Visual system: Adaptive regression and progression in subterranean mammals

The extensive convergent evolution of subterranean mammals across the planet represents a global, natural experiment of life without light under darkness stress. Adaptive convergence comprises structural and functional reductions and expansions (hypertrophies) through molecular and organismal evolutionary tinkering involving regression, progression, and convergence of eye evolution advanced differentially in subterranean mammals, climaxing in the blind subterranean mole rat, Spalax. The mosaic evolution of the Spalax eye is described in terms of morphology, physiology, and molecular biology. Eye development is initiated normally but at an early stage displays degenerative features - though neurogenesis develops normally. The main regressive feature is the drastic, relative reduction of retinal input to the superior culliculuis. By contrast, the structure of the 'non-image-forming' visual pathway involved in photoperiodic perception is well developed in Spalax. The retinal photopigments, coneopsin, rhodopsin, and melanopsin, function in photoentrainment, and the somatosensory brain cortex is remarkably developed. The differential regression and progression of visual structures, photopigments, and brain underwent adaptive evolutionary tinkering through natural selection to life underground at both the molecular and organismal structures.