11) Evolutionary history matters
Evolution is often thought to be about finding optimal solutions to the problems that life throws up. But natural selection can only work with the materials at hand — materials that are themselves the results of many millions of years of evolutionary history. It never starts with a blank slate. If that were the case, then tetrapods faced with the task of moving on land would not have had their fins transform into legs; they might perhaps have evolved wheels.
A real-life case of the ingenuity of adaptation concerns a moray eel (Muraena retifera), a long, snake-like reef predator. Historically, bony fish use suction to catch their prey. A fish approaching food opens its mouth wide to create a large cavity into which prey and water flood. As the excess water leaves through the gills, the fish sucks the prey down into its throat and pharyngeal jaws, a second set of jaws and teeth derived from the skeleton that supports the gills. But morays have a problem because of their elongated, narrow shape. Even with their jaws agape, their mouth cavity is too small to generate enough suction to carry prey to their
pharyngeal jaws. The solution to this conundrum was documented in 2007.
Through careful observation and X-ray cinematography, Rita Mehta and Peter Wainwright from the University of California, Davis, discovered evolution’s breathtaking solution. Rather than prey coming to the pharyngeal jaws, the pharyngeal jaws move forwards into the mouth cavity, trapping the prey and dragging it backwards. This, the researchers say, is the first described case of a vertebrate using a second set of jaws to both restrain and transport prey, and is the only known alternative to the hydraulic prey transport reported in most bony fish — a major innovation that could have contributed to the success of moray eels as predators.
The mechanics of the moray’s pharyngeal jaws are reminiscent of the ratchet mechanisms used by snakes — also long, thin and highly predatory creatures. This is an instance of convergence, the evolutionary phenomenon in which distantly related creatures evolve similar solutions to common problems. This study demonstrates the contingent nature of evolution; as a process it does not have the luxury of ‘designing from scratch’.
Reference
Mehta, R. S. & Wainwright, P. C. Nature 449, 79–82 (2007).
Additional resource
Westneat, M. W. Nature 449, 33–34 (2007).
Author websites
Rita Mehta: http://www.eve.ucdavis.edu/~wainwrightlab/rsmehta/index.html
Peter Wainwright: http://www.eve.ucdavis.edu/~wainwrightlab
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