics and genomics. I present a qualitative model of the processes (...) that cause de-coupling of morphological and molecular evolution by developmental system drift (DSD), and hypothesize that DSD is a widespread and regular stochastic process that is predictable from parameters such as population size, pleiotropy, and regulatory redundancy. These theoretical findings support an integrative approach in which models of DSD aid in determining when developmental-genetic explanations of homology apply and when other explanations such as stabilizing selection are needed. I argue that a phylogenetic monism about the definition and criteria of homology supports the integration of different explanations into a theory of homology better than pluralism does. (shrink)

This paper describes a pattern of explanation prevalent in the biological sciences that I call a ‘lineage explanation’. The aim of these explanations is to make plausible certain trajectories of change through phenotypic space. They do this by laying out a series of stages, where each stage shows how some mechanism worked, and the differences between each adjacent stage demonstrates how one mechanism, through minor modifications, could be changed into another. These explanations are important, for though it is widely accepted (...) that there is an ‘incremental constraint’ on evolutionary change, in an important class of cases it is difficult to see how to satisfy this constraint. I show that lineage explanations answer important questions about evolutionary change, but do so by demonstrating differences between individuals rather than invoking population processes, such as natural selection.1. Introduction2. Turning a ‘Scale’ into a ‘Plume’3. Lineage Explanations in Biology3.1. The evolution of eyes3.2. The evolution of feathers4. The Two Dimensions of a Lineage Explanation4.1. The production dimension4.2. The continuity dimension4.3. The dual role of the parts5. Constraining the Explanations6. Operational and Generative Lineages7. Explaining Change Without Populations8. Conclusion. (shrink)