The structural coupling between an organism and its ecological niche is mediated by various kinds of signs. The most immediate or purely physical form of this coupling occurs at the molecular level when allosteric proteins fold into one of two possible three-dimensional structures depending on the presence of a molecular ‘partner.’ These proteins couple with complementary shapes, and this allows them to act as ‘switches’ to facilitate chemical reactions within living cells. Some of these reactions act as signals for other actions, contributing to the guidance system at the molecular level. At this level, ‘everything that gets done in an organism or by an organism is done by proteins’ (Loewenstein 1999, 72). But the system guiding the behavior of the whole organism is irreducibly semiotic.