1. Does light environment have an influence on the development of color vision? The accompanying data are measurements of the relative abilities of bluefin killifish from two wild populations to detect short wavelengths of light (blue light in our own visible color spectrum). One population was from a swamp, whose tea-stained water filters out blue wavelengths, whereas the other population was from a clear-water spring. Fish were crossed and raised in the lab under two light conditions simulating those in the wild: clear and tea-stained. Sensitivity to blue light was measured as the relative expression of the SWS1 opsin gene in the eyes of the fish (as a proportion of total expression of all opsins). Opsin proteins in eyes detect light of specific wavelengths; SWS1 is so named because it is shortwave sensitive. The data are from a single individual from each of 33 families. Because the fish were raised in a common lab environment, population differences are likely to be genetically based, whereas differences between fish under different water clarity conditions are environmentally induced.