[DoctorBeaver (OP)]

While I was at the stable yard earlier I was listening to the cacophony of animal noises - horses, dogs, sheep, goats, pigs, chickens, geese - and I got to wondering why they all make different noises. What evolutionary process has driven it?

All of the above animals apart from dogs are prey for carnivores (yes, I know dogs can be predators). Would it not have been to their evolutionary advantage to all have the same alarm calls so they could warn each other of impending predation? Predators would then have had a lot more difficulty catching their prey and their numbers may well have dwindled. Evolutionarily, that would have been very advantageous to prey animals.

[blakestyger]

Good question - but if all animals made the same noise then they would be unable to distinguish between each other both at individual and therefore species level.
That would be OK if they were giving alarm signals - all creatures would be warned of the predator - but for other communications, such as territory marking, calls associated with selecting a mate etc, there would be nothing but confusion (rabbits would be wasting time trying to locate calling voles)and breeding would come to a halt and with it the natural world - except mute creatures who would carry on just the same.
Would you include humans in this too? That could be hilarious.

[DoctorBeaver (OP)]

I think evolution would preclude animals making ALL the same noises; but a common alarm call makes sense.

[blakestyger]

Sure, it would but evolution acts at species level; it's intraspecific rather than interspecific.
Off hand, I can't think of an example of an adaptation by one species that benefits another - but the mind's open.
Also, not all the adaptations that might appear to us to be 'best' take place. I think for that to happen there would need to be a teleological component to natural selection which science doesn't allow.
Thought provoking.

[DoctorBeaver (OP)]

Off hand, I can't think of an example of an adaptation by one species that benefits another - but the mind's open.

Remoras and sharks.

[blakestyger]

They're hitch hikers - does that count? Lichen are a good example though; an alga and fungus in a totally symbiotic relationship.

[DoctorBeaver (OP)]

There are quite a few symbiotic examples in nature.