EVOLUTION IN THE DARK: DARWIN'S LOSS WITHOUT SELECTION

Colonizations have been sudden, and many are recent and so different that populations have not diverged enough to become reproductively isolated. This is different from most cavefish, which have been isolated so long that their putative ancestors do not exist. Wilkens explains how the surface-living Astyanax Mexican tetras are different from the ancestors of most cavefish, such as catfish, because they are only minimally pre-adapted to life in caves. As day-active schooling fish they strongly rely on vision. They do not have great elaboration of sense organs that are important for finding food in caves, like lateral line neuromasts, taste buds, and olfactory rosettes. But we know now that the surface fish do have taste buds and lateral-line sense organs and that they have standing genetic variation for these traits. Those surface fish individuals with the greatest lateral line, taste, and smell senses may have been the only ones to survive when first isolated in caves. They colonized successfully, whereas those colonizing later die of starvation. Astyanax provides a huge advantage for reconstruction of the evolution of cave adaptation because it has had many independent colonizations of caves that range from old to recent. All populations are inter-fertile, and this has allowed a full range of classic genetic and molecular genetic studies in the laboratory. It has also allowed paleo-biogeographic reconstruction of past and ongoing colonizations. The dynamic evolution of caves and karst along with Pleistocene climatic change has resulted in continued extinctions and recolonizations, some with introgressive hybridization between surface and cave in at least the distant past. Wilkens and all others also agree that there has been direct natural selection to elaborate sense organs and metabolic economy. But they disagree about mechanisms of regression or reducing of eyes and pigment. Wilkens shows that there have been many mutations in eye and pigment genes and that these have no disadvantage in caves. These mutants are not eliminated by purifying selection, and eyes become more regressed with time through still more mutations. Others argue that there has been both direct and indirect selection, by pleiotropy, against eyes and pigment. Horst Wilkens, Richard Borowsky, and William Jeffery are the three titans of Mexican cavefish study. Each is interested in both regressive changes, especially of eyes and pigment, and adaptations of behavior and physiology to survive in caves. But their conclusions differ in many ways. Beginning in the late 1960s, Horst Wilkens and a few students and close colleagues did by far the earliest research on the biology, genetics, and history of Astyanax surface and cave populations. He is the only one who has consistently studied many surface populations and almost all the cave populaBooK review: evolution in tHe darK: darwin’s loss witHout seleCtion