When I say "all reproduce", I mean all survive conditions that can reasonably expected to occur regularly over the lifetime of the individual in the environment in which it lives and produce offspring that are capable of having offspring themselves with in the parameters provided by the eniviroment. For instance, lung fish have rudimentary lungs so that they can survive the seasonal fluctuations in the water level of their habitat. However, it would be extremely unlikely for any lungfish to survive if all the water sources in their habitat dried up to the point that the earth was cracked and dry since, as I recall, they have to lay their eggs in water or mud. Such "rare stressors" include things such as bolide impacts, pyroclastic flows, and flash floods as they all select without respect to the individual's phenotype even when variation induced by drift and mutation is taken into account. In other word, "all reproduce" refers to those individuals that are subject to selective pressures that select with respect to phenotypes that can be reasonably expected to exist in the population due to variation induced by drift and mutation.
Would the following, so far theoretical, case fulfil your criterion:
- There is a population which has a trait A which rarely becomes modified to trait A*. Trait A* may or may not be externally visual, so the individuals with trait A may or may not be able to recognise if another individual has trait A or trait A*.
- The population consists of obligate sexuals.
- The occurrence of trait A* is sufficiently rare that two individuals with trait A* will never meet.
- Trait A* may or may not impair the survivability of the individual in common situations. In fact, trait A* may even be beneficial to the survivability of the individual in some cases.
- If an individual has trait A*, it may or may not be able to court an individual with trait A. In fact, trait A* may even make courtship more likely to have a positive outcome for the individual (
i.e., it may make it more likely that the individual gets some...)
- If an individual with trait A* manages to court an individual with trait A, it may or may not be able to copulate with it.
- If an individual with trait A* manages to copulate with an individual with trait A, it may or may not result in offspring which survives to be born, hatched, or similar.
- If a coupling between an individual with trait A* and one with trait A results in offspring, this offspring is never fertile.
I'm sorry if this has been done before, but I don't think I have ever read any of the discussions you've been involved in before.