Brian K. Hall

The return of Atlantic salmon (Salmon salar) to their home river for spawning coincides with drastic skeletal alterations in both sexes. Most prominent is the development of a kype (hook) at the tip of the lower jaw in males. Salmon that survive spawning have to cope with the kype throughout their life, unless it disappears after spawning, as was suggested in the early literature. To understand the fate of the kype skeleton, we compared morphological and histological features of kypes from pre‐spawned mature anadromous males (grilse) with post‐spawned males (kelts). The kype of male grilse is supported by fast‐growing skeletal needles that differ from regular dentary bone. In kelts, growth of the kype skeleton has stopped and skeletal needles are resorbed apically by osteoclasts. Simultaneously, and despite the critical physiological condition of the animals, proximal parts of the kype skeleton are remodelled and converted into regular dentary bone. Apical resorption of the skeleton explains reports of a decrease of the kype in kelts. The conversion of basal kype skeleton into regular dentary bone contributes to the elongation of the dentary and probably also to the development of a larger kype in repetitive spawning males.

Homology is at the foundation of comparative studies in biology at all levels from genes to phenotypes. Homology similarity because of common descent and ancestry, homoplasy is similarity arrived at via independent evolution However, given that there is but one tree of life, all organisms, and therefore all features of organisms, share degree of relationship and similarity one to another. That sharing may be similarity or even identity of structure the sharing of a most recent common ancestor–as in the homology of the arms of humans and apes–or it reflect some (often small) degree of similarity, such as that between the wings of insects and the wings of groups whose shared ancestor lies deep within the evolutionary history of the Metazoa. It may reflect sharing entire developmental pathways, partial sharing, or divergent pathways. This review compares features classified homologous with the classes of features normally grouped as homoplastic, the latter being convergence, parallelism, reversals, rudiments, vestiges, and atavisms. On the one hand, developmental mechanisms may be conserved, when a complete structure does not form (rudiments, vestiges), or when a structure appears only in some individuals (atavisms). On the other hand, different developmental mechanisms can produce similar (homologous) features Joint examination of nearness of relationship and degree of shared development reveals a continuum within expanded category of homology, extending from homology → reversals → rudiments → vestiges → atavisms → parallelism, with convergence as the only class of homoplasy, an idea that turns out to be surprisingly old. realignment provides a glimmer of a way to bridge phylogenetic and developmental approaches to homology homoplasy, a bridge that should provide a key pillar for evolutionary developmental biology (evo‐devo). It will and in a practical sense cannot, alter how homoplastic features are identified in phylogenetic analyses. But rudiments, reversals, vestiges, atavisms and parallelism as closer to homology than to homoplasy should guide toward searching for the common elements underlying the formation of the phenotype (what some have called deep homology of genetic and/or cellular mechanisms), rather than discussing features in terms of shared independent evolution.

A wind of change has swept through palaeontology in the past few decades. Contrast Sir Peter Medawar’s dismissive: ‘palaeontology is a particularly undemanding branch of science’ (as recalled by John Maynard Smith in Sabbagh 1999, p. 158) with ‘Palaeontology: grasping the opportunities in the science of the twenty–first century’, the title of a contribution to a special issue of Geobios by the Cambridge palaeontologist, Simon Conway Morris (1998a). The winds of change have come partly from palaeontologists seeking to broaden the impact of their studies and partly from biologists (neontologists) realizing the contributions that palaeontology can make to their disciplines. Consequently, impressions of past life preserved in stone are coming alive. Fossils are being described and analyzed using new tools and languages as the static fossil record becomes a record of transitions in patterns that can be explained and related to biological, ecological, climatic and tectonic changes. The latest addition is evolutionary developmental biology, or ‘evo–devo’, whose language provides a new basis upon which to interpret anatomical change, both materially and mechanistically. In this review I examine the major contributions made by palaeontology, how palaeontology has been linked to evolution and to embryology in the past, and how links with evo–devo have enlivened and will continue to enliven both palaeontology and evo–devo. Closer links between the two fields should illuminate important unresolved issues related to the origin of the metazoans (e.g. Why is there a conflict between molecular clocks and the fossil record in timing the metazoan radiation; were Precambrian metazoan ancestors similar to extant larvae or to miniature adults?) and to diversification of the metazoans (e.g. How do developmental constraints bias the direction of evolution; how do microevolutionary developmental processes relate to macroevolutionary changes?).

SUMMARY Despite announcements and obituaries, news of the death of the gene has been greatly exaggerated, or so says the gene as it struggles to survive and find a safe haven from which to steer its course through development and evolution. In this short piece, I consider recent claims that the gene is dead. I conclude that the gene is alive and well, living and functioning in the cell, which is both its natural home and a fundamental unit of evo‐devo.

Clavicles (collar bones) are variably present in mammals. Furculae (wishbones)—which may or may not be homologous with clavicles—are variably present and/or fused in birds and present in theropod dinosaurs. In this overview the development of clavicles and furculae is discussed with special attention to modes of skeletogenesis (whether intramembranous or endochondral), numbers of centres of ossification (one in chick furculae; two in murine clavicles), presence of cartilage (primary in clavicles, secondary in furculae), evidence from experimental analysis and from mutations for dependence of both clavicular and furcular growth on mechanical stimulation, and syndromes and mutations affecting clavicular development leading to both under and over development. J. Exp. Zool. 289:153–161, 2001.

Evolution has been integrated with embryology during two great periods: the latter half of the 19th C as evolutionary morphology/embryology, and the latter third of the 20th C as evolutionary developmental biology. My mandate was to use the contributions of three embryologists/morphologists: Francis (Frank) Balfour (1851–1882), Walter Garstang (1868–1949) and Gavin de Beer (1899–1972) to discuss the foundations of evolutionary embryology in the UK from 1870 (when “every aspiring zoologist was an embryologist, and the one topic of professional conversation was evolution,” Bateson, 1922, p. 56), through the 1920s (“ontogeny does not recapitulate phylogeny, it creates it,” Garstang, 1922, p. 81) to the 1970s (“homology of phenotypes does not imply similarity in genotypes,” de Beer, 1971, p. 15). Evolutionary embryology was driven by a comparative embryological approach that sought homology of adult structures in germ layers and ancestry in embryos, and sought to differentiate larval adaptations from retained ancestral characters. An initial emphasis on a phylogenetic mechanism (recapitulation) slowly gave way to more mechanistic approaches that included heterochrony and the integration of embryology with physiological genetics. Germ layers, homology, larval evolution, larval origins of the vertebrates, paedomorphosis and heterochrony underpinned the origins of evolutionary embryology, and so I discuss each of these topics.