Intermediate mutations don’t necessarily need to provide any benefit at all, they just need to not have any detrimental effects.
As I recall, a rattlesnake’s rattles are formed more-or-less by its skin failing to shed perfectly cleanly. That costs practically nothing and is exactly the kind of weird mutation that can crop up in an isolated segment of the population where it doesn’t take long for genes to stabilize.
Then the isolation ends, and it turns out that the weird new trait has some amount of benefit over the population at large, so it spreads.
Intermediate mutations don’t necessarily need to provide any benefit at all, they just need to not have any detrimental effects.
As I recall, a rattlesnake’s rattles are formed more-or-less by its skin failing to shed perfectly cleanly. That costs practically nothing and is exactly the kind of weird mutation that can crop up in an isolated segment of the population where it doesn’t take long for genes to stabilize.
Then the isolation ends, and it turns out that the weird new trait has some amount of benefit over the population at large, so it spreads.