Background: The concept of a tree of life is prevalent in the evolutionary literature. It stems from attempting to obtain a grand unified natural system that reflects a recurrent process of species and lineage splittings for all forms of life. Traditionally, the discipline of systematics operates in a similar hierarchy of bifurcating (sometimes multifurcating) categories. The assumption of a universal tree of life hinges upon the process of evolution being tree-like throughout all forms of life and all of biological time. In multicellular eukaryotes, the molecular mechanisms and species-level population genetics of variation do indeed mainly cause a tree-like structure over time. In prokaryotes, they do not. Prokaryotic evolution and the tree of life are two different things, and we need to treat them as such, rather than extrapolating from macroscopic life to prokaryotes. In the following we will consider this circumstance from philosophical, scientific, and epistemological perspectives, surmising that phylogeny opted for a single model as a holdover from the Modern Synthesis of evolution.

Both realist and anti-realist accounts of natural kinds possess prima facie virtues: realists can straightforwardly make sense of the apparent objectivity of the natural kinds, and anti-realists, their knowability. This paper formulates a properly anti-realist account designed to capture both merits. In particular, it recommends understanding natural kinds as 'categorical bottlenecks,' those categories that not only best serve us, with our idiosyncratic aims and cognitive capacities, but also those of a wide range of alternative agents. By endorsing an ultimately subjective categorical principle, this view sidesteps epistemological difficulties facing realist views. Yet, partly in consequence of the ubiquity of robust processes in our universe, it nevertheless identifies natural kinds that are fairly, though not completely, stance-independent or objective.

The interventionist account of causal explanation, in the version presented by Jim Woodward ([2003]), has been recently claimed capable of buttressing the widely felt-though poorly understood-hunch that high-level, relatively abstract explanations, of the sort provided by sciences like biology, psychology and economics, are in some cases explanatorily optimal. It is the aim of this paper to show that this is mistaken. Due to a lack of effective constraints on the causal variables at the heart of the interventionist causal-explanatory scheme, as presently formulated it is either unable to prefer high-level explanations to low, or systematically overshoots, recommending explanations at so high of a level as to be virtually vacuous.

Among the factors necessary for the occurrence of some event, which of these are selectively highlighted in its explanation and labeled as causes -and which are explanatorily omitted, or relegated to the status of background conditions? Following J. S. Mill, most have thought that only a pragmatic answer to this question was possible. In this paper I suggest we understand this 'causal selection problem' in causal-explanatory terms, and propose that explanatory trade-offs between abstraction and stability can provide a principled solution to it. After sketching that solution, it is applied to a few biological examples, including to a debate concerning the 'causal democracy' of organismal development, with an anti-democratic (though not a gene-centric) moral. 1-Introduction: Explanatory Sparseness and SystematicityOur universe is dizzyingly complex, and everything that happens within it causally depends on innumerable other things. The living world in particular can appear almost horrifically complicated. Though some of this complexity remains beyond our grasp, scientists have unraveled ever-larger portions of it. The combination of this complex world and our increasingly sophisticated theories accounting for it should make two features of our causal-explanatory practice appear surprising: its sparseness and its systematicity.Explanatory practice is sparse in that many apparently legitimate causal explanations are rather thin affairs, in which happenings are accounted for with only the tiniest sliver of information, and not by citing all, or even very many, of an event's causal influences. 2 Explanatory practice is systematic in that those few morsels that sparse explanations feed to us do not seem to emerge higgledy-piggledy, as if they were the output of some 'explanatory lottery' in 1 For helpful comments on this paper, thanks to David Frank, Maria Kronfleldner, Michael Strevens and David Velleman. Special thanks are due to the editors of this volume, PierreAlain Braillard and Christophe Malaterre, both for their patience and for their perceptive editorial suggestions. 2 The distinction between explanations as communicative acts and explanations construed in an 'ontic' mode as sets of facts will not loom large in this paper; throughout, I will presume that the content of communicative acts are the explanatorily relevant facts.

The Tree of Life has traditionally been understood to represent the history of species lineages. However, recently researchers have suggested that it might be better interpreted as representing the history of cellular lineages, sometimes called the Tree of Cells. This paper examines and evaluates reasons offered against this cellular interpretation of the Tree of Life. It argues that some such reasons are bad reasons, based either on a false attribution of essentialism, on a misunderstanding of the problem of lineage identity, or on a limited view of scientific representation. I suggest that debate about the Tree of Cells and other successors to the traditional Tree of Life should be formulated in terms of the purposes these representations may serve. In pursuing this strategy, we see that the Tree of Cells cannot serve one purpose suggested for it: as an explanation for the hierarchical nature of taxonomy. We then explore whether, instead, the tree may play an important role in the dynamic modeling of evolution. As highly-integrated complex systems, cells may influence which lineage components can successfully transfer into them and how they change once integrated. Only if they do in fact have a substantial role to play in this process might the Tree of Cells have some claim to be the Tree of Life.