Visual systems are ideal models for the study of sensory evolution. The cichlids of Lake Malawi possess an elaborated complex of genes (opsins) that encode chromatic visual pigments, which allows us to study the evolution and diversification of chromatic vision in great detail. In this dissertation, we investigated the molecular and behavioral properties of cichlid visual systems in order to more thoroughly understand the diversification of visual systems and the behavioral consequences of these changes. The work is organized into three research projects, with the following results:

(1) Opsin gene sequence variation, with corresponding functional sensitivity changes, were found for the SWS1 (ultraviolet-sensitive), SWS2B (violet-sensitive), RH2Aβ (green-sensitive), and LWS (red-sensitive) opsin genes. Of the two genera profiled, each had two variable genes, suggesting that diversifying selection acts on different opsins in each genus. Furthermore, our data suggest that the variation in the SWS1 gene has arisen recently in Lake Malawi and is under rapid selection.

(2) Intraspecific cone opsin gene expression variation was found in wild populations of multiple species. Expression variation was found primarily for the LWS and SWS1 genes, while the other genes were relatively consistent within species. This finding suggests that expression can be modulated by adding genes to what may otherwise be considered a species-specific expression pattern. Quantitative models suggested that this expression variation was not the result of environmental constraint.

(3) Fish raised in different ambient developmental light environments had different cone opsin gene expression, primarily in the LWS opsin gene. These expression differences caused an increase in behavioral sensitivity in the optomotor response. Furthermore, analyses indicated that the OMR response is determined solely by the LWS cone pigment, rather than a complement of different cone types.

Taken together, these findings shed new light on how visual systems diversify over short evolutionary time-scales, and the possible linkage of early determinants of visual sensitivities (opsin genes) and processes that directly influence speciation (behavior).