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The answer: natural selection. In the evolutionary past, there was strong selection for genes that build nerve circuits in the visual system that are stimulated to fire just by little black moving dots in the frog's visual field. Big dots won't do it, nor will motionless little ones. The advantage of such circuits is obvious. The only moving little black dots in the frog's environment are insects it can eat. There was also selection for (genes that build) other circuits wired to take an inputs the visual system's output and send their own output to other neural circuits that ultimately trigger tongue flicking. The process was just blind variation and environmental filtration. Over the evolutionary history of frogs, variation resulted in a few neural circuits connected to the eyes and the tongue in ways that happened to result in eating; most didn't. the ones that did were selected for. There is nothing special about these neural circuits or different from other neural circuits that can do this job, other than their fortuitously connecting circuits from the eye to circuits for the tongue. These neurons aren't any more about the fly than others that could do the same job but don't because they are hooked up to other circuits. Frogs aren't smart enough to learn how to flick flies by operant conditioning. Their survival is a testimony to how finely Mother Nature (aka genetic natural selection) can tune the appropriateness of responses to the environment.
We are of course, amazed at the environmental appropriateness of the frog's tongue flickering. We say, "The frog knows where the fly is. There must be neurons in its brain that contain information about the fly's speed and direction." It's perfectly natural for us to say this, but it's an illusion that results solely from the environmental appropriateness of the dot-detection to tongue-flicking chain of natural inputs and outputs. The frog's neural circuitry can't be about the fly; they can't be about anything. So, when we say that there are neural circuits in the frog that acquire information about flies, store this information, and make use of it to stimulate, direct, or control tongue flicking, we are either speaking metaphorically or we have succumbed to an illusion. ~ Page 197
When the first report that frog populations were crashing began to circulate, a few decades ago, some of the most knowledgeable people in the field were the most skeptical. Amphibians are, after all, among the planet's great survivors. The ancestors of today's frogs crawled out of the water some 400 million years ago, and by 250 million years ago the earliest representatives of what would become the modern amphibian order -- one includes frogs and toads, the second nets and salamanders, and the third weird limb less creatures called caecilians -- had evolved. This means that amphibians have been around not just longer than mammals, say, or birds; they have been around since before there were dinosaurs. ~ Page 11
Amphibians emerged at a time when all the land on earth was part of a single expanse known as Pangaea. Since the breakup of Pangaea, they've adapted to conditions on every continent except Antarctica. Worldwide, just over seven thousand species have been identified, and while the greatest number are found in the tropical rainforest, these are occasional amphibians, like the sandhill frog of Australia, that can live in the desert, and also amphibians, like the wood frog, that can live above the Arctic Circle. Seven common North American frogs, including spring peepers, are able to survive the winter frozen solid, like popsicles. Their extended evolutionary history means that even groups of amphibians that, from a human perspective, seem to be fairly similar, may genetically speaking, be as different from one another, as say, bat are from horses. ~ Page 11
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