Blind Cave Fish: Can Darwinism Be Credited for Regressive Evolution?
It is a worldwide phenomenon that cave creatures go blind. Some cave fish lose their eyes entirely; in others, the eyes shrivel and lose function. In many cave fish, scale pigmentation also changes. Are these gradual modifications due to natural selection, Darwin’s mechanism of evolution, or to genetic drift? Darwin himself could not see any positive value in functionless eyes. He attributed the blindness to disuse – a Lamarckian idea. Maybe his mechanism was the better explanation after all.
Some American biologists investigated whether the changes in cave fish were due to natural selection or random genetic drift. Their publication in the upcoming issue (Feb. 20) of Current Biology1 was summarized by Science Daily. Basically, they concluded that the pigmentation changes are due to genetic drift, because sometimes the pigments grew lighter and sometimes they got darker. But since the eyes always atrophied, they ascribed the blindness to natural selection – “regressive evolution” as they called it. Evolution selects for blindness because of the high energetic cost of maintaining eyes. Their explanation of this cost brings out some amazing facts about animal eyes in general:
Is it possible that Darwin’s premise was simply incorrect? Are eyes in a cave disadvantageous, and if so, why? In essence, the argument against selection is that the cost of making an eye is trivial compared to the cost of its replacement tissue in the socket or that the developmental cost is paid by cave fish anyway because the eyes start developing and only degenerate after many cell cycles of tissue growth and replacement. However, modern physiology and molecular biology suggest that these arguments might address the wrong costs. The vertebrate retina is one of the most energetically expensive tissues, with a metabolism surpassing even that of the brain. Underscoring this high metabolic demand is the observation that one manifestation of genetic defects decreasing the efficiency of mitochondria is blindness (e.g., Leber’s hereditary optical neuropathy). Thus, maintenance of eyes might pose a significant burden in the cave environment. Increasing this burden, the vertebrate retina uses more energy in the dark than in the light because the membranes of the photoreceptor disks must be maintained in the hyperpolarized state until they are depolarized in response to light. Oxygen consumption by the vertebrate retina is approximately 50% greater in the dark than in the light. Adding further to the retina’s cost is its structural maintenance. Ten percent of the photoreceptor outer disks in vertebrates are shed and renewed each day, and the structure may be completely replaced over 35 times yearly.
So in a sense, they exonerated Darwin’s famous mechanism for its ability to explain the phenomenon. But in another sense, by underscoring the high cost of maintaining eyes with all their parts, they re-opened the question of how such a complex visual system could have evolved in the first place – by a blind process.
1Protas, Conrad, Gross, Tabin and Borowsky, “Regressive Evolution in the Mexican Cave Tetra, Astyanax mexicanus,” Current Biology, online preprint for the Feb. 20, 2007 issue.
The Bible describes a storm at sea endured by Luke and Paul (Acts 27). When the sailors realized the trouble they were in, they knew what to do: lighten the ship. Over the sides went the cargo and the tackle – of little use with a higher priority (survival) in mind. In an Old Testament story of a storm at sea (Jonah 1), the wish to survive drove another crew to toss overboard another piece of costly but cumbersome baggage: Jonah. In neither of these cases could it be claimed that survival of the fittest was helping the ships evolve into speedboats.
According to Darwinian theory, selection can be progressive and regressive. Populations can climb up a fitness peak, and slide down a fitness peak. Natural selection can add new organs and shed useless organs. But think; if the world’s living things are always undergoing neutral genetic drift and regressive evolution, Charlie’s little myth will never produce endless forms most beautiful. Everything will go extinct! Assuming that “regressive evolution” awards Charlie another medal, therefore, gives him only fool’s gold. This is not the way to explain the living world.
What have we learned? Natural selection is real. It is downward! This is the sense in which Edward Blyth (10/10/2002) and even William Paley (12/18/2003) understood it (before Darwin plagiarized their ideas and turned them upside down). Natural selection is a conservative process. It either maintains what exists or gets rid of it. It cannot generate new organs and new genetic information. As Hugo deVries quipped, survival of the fittest does not explain the arrival of the fittest. Removal of the fitless is all this case has demonstrated. Natural selection gets rid of things that inhibit survival in a storm and tosses them overboard. That is not evolution in the sense most people have been taught. Have these scientists, or Darwin, actually demonstrated that random mutations could build an eye or any other complex organ from scratch? Only in their dream-world of imagination (01/17/2007).
More importantly, these scientists have reminded us how precious and costly the organs of sense are to their possessors. Romeo may say Juliet’s eyes are like pearls, but they are much more valuable. They are the lamps of the body. It takes elaborate, costly power plants and extensive maintenance crews to keep them running. The crews must be paid daily in hamburgers, french fries and chocolate. (OK, soy, garlic, and broccoli for some.)
Darwin may be able to explain how eyes break down, but not where the blueprints and programs for eyes came from. To fail to see the sense of this is to enter Plato’s cave, where lingering too long diminishes all sense into shadows. The Darwin Party headquarters is located down there, past the twilight zone. Temptresses at the entrance lure passers by (students) with promises that greater enlightenment lies below (01/12/2007). Victims are usually afraid of the dark at first, but become seduced with the promise that the decreasing daylight will be replaced by a better, inner light of imagination (01/17/2007).
Thus the blind lead the blind into their niche with their bait and switch sales pitch. Inductees (12/11/2006) are taught the ritual: offer the Charlie Buddha, the idol of the cave (07/10/2006 footnote), his daily incense and all will go well (07/18/2006, 08/07/2003 commentary). Once acclimated and accepted by the clan, novitiates find the light of imagination to be bright, beautiful, and liberating, filled with wondrous possibilities (12/21/2005, 12/05/2006). Visions of complex creatures emerging from the void play across the screen of the mind’s eye (12/10/2006, 11/11/2006). Simultaneously, the skin grows extremely sensitive. Any suggestion that a true light can be found above ground produces a violent reaction (01/11/2007, 10/27/2006).
Beware, travelers; while you are able, come to the light. Then learn to walk in the light. Caves are interesting places to visit, but never enter without a reliable flashlight and spare batteries. Read these pages for details.