The Phenomenon of Organ Reduction in the Evolution of Pulmonate Mollusks

Abstract

It has been established that among all organs and systems of pulmonate mollusks only the shell and the elements of the reproductive tract are subjects of reduction changes. It has been noted that a reduction of the shell in different branches of the Pulmonata subclass (mainly within the superorder Stylommatophora) has occurred repeatedly (at least 20 times). Reduction of accessory organs of the reproductive tract occurred either by one-step mutations, or gradually, in a chain of generations. A likely cause for the emergence of accessory organs is the creation of isolating mechanisms preventing introgression. It is assumed that, during evolution, morphological isolating mechanisms are replaced by less energy-consuming ethological, biochemical, and physiological mechanisms that stimulate the reduction of the accessory organs of the reproductive tract.