Contributed by Mahmoud Eljalby
As organisms evolve, they become more complex, right? NO! Organisms continually evolve, this is a fact; but it is far from true to say that organisms evolve to become more complex. Some do, certainly; but others ‘lose the complexity’. Unbelievable, right?!
Well, let’s look at a couple examples. Like seafood? Let’s take a case study in fish. In 1997, researchers found that sightless A. mexicanus cavefish evolved from eyed, surface-dwelling forms! That is not all. The researchers also concluded that the loss of sight evolved independently at least three times! This shows that natural selection is strongly favoring cavefish that lost a complex trait—in this case, eyesight. The next time you eat sightless fish, you’ll know where it came from!
Not only that, but in 2012, researchers at the University of Maryland, College Park, found that natural selection favors sightless cavefish with smaller eyes. Pachón cavefish live in extremely dark environments: they live in caves. As such, having a functional eye that allows you to see in light is of no use—there is no light. Losing your ability to see, on the other hand, will give the cavefish a great advantage. The researchers found that those cavefish developed over time a non-visual sensory system: that is a sensory system that does not depend on light (and hence more useful in a cave). The genes responsible for this system were also found to indirectly promote eye regression. Through a trade-off between the evolution of a non-visual sensory system and eye regression during the adaptive evolution of the fish in the cave environment, nature was selecting for the cavefish that had the non-visual sensory system and hence had smaller eyes.
Not convinced by this example to know that evolution is not always to the more complex? Well, you can always look at many other examples, such as sightless naked mole rats and other fish species—just to name a few.
Also, check out some of the recent research on convergence:
Rétaux S, Casane D. 2013. Evolution of eye development in the darkness of caves: adaptation, drift, or both? EvoDevo, 4:26 http://www.evodevojournal.com/content/4/1/26.
Nikitina NV, Maughan-Brown B, O’Rian MJ, Kidson SH. 2004. Postnatal Development of the Eye in the Naked Mole Rat (Heterocephalus glaber). The Anatomical Record Part A, 277A:317–337.
Peichl L, Nemec P, Burda H. 2004. Unusual cone and rood properties in subterranean African mole-rats (Rodentia, Bathyregidea). European Journal of Neuroscience 19: 1545-1558.
Durand JP. 1976. Ocular Development and Involution in the European Cave Salamander, Proteus anguinus Laurenti. Biological Bulletin, 151 (3): 450-466.
Yoshizawa M, Yamamoto Y, O’Quin KE, William R Jeffery. 2012. Evolution of an adaptive behavior and its sensory receptors promotes eye regression in blind cavefish. BMC Biology, 10:108. http://www.biomedcentral.com/1741-7007/10/108.
Bilandžija H, Ma L, Parkhurst A, Jeffery WR. 2013. A Potential Benefit of Albinism in Astyanax Cavefish: Downregulation of the oca2 Gene Increases Tyrosine and Catecholamine Levels as an Alternative to Melanin Synthesis. PLoS ONE 8(11): e80823. doi: 10.1371/journal.pone.0080823.