What is a Kind?

So I was listening to a sort of informal debate the other day between a creationist and a couple of atheists, in which the creationist refused to engage in discussion of species and instead insisted on speaking of “kinds.” The argument he was making was basically that he believes “microevolution,” which he defines as changes within a “kind,” does occur, but “macroevolution,” which he defines as changing from one “kind” into another, does not. This resulted in the two sides talking past each other for a bit, since the atheists were taking him to mean “species,” when he was adamant that he was talking about “kinds,” but couldn’t or wouldn’t define what he meant.

Now I’m sort of assuming that what he intended it to mean is this thing creationists often talk about as Biblical kinds, referring to how Genesis talks of God’s creation of animals “each producing offspring after their own kind.” Of course, that would seem to fit rather neatly into the common definition of “species” (a group of organisms capable of mating together to produce fertile offspring). But it has been unequivocally demonstrated that it is possible to derive populations of organisms incapable to mating to produce offspring with their ancestor species, but able to do so amongst themselves, by purely natural means (i.e. the evolution of new species does occur). Thus, creationists have taken to insisting that a species is not what they’re talking about when referring to “kinds,” but rather some broader classification. The debater I mentioned above, while unable to provide a definition of a kind, tried to illustrate by example: he said cats are a kind, and dogs are a kind.

It took some further discussion to clarify that what he meant by “cats” was all felines (domestic cats, lions, tigers, cheetahs, pumas, lynxes, etc.) and by “dogs” he meant all canines (domestic dogs, wolves, coyotes, foxes, dingoes, jackals, etc.). This brought us back to his macroevolution point, which was to say that he didn’t believe any organism could evolve into a different “kind.” And he phrased it in terms of the question “How long does a dog need to evolve before it becomes not-a-dog?”

Now, the atheists in this particular debate never really got past the difference between “kind” as the creationist was using it, and “species.” This owes in part to the lack of a clear definition of “kind,” – personally, I suspect the definition of “kind” as creationists use it is “whatever level of classification is necessary to support the user’s incredulity that the members of one could be related to the members of another.” But the other problem is that the creationists’ question doesn’t actually make a lot of sense in the evolutionary perspective.

Let’s start by taking his examples of what a kind is (cats being one, dogs being another). He didn’t explicitly say it, but I think we can reasonably extrapolate that he’d consider bears (including brown bears, grizzly bears, polar bears, etc.) to be a kind, and horses (including horses, zebras, donkeys, etc.) yet another. And hopefully, by way of these illustrations, we can consider that we have a reasonable grasp of the concept he was getting at even if he was unable to define it.

But here’s the thing: those broad categories are, to an extent, arbitrary. Functionally, they really just sort of mean “these animals are similar enough to each other that we feel pretty comfortable lumping them together.” Most of the time, those similarities occur because the species in question are closely related to each other. For example, the reason all felines species share all those similarities is because in the relatively recent past their ancestors were all the same species. I think even the creationist who was making this argument would agree with that, given that he acknowledged that animals of the same “kind” could branch into different species.

Of course, when I say “relatively recent past,” I’m talking about anything between a few hundred thousand to a handful of millions of years.

Now, you may be thinking at this point “If you claim that all cats are related by ancestry, how can you say that grouping them together is arbitrary?” And the answer to that is because our perspective is limited by time.

You see, that last common ancestor shared by all cats that I just mentioned lived several million years before modern humans. By the time humans started recording this kind of stuff, virtually all of the modern cat species already existed. They had changed enough from that common ancestor in enough different directions that we could readily differentiate the species from each other, but were still closely related enough that they still shared a vast array of characteristics in common – what we might refer to as “feline features.” That’s the array of cats that we see today, and have seen throughout human history.

But suppose for a moment that we could go back in time to live when that common ancestor lived. None of the modern cats we see around us today would yet exist. Yet that “first cat” would have still have had relatives running around – other species to which it was closely related that shared a lot of features in common with it. Had we lived back then, with no knowledge of the coming evolution of “cats,” we would have likely referred to that group of closely related species as a “kind.” Let’s call that kind “feliforms.” And here we run into a handy example: the hyena.

Hyenas look a lot like dogs. Most people think they are. But genetically, they are far more closely related to cats, and there are a good number of feline physical and behavioral features in them that corroborate the relationship. Their distant ancestor living at the same time as our “first cat” would also have been part of the “feliform kind.” Hyenas, like cats and dogs, can be further divided into several separate but closely related species – if the definition of “kind” is to have any consistency, hyenas qualify as their own “kind.”

So we have two modern “kinds” (hyenas and cats) who are both descended from the feliform “kind.” So when did feliforms become not-feliforms?

The answer is “they never did.” Hyenas and cats remain feliforms to this day.

The only reason we think of cats as one kind and hyenas as another is that they had already evolved significant differences from each other before we started classifying and recording things (before, even, our ancient ancestors had even developed the capabilities to do so). If we could somehow have been around to observe the whole process from then to now, we’d just think of them as different species of feliforms – there would be no such words as “cat,” and “hyena” because they would be blended together in our thinking. The dividing line is arbitrary, based solely on our perspective in time.

Similarly, the ancient common ancestor of all feliform species would have been part of another “kind” which we’ll call carnivora, from which are descended all large carnivorous placental mammals. Here you would find the common ancestors of feliforms, caniforms (the common ancestors of dogs), ursiforms (the common ancestors of bears). Back then, they all would have been close enough to each other in appearance that we’d have called them a “kind” had we lived in that time. They were just as closely related to each other, at that point in time, as all cats are to each other today. And all of those animals’ descendants remain of the “carnivora kind” to this day. Cats are still feliforms, which in their time were still carnivora. Dogs are still caniforms, which in their day were still carnivora. Bears are still ursiforms, which were still carnivora as well. And if you go back even further, you find a common ancestor for all these “kinds.” According to fossils found in France fairly recently, he probably looked something like this:

All modern carnivorous placental mammals (cats, bears, hyenas, dogs, weasels, etc.) are believed to be descended from something like that. He belongs to a “kind” called carnivoraforms, which were the class of placental mammals that were adapting to a primarily carnivorous diet that lived roughly 55 million years ago. And every single one of those modern species is still a carnivoraform as you read this.

In a very real sense, no species ever becomes a different “kind.” We draw dividing lines between categories based on similarities and differences, which themselves are based on how recently in time those species were related. These “kinds” are just labels we apply to those categories.

So how long will it take a dog evolve to be not-a-dog? About as long as it took carnivoraforms to evolve into not-a-carnivoraform. Which is to say: in one sense, never, and in another sense, roughly until they’ve changed enough from what we traditionally consider to be dogs that we arbitrarily decide to call them something else.

An image began to form in my mind as I was trying to put this post together that may help illustrate the point. Imagine standing on a plain dotted with bushes. Let’s pretend that each bush represents a “kind,” with each branch representing a single species. Each branch clearly belongs to its own bush, and you know it can’t ever grow to be part of one of the other bushes. The ground here represents the point at which people started making recorded observations.

Now let’s say you start digging at the base of one of the bushes to see what the roots look like. You go down a few feet and discover that there’s another branch off of the trunk hidden under the dirt. So you dig out around that branch and discover that it actually is the trunk of the bush right next to the one you started at. Those two bushes that looked so distinct from each other above ground turn out to be part of the same plant!

You dig a little further, and continue to find branches. Some of them peter out before reaching the surface, but others turn out to be the trunks of all the bushes in your immediate area. Every one of them, for all that they looked from the surface like distinct individual bushes, turn out to be the same thing, and the only real difference between them is what direction they branched and how deeply under the earth (i.e., in the past) they happened to branch off.

Of course, the bushes continue to grow. And soil continues to build up on the plain. Maybe one day, the soil reaches high enough to cover the branch point on one of the bushes – at which point we might call those things two different bushes. But, knowing the truth about how they are related now, maybe we won’t. It will be our decision.

An evolutionist is someone who looks into the hole and recognizes what he is observing. A creationist is someone who stands on the plain, refuses to look into the hole, and continues to pretend that the bushes are separate things.