Carl Zimmer blog essays

Originally, the evidence indicates, many of the molecules found in eyes today were only produced in other parts of the body. But then, thanks to a mutation, the same gene began producing its molecule in the developing eye. It just so happened to have the physical properties that made it well suited to being in an eye. In later generations, natural selection favored mutations that made it work better in the eye.

But this new job in the eye may have posed a trade-off for the molecule's original job. Further fine-tuning may have only been possible when the gene went through a particularly drastic (but common) mutation: it duplicated. Now one copy of the gene could adapt to the eye, while the other continued specializing in its original job. (I wrote an essay a couple years ago about some of Piatigorsky's work in Natural History.)

Darwin didn't know about gene sharing or gene duplication, but he still managed to make some important observations about how the human eye could have evolved from a simpler precursor. Early eyes might have been nothing more than a patch of photosensitive cells that could tell an animal if it was in light or shadow. If that patch then evolved into a pit, it might also have been able to detect the direction of the light. Gradually, the eye could have taken on new functions, until at last it could produce full-blown images. Even today, you can find these sorts of proto-eyes in flatworms and other animals.

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Once scientists began to decipher the molecules of heredity, such an explanation became obsolete. Instead, some scientists translated the notion of "disuse" into the language of mutations. Like any animals, a cavefish has a small but real chance of undergoing a mutation to its DNA. In some cases, these mutations can impair the fish's eyes. In a population of surface-dwelling fish, this sort of mutation would probably make it hard for a fish to find food, and might even make it an easy target for predators. The chances of the fish passing down that mutant gene to a new generation of fish would be pretty slim. But in a cave, such a mutation would have no effect on the reproductive fortunes of a fish. Over time, the population of cavefish would accumulate lots of eye mutations, until their eyes were rendered useless.

But this "neutral mutation" hypothesis isn't the only possibility. Scientists have also proposed an "energy conservation" hypothesis. Mutations that prevent cave fish from developing eyes let them save energy, boosting their odds of survival.
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Jeffery thinks that Hedgehog may be the key to understanding what's really driving the evolution of cavefish. Like many genes involved in development, Hedgehog has many different jobs. It is known to be essential for the development of tastebuds, for example, as well as teeth and the bones that make up the head. And in cave fish, all of these features are significantly different from surface fish. It's possible that these changes are adaptations that help the cave fish feed more efficiently. These changes were only made possible by cranking up the production of Hedgehog. A side effect of this increase was the destruction of the cave fish eyes. But because eyes aren't essential in the dark, this wasn't such a big price to pay. If Jeffery is right, Darwin's real mistake with cave fish wasn't falling back on a Lamarckian explanation. It was not recognizing how powerful natural selection could be.

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In 1990, Steven Pinker (now at Harvard) and Paul Bloom (now at Yale) published a paper called "Natural Selection and Natural Language." They laid out a powerful argument for language as being an adaptation produced by natural selection. In the 1980s some pretty prominent scientists, such as Stephen Jay Gould, had claimed that the opposite was the case--namely, that language was merely a side effect of other evolutionary forces, such as an increase in brain size. Pinker and Bloom argued that the features of language show that Gould must be wrong.
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Hauser et al see three possible explanations for how this three-part system evolved. One possibility is that all three parts had already evolved before our ancestors diverged from other apes. They introduce this hypothesis and then immediately abandon it like a junked car. The second possibility they introduce could be called the uniquely-human hypothesis: the language faculty, including all its components, has undergone intense natural selection in the human lineage. Pinker and Bloom's argument fits this description. The final hypothesis Hauser et al consider is that almost everything essential to human language can also be found in other animals. Perhaps only a minor addition to the mental toolkit was all that was necessary to produce full-blown language.

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