Cx = ln( minimum number of bytes to completely describe all the members of the species ),
where minimum number = current ever-changing best compression algorithm result, deployed upon on any complete stream of bytes describing the species.
note that any descriptive additional clauses needed to account for individuals who have been modified by things like getting an ear eaten off or losing a limb to an ice storm or getting the same tattoo as everyone else in the gang or being born with AIDS are NOT included.
but blue eyes/brown eyes/hazel eyes - that sort of thing are included. the iris in fact is probably as complex as a fingerprint, retinal scans are more so, and so on. things that get passed along from generation to generation.
so already, at the human level, just to pick one of the many amazing living things on this planet, this number is huge. hence the logarithm step. True, it thus corresponds loosely to the definition of thermodynamic Entropy, S, but that is a Red Herring.
Evolution does not care about Complexity. never has, never will.
so, in a nutshell, it's the order of magnitude of the description. now, whether it's useful or not - that is another story.