A computational model (shown below) designed to simulate the evolution of an eye. It starts with at flat sheet of photosensitive cells lying above a flat layer of pigmented cells and below a monolayer of transparent cells. Using a sequence of small modifications in shape, the originally flat patch gradually changes into a cup, which then acquires a lens in its opening. The end result is a focused, light-imaging organ with the geometry typically seen in a fish eye. The mathematical modeling procedure chosen was such that each 1% increment of change produced a maximal increase in visual acuity. Using conservative assumptions as to heritabilities (q.v.) and selection pressures (q.v.), the times taken (in generations) to perform each evolutionary stage are shown in the diagram. To complete an eye for a species with one generation per year, it would take a total of only 364,000 years, a relatively short time from a geological prospective. So it is not surprising that eyes have been produced independently at least 40 times during the evolution of Eumetazoa (q.v.).
See Chronology, 1994, Nilsson and Pelger.
Drawing by Dan-E. Nilsson, Lund University, Lund, Sweden. Reproduced with permission of D.-E. Nilsson.
Subjects: Genetics and Genomics.