We conceive of limits to variation in animal behaviors and morphology. When we deduce something about the constraints acting on the development of an organism and on the evolution of a species we must distinguish between two kinds of observed limits. The first kind is a limit due to actual biological limitations, whether on embryological development, lack of viability (i.e., animals having the variation die before they can reproduce), or lack of fecundity. The second kind is a limit on what we as scientists can actually observe due to lack of a fossil record either for the animal as a whole or for the specific variation in question, or due to limitations in our methods or devices. The upshot of this difference is that the first kind of limit is fit for building our philosophy of nature while the second kind is misleading and has the power to render our predictive tools worthless.
There are, of course, mitigations to the perfidy of the second kind of limit. For instance, we can determine in some cases that a variation, such as a distinctive birdsong sung by an ancient bird, would not be visible to our tools. In addition, constraints determined in one area may depend on constraints in another area which confound limits of the second kind. Ultimately, these ways of thinking lead to the practice of integrating heterogeneous models of living systems into a single model. The hope is that there is a dependence between parts of the system which constrain those variations which cannot be constrained through observation. In the limit, this approach frees us entirely from the "black swans" since it produces an emulation of reality. In the real world, however, where computational limitations prevent perfect copies of reality, there may yet be independent pockets of variability that we will always fail to pick up either in observations or in our models.