Why don’t humans have tails or gills? Why aren’t we covered in hair? Why don’t we have all the traits of our primitive ancestors? When we think of the advance of evolution, we may think of increasing complexity, but many traits are lost due to regressive evolution. Regressive evolution is the disappearance of an ancestral trait or characteristic over time.
 
A. mexicanus Surface Fish and Cavefish; Courtesy of William Jeffery |
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In order to better understand how regressive evolution occurs, scientists have found a model organism among cave-dwelling animals. Cavefish, like most cave-dwelling animals, have lost their eyes and pigmentation over time. And interestingly, they can interbreed with their surface-dwelling counterparts who still have eyes and normal pigment, making them a model tool for understanding the genetics of trait loss.
How are traits lost in evolution? It may be that genes responsible for these traits are mutated or lost due to their lack of advantage. Or perhaps it is somehow more beneficial to not have the lost trait due to energy conservation. A third theory is that lost traits may be negatively linked to other traits that now benefit the organism in a new environment. When one gene affects many traits, this is known as pleiotropy.
In a recent review in the journal Annual Review of Genetics, William Jeffery, a professor at the University of Maryland’s Department of Biology, discusses regressive evolution in the cavefish Astyanax mexicanus. Known as the Blind Tetra, A. mexicanus is native to 30 known caves in Mexico but can now be found in laboratories around the United States due to its short reproduction time, simple diet, and frequent spawning. Although cavefish appear eyeless, they actually have small nonfunctional eyes in a smaller eye socket completely covered with skin. In Jeffery’s lab, when a cavefish embryo is transplanted with a surface fish lens, the cavefish is able to develop an eye, containing a retina lacking pigment. The lens of the eye prevents cell death in the retina. Because the surface fish lens was able to rescue eye development, this suggests that the lens has a fundamental role in regulating eye degeneration and may be the crucial trait that is lost in the regressive evolution of eyesight.
From genetic studies, it is estimated that up to twelve separate genes are responsible for eye loss. One of these genes (known as shh), affects both eye degeneration and the enhancement of oral and taste bud development, traits that would be advantageous to a cave-dwelling fish. This indicates that pleiotropy may play a role in eye regressive evolution. Jeffery stresses that more than one of the three possible theories for regressive evolution may be responsible for eye loss in cavefish. The search to identify all of the genes that affect eye degeneration in A. mexicanus is underway and may lead to insights about our own regressive evolution.
I wish I could trade in my spleen for a tail…
