Tree Climbing

Readers were busy this weekend, posting over fifty comments to my last post about HIV. Much of the discussion was sparked by the comments of a young-Earth creationist who claims that the evolutionary tree I presented was merely an example of microevolution, which–apparently–creationists have no trouble with. This claim, which has been around for a long time, holds that God created different "kinds" of plants and animals (and viruses, I guess), and since then these kinds have undergone minor changes, but have never become another "kind."

Some readers expressed frustration that the comments were getting side-tracked into arguments about creationism. I take a pretty relaxed attitutde to what goes on in the comment threads, though. Part of that attitude, I’ll admit, comes from the fact that I don’t have the time to hover over the comments all day. But I also don’t relish the thought of shutting down discussion, except of course when comments come from pornography-peddling bots.

I myself find that objections to evolution frequently turn into good opportunities to discuss interesting scientific research. For example, let’s take the claim that an evolutionary tree of HIV merely documents microevolution.

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Here’s the tree from my last post, published in The Lancet. It compares a new aggressive, resistant strain of HIV to strains taken from other patients. These viruses all descend from a common ancestor. The descendants mutated, many mutants died, and some mutants thrived, thanks to their ability to evade the immune systems of their hosts. Strains that share a closer common ancestor fall on closer branches.

This new strain belongs to a group of strains known collectively as HIV-1. What happens if you compare HIV-1 to viruses found in animals? Is it impossible to link these viruses together on a single tree? Were they all created separately, each to plague its own host? That’s what one might expect if indeed the "microevolution-yes, macro-evolution no" idea was true. After all, viruses that infect different animals are generally different from one another. They can only survive if they have biological equipment suited to their host species, and different species offer different challenges to a virus.

It turns out that the same approach used to compare HIV strains found in individual people works on this larger scale. Scientists can draw a tree.

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Here is the most up-to-date version of the tree, which appears in the latest issue of the Journal of Virology. The different branches of HIV-1 are marked in black. The red branches are viruses known as Simian Immunodeficiency Virus (SIV) found in certain populations of chimpanzees. The blue branches also represent chimp SIV’s, but these are more distantly related to HIV-1. (A side note: the Lancet paper doesn’t specify exactly which HIV-1 group the nasty new strain belongs to. That’s a matter of ongoing research.)

It appears, then, that HIV-1 evolved into a human scourge not once but several times from chimp SIV ancestors. One likely route is the increasing trade in chimpanzee meat in western Africa. Hunters who get chimpanzee blood in their own wounds can become infected, and certain strains that manage to survive in our species can then evolve into better-adapted forms.

Of course, tracing back HIV-1 evolution this far leads to the question, where did the ancestors of HIV-1 come from? The authors of the review in Journal of Virology takes another step back, comparing chimpanzee SIV to SIVs from other monkeys. Does this enterprise now finally collapse? Does "microevolution" finally hit the wall, unable to explain "macroevolution"?

Nope. Here’s what they find. The tree on the left is based on studies of one HIV/SIV gene called Pol, and the one on the right is based on another called Env. SIVcpz refers to chimp SIV, and the other abbreviations refer to SIV’s found in various monkeys.

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It turns out that different genes in chimp SIV have different evolutionary histories. This is no big surprise. Virologists have known for a long time that a single animal can get infected by two different viruses, which–on rare occassion–may combine their genetic material into a single package. The scientists hypothesize that chimp SIV evolved from SIV found in red-capped sooty mangabeys as well as SIV that infects greater spot-nosed, mustached, and mona monkeys. Just as humans hunt chimpanzees, chimpanzees hunt and eat monkeys. So they may have been infected in this manner.

You can take the same walk back in time with any virus that’s been studied carefully–or any species of animal or plant. Take us. Scientists publish evolutionary trees all the time in which they compare the DNA of individual people. They also use the same methods to demonstrate that chimpanzees are our closest living relatives, that primates descend from small shrew-like mammal ancestors, that mammals and other land vertebrates descend from fish, and so on. (I don’t have time this morning to grab examples of these trees, but if I have time tonight I will.) Certainly there are parts of these trees that are still difficult to make out. DNA sometimes evolves so much that a gene can wind up obscuring its own history, for example. But scientists have never hit the wall that creationists claim exists.