The "cognitum" is a concept in creation taxonomy that groups animals according to the perceptions that humans have about those creatures. I have been a fan of the idea of the cognitum since I first heard about it from Sanders and Wise's paper at ICC. The goal is to develop a standard of taxonomy based specifically on human perception, and not at all on other standards such as genetic data or morphology.
I found this idea extremely intuitive. There is obviously the Biblical reason that Adam was given to naming each kind that God created. Therefore, perhaps God gave Adam (and by extension the rest of us) the power to discern important relationships. It is interesting, for instance, that even children can usually tell, from a simple drawing, the difference between a cat and a dog, despite their relatively similar morphology, combined with the simpleness of the drawing. The same child can, at least by Sanders and Wise's paper, look at a more bizarre representative of the cat family and still identify it as a cat.
But, I think there is another point worth looking at. When there is a debate about the phylogeny of a species on whether it should be grouped according to its morphology vs. its DNA sequences, how is such a decision decided (or for that matter, when any two trees are in conflict)? I think few people think about how tough a question this is. No one saw the type of animals who were the current animals' ancestor. Therefore, we must lean on secondary evidence. But if the secondary evidence is in conflict, there seems to be some sort of a faculty in the human mind that makes such discernments. It is neither perfect nor consistent, but nonetheless it is there.
Sanders and Wise's paper has a whole host of interesting points:
Anyway, as you can see, there were a lot of things that jumped out at me.
I also had a thought - I wonder if the "fuzzy boundary" organisms might have originated in locations with a low diversity of species. So, basically, an organism "sensed" the lack of biological character space, and then morphed to fill it.
It is interesting to compare this notion of taxonomy with a study on <a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1635482/?tool=pubmed">adaptive radiations in cichlid fish</a>. I have not read it in detail (a commenter on UD pointed me to it), but the abstract says, "The evidence suggests that speciation rate declines through time as niches get filled up during adaptive radiation: young radiations and early stages of old radiations are characterized by high rates of speciation, whereas at least 0.5Myr into a radiation speciation becomes a lot less frequent."
But even more interesting is this statement -- "The available data suggest that the propensity to undergo adaptive radiation in lakes evolved sequentially along one branch in the phylogenetic tree of African cichlids, but is completely absent in other lineages."
This indicates that there might be a "basal-type" species which is presumably more similar to ark-based species than others, from whom adaptive radiations tend to take place. This would be super-awesome if it is true.
Sanders also has a newer paper out on the application of the cognitum, but I haven't had time to read it yet. Wood has a basic overview for those interested. A quote from Wood quoting Sanders:
What is most striking from the results compiled in Appendix A is the high level of support by the molecular data for the circumscription of the core groups of most of the primary cognita identified. ... This suggests that the core groups of primary cognita are units that are generally internally consistent morphologically, as well as genomically. ... The decoupling of molecular similarities from morphological similarities just above the family/order level suggests that the circumscribed core groups of cognita at this level or the subfamily/family level may closely reflect the constitution of holobaramins represented by them. In fact, more precise methods of documenting both the decoupling of morphological and molecular characters and mosaic recombination of these characters, so easily depicted in a cognitum system, may eventually prove to serve as a criterion in delimiting holobaramins.