Increased chemosensory ability and eye regression: a “Just-So” fish story
Evolution of an adaptive behavior and its sensory receptors promotes eye regression in blind cavefish Yoshizawa M, Yamamoto Y, O’Quin KE, Jeffery WR BMC Biology 2012, 10:108 (27 December 2012) [Provisional PDF]
Excerpt: “…loss of eyes as a result of selection for increased tactile sensitivity is an attractive general hypothesis for the convergent evolution of eye regression in cave fauna.”
My comment: The authors start with eye regression and increased chemosensory abilities required for foraging in the dark / nutrient poor environment of the cave fish. They later suggest that the same quantitative trait loci (QTL) clusters, which are involved in eye regression and increased chemosensory abilities are the clusters that control groups of potentially advantageous traits during the rapid evolution and species diversification of African cichlids (with eyes) and three-spined sticklebacks (with eyes). In the species with eyes, the QTL clusters somehow came to be involved in sex determination, body color, and body shape. But in the species with eyes that regressed increased tactile sensitivity evolves for nutrient chemical acquisition and mate choice.
Does that make sense? Loss of eyes associated with increased chemosensory abilities leads to a hypothesis of increased tactile sensitivity for food choice and mate choice. However, adaptive evolution of eyes that enable sexual reproduction associated with visual input has nothing to do with either tactile or chemosensory ability.
Here’s the problem I have with their hypothesis. The blind Astyanax cave fish and the sighted Astyanax surface fish produce viable offspring because they are the same species. In my model, species-specific pheromones control nutrient chemical-dependent reproduction. Visual, auditory, and tactile stimuli do not control nutrient chemical-dependent reproduction in any species.
Moving across an evolutionary continuum of eye development and/or regression while explaining that eye regression in cave fish is due to increased tactile sensitivity but normal eye development in other fish is required for visual perceived physical features important to sexual reproduction is confusing. It is also amazing because there is no mention of the role of olfaction or pheromones that enable the response to food and potential mates in every other species on the planet, whether or not the species has eyes that evolved, or first evolved and then regressed.