Evolution as a process: constant convergence (Introduction)

by David Turell , Friday, March 01, 2024, 19:04 (60 days ago) @ David Turell

Chitons demonstrate it:

https://www.quantamagazine.org/mollusk-eyes-reveal-how-future-evolution-depends-on-the-...

"A new paper published today in Science describes a rare and important test case... which is fundamental to understanding how evolution and development interact. A team of researchers at the University of California, Santa Barbara happened upon it while studying the evolution of vision in an obscure group of mollusks called chitons. In that group of animals, the researchers discovered that two types of eyes — eyespots and shell eyes — each evolved twice independently. A given lineage could evolve one type of eye or the other, but never both.

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"Chitons, small mollusks that live on intertidal rocks and in the deep sea, are like little tanks protected by eight shell plates — a body plan that’s remained relatively stable for some 300 million years. Far from being inert armor, these shell plates are heavily decorated with sensory organs that enable the chitons to detect possible threats. (my bold)

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“'Independently, chitons evolved eyes — and, through them, what we think is likely something like spatial vision — four times, which is really impressive,” Varney said. “They also evolved that incredibly quickly.” The researchers estimated that in the neotropical genus Chiton, for example, the eyespots evolved within just 7 million years — a blink of an eye in evolutionary time. (my bold)

"Varney saw that twice, independently, lineages with 14 or more slits in the head plate evolved eyespots. And twice, independently, lineages with 10 or fewer slits evolved shell eyes. She realized that the number of slits locked into place which kind of eye type could evolve: A chiton with thousands of eyespots needs more slits, whereas a chiton with hundreds of shell eyes needs fewer. In short, the number of shell slits determined the evolution of the creatures’ visual systems."

Comment: an ancient organism develops eyes by itself. This is what is called convergence, that is, the same organ appearing in many new places in evolution independently. It is as if the organisms reach into a box for a readymade tool and grab it. If that is the case, how did that naturally occur? A chance event is highly improbable as it requires many hundreds of new integrated mutations. This is where Simon Conway Morris, quite an authority, steps in and offers a designer.