Tuesday, 21 June 2022

Evolution News - Why Cave-Dwelling Species Appear to be Degenerate

The olm, Proteus anguinus

Photo: Nacionalni park Una/Wikicommons (CC BY-SA 4.0)

White, fat and blind — Economy and evolution in caves: Current Biology

One of the aspects of evolution that creationists find difficult to cope with is the fact that, in the right environmental conditions, evolution can result in a loss of complexity, whereas, to make it easier to attack, creationist dogma defines evolution as an increase in complexity (which they then claim is impossible due to misrepresentation of the Second Law of Thermodynamics and Shanning Information Theory). For example, many parasitic organisms, particularly intestinal worms, have lost their digestive systems and absorb nutrients directly from the pre-digested contents of their host’s gut.

Other examples can be found in species adapted for living in caves which often lose their eyes and/or pigmentation. Once it was thought that these were examples of regression as these were the remnants of competition with their fitter, surface-dwelling relatives which had taken shelter in the safety of caves. Darwin found them difficult to explain because there seemed to be no obvious way in which a loss of function could be advantageous. At one point, Darwin seemed to accept a Lamarckian explanation for the loss of eyes in that their loss was a direct consequence of their lack of use.

Cavefishes of the genus Sinocyclocheilus, showing various degrees of troglomorphism, such as eye reduction, pigment loss and extended barbules, from top to bottom.

Image: © 2022 Danté Fenolio, www.anotheca.com.)

However, in an article published yesterday in Current Biology, Senior Reviews Editor, Florian Maderspacher, points out that these changes are a response to the harsh environment and low level of resources in a cave environment and so are the result of environmental selectors, just as much as other evolutionary changes are. It's all a matter of cave economics.

Without light, there is no basis for generating energy locally by photosynthesis, so, with the few exceptions of caves populated by chemosynthetic bacteria which can use the minerals dissolved from the cave walls, all energy must be imported from outside, either in the water which permeates through the rock, or by species that come and go such as bats and cave crickets which leave nutrient rich droppings. The basic principles of household economics apply in this situation - either increase the income or reduce the expenditure.

An additional feature is that, especially where the main source of energy is water, the supply can be subject to boom and bust, as a flood can bring in a large supply, followed by perhaps prolonged periods of little or no income, so the cave economy can follow repeated boom and bust cycles.

The Mexican cave tetra, Astyanax mexicanus.

Photo: Nicolas Rohner.
Another feature can be the absence of predators, especially for those species which are apex predators in the cave. Without the cost of predation, there is little use for defensive adaptations, of which eyes are one example since they enable a predator to be detected and avoided. In the absence of light, the use of eyes to detect predators and find prey is obviously redundant, so alternatives such as elongated barbules and feelers are advantageous.

The absence of predators changes the economic dynamics quite radically. For example, cavefish like Astyanax are at the top of the short food chain whereas their surface relatives are way down the chain and need to expend energy avoiding predators. The cavefish, by contrast, can spend less time evading predators and more time and energy finding food and mates.

The evolution of blindness is still difficult to explain although the evidence is that there is selection pressure against sight and that in different populations these selectors have operated against different alleles. Crossing related blind cavefish can produce sighted offspring indicating that blindness is simple Mendelian condition. In fact, embryonic cavefish begin to form eyes, but this is then stopped, usually due to the failure of a lens to form. Grafting a lens onto the developing embryo can result in the growth of an eye, but the eye lacks the necessary neurology to function.

So what are these selectors?

The most likely explanation is that there are no direct selectors as such, other than the economic advantage of evolving alternatives and reducing the expense of growing a useless organ. The loss of sight isn't advantageous as such, but then neither is it deleterious. It seems most probable that random mutations have resulted in loss of eyes by breaking the genes that controlled their development in the embryo. Since these mutations, that in surface-dwelling fish would be seriously maladaptive, have no deleterious effects in cavefish. The same explanation probably accounts for loss of pigmentation.

In economic terms, the cave creatures have reacted to a low income by reducing their expenditure. In short, these are examples of evolution by the interplay of cave economics and random mutations - no different in principle to any other evolutionary change which is the product of the environment acting on random mutation.

In evolutionary terms of course, there is no such thing as degeneration, devolution or regression or whatever sciencey-sounding buzzword with which Michael J Behe and the Deception Institute are currently fooling their target dupes. All evolution is progressive and adaptive in the context of the prevailing environment.

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