| Abstract
| - This paper describes a pattern of explanation prevalent in the biological sciences that I call a ‘lineage explanation’. The aim of these explanations is to make plausible certain trajectories of change through phenotypic space. They do this by laying out a series of stages, where each stage shows how some mechanism worked, and the differences between each adjacent stage demonstrates how one mechanism, through minor modifications, could be changed into another. These explanations are important, for though it is widely accepted that there is an ‘incremental constraint’ on evolutionary change, in an important class of cases it is difficult to see how to satisfy this constraint. I show that lineage explanations answer important questions about evolutionary change, but do so by demonstrating differences between individuals rather than invoking population processes, such as natural selection. Introduction Turning a ‘Scale’ into a ‘Plume’ Lineage Explanations in Biology 3.1 The evolution of eyes 3.2 The evolution of feathers The Two Dimensions of a Lineage Explanation 4.1 The production dimension 4.2 The continuity dimension 4.3 The dual role of the parts Constraining the Explanations Operational and Generative Lineages Explaining Change Without Populations Conclusion
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