I’m going to take a break from our regularly scheduled debunking of John Everett’s Senate testimony, to pose a question for creationists and cDesign Proponentsists: Why do people catch swine flu but not tobacco mosaic virus?

I’m not asking why people get sick. I’m not interested in a rehashing of the tired old arguments about the coexistence of god and human suffering. I want an explanation of the fact that, despite the myriad pathogens which infect other branches of the tree of life, it’s only pathogens from other animals (usually other vertebrates) which make humans sick. Poxes, tuberculosis, and anthrax infect cattle. HIV is a mutant variant of Simian Immunodeficiency Virus, which infects other primates. Why don’t we fall ill from Partitivirus, pathogen of fungi? Or T4 phage, parasite of bacteria?

It’s a common misconception that the similarities between life forms are evidence for evolution. This misconception has been exploited by creationists to advocate for common design rather than common descent:

“… a naturalist looks at similar DNA and concludes all of life comes from a common ancestor; a theist looks at similar DNA and concludes all of life had a common Creator. Same data interpreted differently because of different presuppositions.” -Tom Short, Campus Minister. [link down temporarily (?)]

Evolution is a powerful scientific explanation for biology not merely because life forms are similar, but because the similarities are patterned. Humans, chimps, lizards, crickets, and bacteria are all similar in some way or other: Humans and chimps are genetically virtually identical; humans and lizards share a vertebrate body plan; humans and crickets share genetic developmental architecture; humans and bacteria share biochemical universals. This is interesting, but by itself it’s not the point. The point is that humans and chimps are more similar than either are to lizards, and that vertebrates are all more similar to each other than they are to invertebrates, and that animals are all more similar to each other than to bacteria. The similarities and differences are structured; they’re ordered.

The tree of life, showing a pattern of nested hierarchy. You can see that each subgroup (eg, mammals) is completely contained within a parent group (eg, animals). Click for sauce.

This sort of pattern is called a nested hierarchy.  It works like this: if two organisms share a common ancestor, they also probably share traits which they have inherited from said ancestor. The more recent the common ancestor, the more similarities they share: humans and chimps went their separate ways about 6 million years ago and are biologically almost indistinguishable. The last common ancestor of humans and E. coli existed about 2 billion years ago, and we share only basic molecular machinery. Life can be divided into a collection of groups and subgroups, with each subgroup strictly contained by its parent group. All mammals are vertebrates- there are no invertebrate mammals.

We see this sort of pattern in things which organically evolve by descent with modification. Languages, for example, fit into nested categories. On the other hand, products of human agency (the only uncontroversial example of intelligent design) tend not to fall into this pattern.*

Another principle of evolution is that populations will adapt to their environment. For peppered moths, this might mean that dark moths will become more common than light moths when their habitat is covered in soot. But for pathogens, this means evolving strategies to exploit their hosts, while avoiding their hosts’ defenses. And for hosts, this means evolving defenses to protect against pathogens in their environments. As a result, host-pathogen relationships, like any other adaptation, will tend to be organized into nested heirarchies.

Here’s an example. This is a chart of diseases infecting important crop plants. I’ve color coded the effected plants according to their families. Nightshades (tomatoes, potatoes, tobacco, eggplant…) are green; grasses (corn, wheat, rice, sugarcane…) are red, and so on.

Plant diseases and their hosts, colored according to the hosts’ family. Taken from Finding New Ways to Fight Plant Diseases, Anne Simon Moffat Science 22 June 2001: Vol. 292. no. 5525, pp. 2270 - 2273. Click for sauce (subscription required, sorry :( )

The diseases only strike within a level of the hierarchy. You see pathogens which infect grasses, or nightshades, or stone fruits. What you don’t see is a disease which “jumps” the hierarchy. You never see, for example, a pathogen which infects some grasses (say, corn) and some nightshades (say, potatoes) but not other members of those families (say, wheat and eggplants).

Going back to my original question: why are humans infected by swine flu but not tobacco mosaic virus? The only answer I’ve been able to get from the antievolution crowd is some variant of:

“God/the Intelligent Designer made it that way.”

This is a really, really bad answer. For one thing, if the Designer is making life look as though it evolved, why bother with a Designer in the first place? If all we can say about a Designer is that its Design looks like the product of evolution, then evolution is sufficient to explain biology, and Design becomes an unnecessary hypothesis.

Even worse, imagine that we travelled to a parallel universe in which humans caught tobacco mosaic virus, but not swine flu. Why, we ask?

“God/the Intelligent Designer made it that way.”

The answer works equally well for the exact opposite of the original question! Science looks for explanations for our observations; creationism and intelligent design merely accommodate any given observation, and explain nothing.

*Strictly speaking, you could choose some characteristics and order human design into a nested hierarchy, but the specific pattern you got would depend on which characteristics you chose- the hierarchy you get from ordering cars according to color would be different from one according to the number of doors. The biological phylogenetic tree is independent of what characteristic you use to create it- the hierarchy based upon body structure is the same as the one based upon genetics, etc. For further reading, check out Douglas Theobold’s primer at TalkOrigins.