Haeckel's ABC of evolution and development

Abstract

One of the central, unresolved controversies in biology concerns the distribution of primitive versus advanced characters at different stages of vertebrate development. This controversy has major implications for evolutionary developmental biology and phylogenetics. Ernst Haeckel addressed the issue with his Biogenetic Law, and his embryo drawings functioned as supporting data. We re-examine Haeckel's work and its significance for modern efforts to develop a rigorous comparative framework for developmental studies. Haeckel's comparative embryology was evolutionary but non-quantitative. It was based on developmental sequences, and treated heterochrony as a sequence change. It is not always clear whether he believed in recapitulation of single characters or entire stages. The Biogenetic Law is supported by several recent studies -- if applied to single characters only. Haeckel's important but overlooked alphabetical analogy of evolution and development is an advance on von Baer. Haeckel recognized the evolutionary diversity in early embryonic stages, in line with modern thinking. He did not necessarily advocate the strict form of recapitulation and terminal addition commonly attributed to him. Haeckel's much-criticized embryo drawings are important as phylogenetic hypotheses, teaching aids, and evidence for evolution. While some criticisms of the drawings are legitimate, others are more tendentious. In opposition to Haeckel and his embryo drawings, Wilhelm His made major advances towards developing a quantitative comparative embryology based on morphometrics. Unfortunately His's work in this area is largely forgotten. Despite his obvious flaws, Haeckel can be seen as the father of a sequence-based phylogenetic embryology.

Head-tail patterning of the vertebrate embryo: one, two or many unresolved problems?

“…A characteristic body plan emerges when most of the trunk paraxial mesoderm has become segmented into somites, the body wall has folded to enclose the gut and a tail bud has formed (even if in some organisms this later regresses). This is the pharyngula (or phylotypic) stage, at which all vertebrates were once thought to resemble each other more than at any other stage of development (Haeckel, 1874;Richardson and Keuck, 2002). Somites arise from paraxial mesoderm emerging from the remnants of the blastopore/shield/primitive streak region, while the hindbrain/spinal cord are derived from small primordia located next to this.…”

Karl Ernst von Baer (1792-1876) and Evolution

“…The recent spate of scholarship on the German biologist Ernst Haeckel, known for enunciating the biogenetic law (Richardson & Keuck, 2002;Richards, 2008;Gliboff, 2009) and the gastraea theory (Brauckmann & Gilbert, 2004) has revived discussion of earlier biologists who influenced Haeckel, such as the embryologist Karl Ernst von Baer (Raikov, 1968;Gould, 1977;Brauckmann, 2008). Baer has received attention because Haeckel synthesized Darwin's theory with Baer's developmental history in order to create his modern theory of recapitulation.…”

“…When scrutinizing the research problems that were disregarded by neo-Darwinism (Gilbert et al, 1996;Hall, 1999;Raff, 2000;Wagner, 2000;Amundson, 2002;Arthur, 2002;Love & Raff, 2003;Minelli & Fusco, 2008), Entwickelungsgeschichte (developmental history) provides an important and up to now neglected topic. Baer's reflections about developmental history (Baer, 1819(Baer, , 1823(Baer, , 1827(Baer, , 1828 offer a most appropriate point of departure, particularly since his ideas are referred to and propagated among scientists dealing with the genetic connection between evolution and development (see most recent research on phylotypic stage, hourglass model), despite debates about the validity of his research programme (Ballard, 1976;Kluge & Strauss, 1985;Richardson & Keuck, 2002;Poe, 2006). …”

Kowalevsky, comparative evolutionary embryology, and the intellectual lineage of evo‐devo

“…The research program of Haeckel was fundamentally about discerning phylogenetic relationships among organisms using embryological patterns for data to establish relationships of homology, especially where paleontological help was unlikely to be forthcoming, such as in the origin of vertebrates, with all of its purported implications for Homo sapiens (Sander, 2002;Hobfeld and Olsson, 2003). Haeckel's interest in the mechanistic details of the evolutionary process was less thorough, though it is often not carefully characterized (Gould, '77;Richardson and Keuck, 2002). The work of his students, collaborators, and fellow travelers continued to emphasize problems of phylogenetic classification, especially the nature of 'ancient' and 'transitional' ancestors, including metazoan precursors and the origin of paired limbs in vertebrates.…”