One familiar argument an evolutionary proponent will encounter when dealing with creationists is the “species problem”. Essentially the argument is that there is not a definitive definition for a species. This almost inevitable argument comes up because the evolutionary proponent asks the creationist if they could define kind. Creationists believe that since biologists cannot come up with a consensus on species that applies to every organism, it gives them a free pass to not define kind.
The problem with this argument and the reason we have a “species problem” in biology is that different forms of life reproduce differently. For example, a definition for bacteria will not work for a population of mammals because they reproduce in a different manner. Thus, one is able to produce a robust definition of a species for organisms that reproduce sexually (i.e. reproductive isolation), but have a more fluid definition for species when it comes to asexually reproducing populations.
However, this is all beside the point and can be considered a red herring, thus one does not even have to address it. The main issue with this creationist argument is that the definition for kind should be vastly more robust than any definition of species. Young Earth Creationists believe that their God came to Earth, seeded all life on this planet, and made sure that each kind would reproduce after its kind. Thus, if a god(s) truly wanted to do this we would be able to see distinct genetics unique to certain kinds, which are not shared with any other animal outside of their kind. That is, there should be genetics shared only between the cat kind or dog kind that are not found in other organisms and we should not be able to find shared genetics between the two kinds.
Nevertheless, this is not what we see when we look into the genetics of life. Every time we look into the genome of an organism, we can see its shared life history with every other living organism on Earth. To date, we have not found a gene sequence unique to a group of organisms except at the species level, and those unique genes are what make that species different. It is this fact that is the real reason creationists refuse to define kind and would rather hide behind the “species problem” when asked to define kind. If the creationists were correct, and god(s) created different kinds then geneticists would be unable to create phylogenetic trees linking all organisms into clades based on their evolutionary history. To make this problem worse, other phylogenetic trees, based on morphology, embryology, etc…, should not be able to produce similar (statistically the same) trees. One would think that their genetics would be different, since all the kinds were created separately with no relation to the other.
Thus, the next time a creationist refuses to define kind, kindly remind him that comparative genetics has definitively proven universal common descent and that there have been no genetic markers to indicate that there ever were unique kinds. The “species problem” is not equivalent to the lack of a definition of kind.
Edited by Dean, 25/02/2013
Reason for edit: Minor corrections of grammar & punctuation.