International Society for the History, Philosophy, and Social Studies of Biology Session. This symposium will explore the conceptual foundations of biological lineages in the light of contemporary biology. Particularly, we will study the extent to which phenomena like genealogical discordance, incomplete lineage sorting, developmental trait evolution, small RNA inheritance, extended inheritance, horizontal gene transfer, and symbiosis relationships challenge the idea that lineages ought to be conceived monistically (i.e., as a single concept). The different parts of this session will explore various consequences of adopting this stance. That includes analysing epistemological and ontological challenges associated with multiple discordant lineages questioning the very necessity of the concept of lineages for conceiving of biological reproduction, and suggesting an array of new perspectives for thinking about inheritance, reproduction and biological evolution.