Friday, December 13, 2013

New mechanism for rapid evolution

Two specimens of Astyanax mexicanus, one that lived
above ground (top) and one that lived in a cave (bottom).
Credit: Nicolas Rohner.  Image source.
The Mexican tetra (Astyanax mexicanus) is a freshwater fish that lives in the rivers, ponds, and caves of Mexico and Texas.  The river-dwelling members of this species are rather small and unremarkable, having large eyes on its small head and a translucent, slightly greenish body.  The cave-dwelling members, on the other hand, look quite different and give the Mexican tetra its alternate colloquial name, the blind cave fish.  This variant possesses no eyes and no pigmentation, despite being of the same species as the surface variety.

A scientific study, published on Science magazine's website today, investigated the genetic changes responsible for the difference between the two types.  From the perspective of classical evolution, one would expect that random mutations built up over long periods of time, and those fish with beneficial mutations (no eyes, no pigment, and more sensitive pressure detection) were gradually selected.  However, this study found something quite different.  In laboratory experiments, stress related to temperature changes resulted in greater variation in the physical forms of the fish very rapidly.  Followup experiments found that one protein, called HSP90, maintained the eye size of the fish.  When surface-form fish were raised in an environment that inhibited HSP90, there was a much greater variety of eye sizes.  In nature, this protein is indirectly affected by the salinity of water, which varies between rivers and caves.  It appears, then, that when the river forms found their way into the caves, the reduced water salinity caused the fish to display more variability, allowing for a greater selection to survive through ensuing generations.

The important part to note here is that the genetic information necessary for cave life was already present in the genome; it just wasn't expressed until a particular protein was turned off.  Even then, most of the mutations necessary for such conditions, such as the loss of eyes and pigment, constitute a net loss of genetic information.  This form of "evolution" is the only kind that is observed and supported by science, and it should not be confused with the type of information-adding evolution that would be necessary for all life to have a single common ancestor.  Creationists expect this observed evolution to be able to happen quite rapidly, as we believe that the current diversity of land vertebrates originated from the pairs (or sevens) of animals on Noah's Ark around 4400 years ago.

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