Biological Theory’s new issue, 20(4), features a Thematic Issue on “The Concept of Lineage in Biology” guest-edited by Javier Suárez and Matthew H. Haber. It discusses the concept of “lineage” as continuous lines of ancestry, which in biology can be applied to cells, organisms, species, and other entities. The idea dates back to Darwin, treating lineages as the basis of phylogeny and explaining why species share forms and how they diverge. While this notion is central to biology, it raises deep conceptual questions: which entities genuinely form lineages (e.g., cells, organisms, holobionts)? Do lineages at different biological levels differ in kind or only in degree? And can one unified concept of lineage accommodate all these cases? This topical collection examines these and other conceptual questions about lineages in light of recent empirical and theoretical work.

Javier Suárez and Sophie Veigl argue that the concept of “lineage,” introduced to explain social evolution and cumulative selection, is now epistemically dispensable and sometimes obstructive. The authors propose that, rather than seeking a specific lineage for entities with certain biological traits, biologists should investigate the processes that could generate those traits, treating lineage formation as only one possible mechanism among many.

Lucie Laplane critiques the clonal evolution model of cancer, which treats clones as lineages of cells sharing driver mutations, for oversimplifying the complexity of cancer cell populations. She shows that this model underestimates epigenetic and contextual factors, relies on a rigid driver/passenger and genotype–phenotype mapping, and that refining these assumptions could improve both the definition of clones and cancer treatment.

François Papale develops a concept of the “unit of selection” compatible with natural selection without reproduction, where entities need not form lineages. Classical notions (replicators, interactors, manifestors, reconstitutors) all presuppose reproduction or misidentify the target. He instead defines minimal units of selection as entities that influence the ratio of entities with similar traits in the population through interactions with their environment.

Kate MacCord, focusing on germline lineages, and Matthew H. Haber, examining large-scale phylogenetic patterns, both argue that entrenched lineage frameworks (e.g., the Weismann barrier, strictly branching evolutionary trees) unduly constrain inquiry. They advocate more flexible, empirically informed concepts—such as gradients between germ and soma and context-dependent “lineal frames of reference”—which yield richer, more accurate accounts of biological history.

The December issue 20(4) opens with a farewell editorial by our departing Editor-in-Chief, Stuart Newman, whom we sincerely thank for his outstanding ten years of service and dedication to Biological Theory! It concludes with the acknowledgments of our reviewers.