The general significance of variability in cave regressive traits for evolution.

The article focuses on the variability of rudimentary traits, such as eye and pigmentation reduction, in cave-dwelling species and explores the evolutionary mechanisms underlying this variability. It presents case studies of invertebrate and vertebrate cave species—including the isopods Asellus aquaticus and Asellus kosswigi, the amphipod Gammarus minus, and the fishes Poecilia mexicana and Astyanax mexicanus—demonstrating that despite genetic separation from their surface ancestors, regressive traits exhibit high phenotypic and genotypic variability. This variability is attributed primarily to the loss of stabilizing (purifying) selection on biologically functionless traits, allowing accumulation of neutral or deleterious mutations, rather than to hybridization. In contrast, constructive traits that confer adaptive advantages in cave environments maintain variability levels similar to surface populations due to ongoing selection. The study highlights that such relaxed selection and resulting variability are important evolutionary processes beyond cave species, contributing to broader patterns of evolution including adaptive radiations and domestication.