John Hawks on confusing necessary for sufficient, broad for narrow

The Nature News story on the gorilla genome includes this section relevant to the evolution of hearing in gorillas and humans:

Some of these rapid changes are puzzling: the gene LOXHD1 is involved in hearing in humans and was therefore thought to be involved in speech, but the gene shows just as much accelerated evolution in the gorilla. “But we know gorillas don’t talk to each other — if they do they’re managing to keep it secret,” says Scally.

This weakens the connection between the gene and language, says [Wolfgang] Enard. “If you find this in the gorilla, this option is out of the window.”

This is one of the genes that I have been working on with reference to its acceleration on the human lineage. It is a mistake to view the evolution of hearing to be directed specifically to language; instead human and gorilla lineages are both adapting to a aural environment different from ancestral hominoids. In both these lineages, there was an increase in body size and reduction in the mean frequency of vocalizations, enough to prompt adaptive changes. In humans, we have had additionally the addition of language as a communication system, which has its own auditory requirements. The connection with language is only indirect, in that human-specific changes to this and other genes provide evidence of adaptive change in the auditory system.

Hawks, eagle-eyed as usual, spots a common error here: The assumption that when we talk of a gene or a trait and its role in evolution or adaptation, it’s easy to think that because that gene or trait helped create an outcome then it more or less codes FOR that outcome: Thus a gene presumably necessary for understanding speech, Hawks argues here, has been mistaken as essentially coding specifically for that ability — rather than creating sub-abilities, as it were, that underlie the “final” trait (in this case, comprehending speech) that we happen to focus on.

Implicit here is another mistake: overlooking the multigenic nature of complex traits and abilities. How might the LOXHD1 gene be crucial to both gorillas and humans but (help) generate different auditory traits in each? Because it works with different sets of other genes in the two species, and, of course, vastly different physical and social environments.

This isn’t a dumb mistake; it’s quite understandable, and it’s easy to make. All the more reason to be aware of it.


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