I wonder what are examples of organs/structures/behaviours/cooperation that evolutionary biologists themselves find most difficult to explain -- to explain how they could appear evolutionarily -- within known mutation rates, generation frequency, and known time frames. Excluding issue of appearance of life itself.
Knowing rates of mutation and generation cycle (and population size), it would be possible to estimate time needed for appearance of new feature.
Thus it would be possible to spot counter-examples, such examples that are probabilistically beyond possibility to appear in given evolutionary time. Are such examples known ?
I might be naive, but I think some rare behavioral trait can be a candidate, behavior that does not trigger even once in a lifetime of organism, on average, that triggers really rarely.