CEFISES Seminar: Wen Shichao, « Types, Populations and Variance Decomposition: The Case of Johannsen’s Pure Line Experiments »

Livestream https://www.youtube.com/@CEFISES

Series: Work-in-Progress

Speaker: Wen Shichao (UCLouvain)

Title: « Types, Populations and Variance Decomposition: The Case of Johannsen’s Pure Line Experiments »

Abstract

At the turn of the twentieth century, debates over heredity and evolution were shaped by contrasting ways of conceptualizing variation and its statistical treatment. Building on earlier work, which treats the Mendelism–biometry controversy as an analytic site for showing how thinking in terms of types and thinking in terms of populations could operate in evolutionary reason, this paper takes up a further question: whether, and in what sense, the two ways of thinking can be made compatible. I argue that Wilhelm Johannsen’s pure line experiments (1901–1903) re-specified the relation between type and population by decomposing population variance into distinct levels—fluctuating variability within pure lines and hereditary differences between pure lines. By showing that selection within a pure line does not shift the line’s mean character, Johannsen reconceived population as a mixture of reproductively stable lines. This intervention transforms an apparent opposition between types and populations into a layered statistical framework in which types are not alternatives to population analysis but components within it. On the reconstruction proposed here, Johannsen’s experiments make visible a conditional compatibility between type-based and population-based reasoning, while preserving their differences. This paper closes by identifying two interpretive directions opened by this reorganization: one emphasizing pure lines (and later genotypes) as elementary hereditary units, the other treating types as descriptors of population structure within a broader statistical account.