One fish, the Mexican Tetra (Astyanax mexicanus), is being studied extensively to help understand the evolutionary mechanisms at work. This species has two distinct forms. Fish living in streams above ground have normal eyes, while those that live in deep caves are blind.
Loss of Sight in the Dark
Presumably fish that were swept underground many years ago gradually evolved to fit their new (cave) environment. Eyes serve no purpose in the dark, but energy is required to build and maintain them – fish without eyes are a little more energy efficient, so the population evolved by doing away with these superfluous structures. Animals evolve in ways that fit them to their environment and way of life, and often a simpler body-plan is more efficient – why have eyes when they are not needed?
The really interesting thing is that there are about thirty different populations of Blind Mexican Tetras, all in different cave systems, and presumably all evolving the blind state independently. In each cave system the ancestral (‘normal’) fish were washed underground and evolved blindness. All the blind fish look very much the same, but differences at the molecular level show that they have evolved independently.
When Cave Roof Collapses Eyes Return
In one case (in Caballo Moro, Mexico) part of the cave roof collapsed a long time ago, letting light in to part of the system. In this situation it would be beneficial to have functional eyes in the illuminated regions, while they would be useless in the permanently dark parts. Here – in this unique case – both forms of the Mexican Tetra are found (‘normal’ and blind).
Molecular studies suggest that the fish with eyes evolved from within the blind population, and are not another batch of ‘normal’ fish washed in after the roof collapsed. Both forms in this cave seem to be closely related, and, while they are quite capable of interbreeding, seem to avoid one-another. The blind fish stick to the dark regions while those with eyes stay where there is light.
Evolutionary Mechanisms
Richard Borowsky is studying the DNA of these fish to see how closely related the different populations are. This is a sort of ‘natural experiment’ – where researchers can glimpse evolution in action. It is happening in thirty different places, and detailed comparisons (of the distinct populations) might shed light on the fine detail of evolution by natural selection.
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