This article defends lineage pluralism; the view that biological lineages are not a single, unified type of entity. I analyze aspects of evolutionary theory, phylogenetics, and developmental biology to show that these areas appeal to distinct notions of lineage. I formulate three arguments for lineage pluralism. These arguments undercut the main motivations for lineage monism; the view that biological lineages are a single, unified type of entity. Although this view is rarely made explicit, it is often assumed in philosophy and (...) biology. Hence, this article sheds light on this monistic assumption and shows why lineage pluralism should be adopted instead. (shrink)

At the end of the nineteenth century, approaches from experimental physiology made inroads into embryological research. A new generation of embryologists felt urged to study the mechanisms of organ formation. This new program, most prominently defended by Wilhelm Roux, was called Entwicklungsmechanik. Named variously as “causal embryology”, “physiological embryology” or “developmental mechanics”, it catalyzed the movement of embryology from a descriptive science to one exploring causal mechanisms. This article examines the specific scientific and epistemological meaning of the mechanistic approaches of (...) embryological development by focusing on Wilhelm His’ histogenetic work. Roux was neither the first, nor the only one to argue for an experimental exploration of causes in embryology. At the time of Roux, physiological explanations of the genesis of the anatomical forms were developing in parallel, not only in German-speaking countries, but in France, Switzerland and English-speaking countries as well. The experimental approach and the cellular descriptions of embryogenesis were already omni-present when Roux proposed his Entwicklungsmechanik. However, these approaches remained disjointed. It appears that it was Wilhelm His who first succeeded in combining the question of the causal factors determining epigenesis, which was closely connected with experimentation on, and cellular descriptions of, development, in a coherent and concrete synthesis, making him one of the true initiators of the developmental mechanics. (shrink)

Sex is fundamental to many organisms. It is through sexual reproduction that humans, and many metazoans (multicellular eukaryotes in the animal kingdom), propagate our species. For more than 150 years, sexual reproduction within metazoans has been understood to rely on the existence of a discrete category of cells (germ cells) that are usually considered uniquely separate from all other cells in the body (somatic cells), and which form a cell lineage (germline) that is sequestered from all somatic cell lineages. The (...) consideration of germ cells and germline as the lone source of reproductive potential within metazoans has allowed many investigators to place the hereditary and evolutionary burdens of sexually reproducing lineages solely within these cells and cell lineages, making them central to many important topics within biology, such as units of selection, transmission and population genetics, Darwinian evolution, and individuality. Regarding these topics, there is a predominant and shared understanding of germ cells, somatic cells, and the ways in which these two relate to each other that is rarely critically evaluated. In this article, I lay out how germ cells and germline within metazoans are understood by a majority of scientists and philosophers, both now and historically, by sketching out what I call the predominant epistemic framework of germ. I show how this framework conflicts with empirical evidence, propose a series of revisions to realign it with this evidence, and indicate why such revisions are urgently needed by highlighting the case of somatic cell genome editing. (shrink)