How can biologists understand embryonic development, i.e., the incredibly complicated process by which a single cell transforms itself into a complex multicellular organism through a series of cell divisions and movements? The short answer to this question, of course, is through intricate experimental manipulations that reveal causal dependencies between different parts of the embryo as well as between the embryo and its maternal environment. Given the enormous number of such causal dependencies present in a developing organism, which kinds of causes are developmental biologists particularly interested in? One class of causes that have taken center stage in explaining development are gradient-forming “morphogens” that appear to structure the embryo along some body axis, normally anterio-posterior or dorso-ventral. Morphogen gradients were postulated as early as 1901 and even caught the interest of mathematicians, most famously Alan Turing. However, experimental confirmation of the first morphogens had to await the development of advanced molecular techniques in suitable model organisms (namely Drosophila, Xenopus, and zebrafish) in the 1980s. In this talk, Marcel Weber will take a look at the very concept of a morphogen from the perspective of the philosophy of causality. What kind of cause are morphogens and why did and do they draw so much interest from developmental biologists? How are putative morphogens confirmed experimentally? And why are they often referred to as “signals” or as media of “positional information” that embryonic cells “interpret”? Weber will argue that this has to do with the fact that morphogen concentrations are hubs in complex causal nets that allow experimental biologists (and perhaps also evolution) to manipulate embryonic patterns along the main body axes by intervening on just a single variable.

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