People with mutation in this single gene lose the ability for vocal learning and speech. How do you explain that?

I would guess it is responsible for one crucial part for the "speech apparatus", but that doesn't make it the speed apparatus. A car without the ignition loses the ability to drive, yet the ignition hardly seems the most important part in the car.

I feel the same uneasyness when I hear "hormone X is the cause of ability Y" - sure, maybe if Hormone X is missing, ability Y is missing. But Hormone X does not manufacture Y, it is just a messenger that makes something else produce Y. Likewise I suspect speech is possible because of the combined effects of several genes. If one is missing, maybe the system fails - but perhaps it could be replaced with something else, given time.

Sorry, it is late, I feel unable to express myself more clearly atm.

An ignition indicates that the car has an internal combustion engine.

The speech-enabling gene FOXP2 indicates that there are brain circuits that make vocal learning possible.

The question is why this gene is present in aquatic mammals and is absent in terrestrial apes

"The speech-enabling gene FOXP2 indicates that there are brain circuits that make vocal learning possible."

That's kind of what I meant: surely that speech gene is not a gene that describes a whole brain?

"The question is why this gene is present in aquatic mammals and is absent in terrestrial apes"

Doesn't that refute the "speech gene" theory? Surely some terrestrial apes also communicate by "speech"?

I don't know enough about genetics to confidently discuss this more. Presumably if several species share a gene, they are likely to have a common ancestor?

"Vocal learning, the substrate of human language, is a very rare trait. It is known to be present in only 6 groups of animals: 3 groups of birds (parrots, songbirds, and hummingbirds) and 3 groups of mammals (bats, cetaceans[whales/dolphins], and humans).

All other groups of animals are thought to produce genetically innate vocalizations.

To understand this concept, it is important to distinguish vocal learning from auditory learning. Auditory learning is the ability to make sound associations, such as a dog learning how to respond to the sound "sit". All vertebrates have auditory learning.

Vocal learning is the ability to imitate sounds that you hear, such as a human or a parrot imitating the sound "sit". Currently only vocal learners have been found to have forebrain regions dedicated to vocal learning and production of these learned vocalizations. Vocal non-learners only have been found to have non-forebrain vocal regions responsible for the production of innate vocalizations." http://www.jarvislab.net/Evolution.html