When there is a rational necessity to a step occurring despite the chance of that step occurring being random.
This refers mainly to a forced move in evolution, where an organism is forced to select for a particular trait despite the mutation leading to traits being random. Frequently these are circumstances that are contingent in the realms of physics and logical possibility but necessary in the realm of biology.
“So at least some ‘biological necessities’ may be recast as obvious solutions to most general problems, as forced moves in Design Space. These are cases in which, for one reason or another, there is only one way things can be done. But reasons can be deep or shallow. The deep reasons are the constraints of the laws of physics — such as the Second Law of Threnodynamics, or the laws of mathematics or logic. The shallow reasons are just historical. There used to be two or more ways this problem might be solved, but now that some historical accident has sent us off down one particular path, only one way is remotely available; it has become a ‘virtual necessity,’ a necessity for all practical purposes, given the cards that have been dealt. The other options are really no longer any other options at all.
The marriage of chance and necessity is a hallmark of biological regularities. People often want to ask: ‘Is it merely a massively contingent fact that circumstances are as they are, or can we read some deep necessity into them?’ The answer is almost always: Both. But note that the type of necessity that fits so well with the chance of random, blind generation is the necessity of reason. It is an inescapably teleological variety of necessity, the dictate of what Aristotle called practical reasoning” (129, Darwin’s Dangerous Idea)
Darwin’s Dangerous Idea by Daniel Dennett