In the cave of the blind, the no-eyed crab is king

Cave dwelling creatures are often blind. The prevailing view is that, in such species, mutations in the visual system have little or no effect on fitness and vision is lost as these mutations gradually accumulate. There are several other types of characters that we can be reasonably confident are adaptations to life in caves, such as elaboration of structures for touch or smell. However, it is often hard identify which population cave adapted species are descended from and, therefore, how long ago they invaded caves. Without this information it has been hard to test ideas about the evolution of traits associated with life in the dark.

A cave form of the fish, Astyanax mexicanus, which is eyeless and unpigmented, traits typical in caves. It is a commonly used model species in studies of adaptation to cave environments (photo Wikimedia Commons).

Sebastian Klaus and colleagues from the National University of Singapore and Goethe University examined five species of freshwater crab in the genus Sundathelphusa, which occur on Bohol Island in the Philippines. Four species are only found in caves and the other has established several populations in caves. The repeated invasion of caves by the crabs has led to varying degrees of adaptation to life in the dark within the group. 

Freshwater crabs in the genus Sundathelphusa from Bohol Island. Thy are arranged from least cave adapted (top) to most cave adapted (bottom). From top to bottom the species are Sundathelphusa boex, S. vedeniki, S. urichi, S. sottoae and S. cavernicola (from Klaus et al. 2013).

The team used genetic data to estimate the time at which each species and population last shared a common ancestor. They then compared several features of cave-adapted crabs with their closest terrestrial relatives. Reductions in the visual system were just as pronounced as changes in cave-adapted features, indicating that evolution occurs at similar rates. The authors argue that this is a clear sign that eye loss is under directional selection because changes should appear more slowly if they are a result of selectively neutral mutations. 

They don’t speculate at all about what might favour eye-loss in the Bohol crabs, but hint in the introduction that it could be due to trade-offs between vision and other sensory systems. Trade-offs occur where increasing one aspect of fitness necessarily requires the reduction of fitness in another. If eyes are energetically costly to build and maintain then retaining functional eyes might prevent greater investment in other senses. Trade-offs are ubiquitous in biology and have been implicated in the loss of eyes in other cave dwelling species.

While I was doing research on this study I came across several creationist websites that argue cave adapted creatures are strong evidence that evolution is false because a trait is lost. According to them this shows evolution progressing in the wrong direction to what is predicted. They argue that evolution should progress towards more information and greater complexity. This is incorrect and shows, yet again, that creationists typically have a poor understanding of evolutionary theory.

The 'logic' of this argument is similar to the idea of a "Great Chain of Being", which pervaded early thinking about biology. This type of thinking is where we get several antiquated, but persistent terms, such as "missing link" and "highly evolved". It continues to dog evolution in the way that evolutionary information is often presented, such as the placement of organisms more closely related to us at the right or top of phylogenetic trees and at the end of textbooks.

The phylogeny of primates with humans at the top and less related groups at the bottom (from Wikipedia).

Linear descent was never part of Darwin's theory, nor was an increase in information ever a necessary assumption on which evolutionary theory rests. When you look at an evolutionary tree (like the primate tree above), all of the living species at the branch tips have an equally long evolutionary history. They are not descended from each other, they are descended from a common ancestor. You could say that they are equally evolved.

The first evolutionary tree drawn by Darwin over 20 years before the publication of On the Origin of Species.

Evolution doesn't prevent information from increasing, but contrary to the creationist claims it does predict that there will be strong limits on it. Both single trait and multi-trait trade-offs are thought to prevent organisms from becoming perfectly adapted. Single trait trade-offs occur where elaboration of a structure increases fitness in one environment, but reduces it in others. Multi-trait trade-offs occur where two or more structures are dependent on a shared finite resource.

Blind crabs are not evolving in the wrong direction. There is no wrong direction, they're just evolving under the constraint of trade-offs. Eye reduction and loss of pigmentation are not the only evolutionary changes that are occurring either. Other traits are becoming more elaborated, such as the length of their legs and the hairs on their claws, suggesting a multi-trait trade-off. This result is not only consistent with evolutionary theory, but expected.

An abbreviated version of this post is published on the Australasian Evolution Society website in the Research Highlights section.

Reference

Klaus, S., Mendoza, J., Liew, J., Plath, M., Meier, R., & Yeo, D. (2013). Rapid evolution of troglomorphic characters suggests selection rather than neutral mutation as a driver of eye reduction in cave crabs Biology Letters, 9 (2), 20121098-20121098 DOI: 10.1098/rsbl.2012.1098