EVOLUTION AND COEVOLUTION OF ODOR LANDSCAPES AND CHEMORECEPTION

The ability to detect, distinguish, and respond to chemicals in the external environment is essentially universal among all living things. For many organisms, it is the principal means of obtaining information about their surroundings, such as the presence of predators, prey, or potential mates. Organisms emit and detect a huge array of chemical products (odorants) with a correspondingly wide range of molecular properties, including their ability to disperse in the environment. The mix of odorants can include both pheromones and allelochemicals. For a given odorant, its distribution is dependent upon the original spatial distribution of odor sources and the associated processes of fluid flow, such as diffusion and turbulence, which are strongly scale and environment dependent. The odorant sources can be fixed or moving and their output can be stable or ephemeral. To complicate matters further, different organisms can occupy the same general spatial location; for example, insects living on a host plant. The complex, three dimensional, and dynamic distribution of odors in the environment is the odor landscape; it is the sensory environment in which chemoreceptive systems function and have evolved. The odor landscape, and the corresponding chemosensory systems, should have evolved in concert with major transitions in the history of life, including the origins of eukaryotes and of multicellular organisms, and the transition to land. Changes in the odor landscape itself would be produced by the development of biotically associated spatial heterogeneity, increasing the localization of odor sources, and increases in the diversity of odorants as organism diversity increased. Organismal responses would occur at multiple hierarchical levels, including the development of olfactory organs and the proliferation of odorant receptor gene families. These changes would also be linked to the evolution of other sensory modalities, in particular vision. Studies with simulation models and experimental organisms suggest how variability in the spatial structure of odor landscapes may control patterns of organismal movement that could be represented in the trace fossil record.