South Florida Sun-Sentinel Palm Beach (Sunday)
Sometimes, evolution goes sideways
Author falters as he tries to debunk Darwin’s family tree of all species
Long before he published “On the Origin of Species,” Charles Darwin jotted down his evolutionary ideas in pocket notebooks. One of the most famous of his notes is a drawing from 1837, not long after Darwin returned to England from the voyage of the Beagle. Below the words “I think” is his sketch of how new species could arise by a branching process — explaining how species are descended from a common ancestor. This was a revolutionary idea that remains central to evolutionary biology.
Key to this idea is vertical inheritance. Traits are passed down from generation to generation: Your DNA comes from your parents, just as, on a larger time scale, humans and chimpanzees inherited their DNA from a common ancestor that lived about 7 million years ago. We can reconstruct the evolutionary past by comparing the sequences of this passed-down DNA because, occasionally, copying errors — mutations — occur between generations, so that within any lineage the DNA sequences change slowly over time.
But in his new book, “The Tangled Tree,” science writer David Quammen sees Darwin’s tree image as simply “wrong,” for inheritance may sometimes be horizontal. Imagine that a chunk of orangutan DNA somehow got incorporated into human DNA, perhaps transferred by a virus that first infected an orangutan and then a human. This would be horizontal, not vertical, transmission of DNA. If you looked only at that one piece of DNA in comparing humans, chimpanzees and orangs, you’d falsely conclude that orangs are closer relatives to humans than are chimpanzees. The one transferred piece of DNA would be identical or, at least, very similar between orangs and humans, suggesting that the two had a very recent common ancestor.
Quammen is right that the horizontal transfer of genetic information does complicate our effort to understand the evolutionary past, but he goes too far in claiming that it undermines any and all attempts to reconstruct the evolutionary past: “The tree of life is not a true categorical because the history of life just doesn’t resemble a tree.” Before accepting this radical conclusion, we must answer two questions: How in practice can horizontal genetic transmission occur, and how common is it?
Viruses are, in effect, genetic parasites that insert themselves into the DNA of other species. They can thus move their own genes from species to species, and sometimes they can also act as couriers, transferring DNA from one host species to another. More remarkably, more complex species can simply incorporate genes from the environment. This process of incorporating environmental DNA is practiced extensively by microbes.
It is useful to think of vertically transmitted DNA as providing the evolutionary signal, while horizontally acquired DNA in the same species is evolutionary noise, potentially obscuring the signal. When we look at the natural world, what do we see? Does noise drown out the signal?
Quammen highlights the discovery in 1977 by University of Illinois at UrbanaChampaign biologist Carl Woese, the book’s central figure, of a major new group of organisms, the microbes placed into the domain of Archaea. Once considered to be regular bacteria, Archaea, often denizens of extreme environments like hot springs and salty ponds, are genetically far removed from “true” bacteria.
Woese pointed out that Archaea are so genetically distinct from the known domains of life that they should inhabit a third domain alongside the two traditional ones: the Eubacteria (regular bacteria like E. coli) and organisms like us with cells that have a nucleus (Eukarya). Woese’s revolutionary “Three Domain” idea depends upon our ability to accurately recon- struct the family tree of these groups. Yes, it’s messy — there’s plenty of noise there; but it’s not so messy that the signal is swamped. Thus Quammen’s thesis is contradicted by one of the important discoveries he highlights.
But what about the situation in complex, multicelled species like our own?
Consider a DNA segment that has been horizontally transferred into human DNA from a weasel; analysis of this will suggest that our closest relatives are weasels. Finally, we take a third segment of DNA — a vertically transmitted one — that shows us to be most closely related to chimpanzees. Which of the three conclusions should we trust?
The solution is to investigate that question using many independent segments of human DNA. Let’s say we analyze 100 segments, finding one showing orangs to be our closest relatives, one showing weasels and 98 showing chimpanzees. The conclusion: Our closest relative is the chimpanzee, despite the orang and weasel noise. And this, in fact, is the way evolutionary trees are constructed: Biologists use many genes that reflect the ancestry of populations and species, not the occasional gene transferred horizontally between distantly related groups.
In the end, Quammen provides us with a lucid guide to a lot of interesting science, but he overstates the impact of horizontal genetic transmission on our ability to reconstruct Darwin’s diversifying evolutionary tree. Today, that sketched-out tree of Darwin still looks good, even if, a la Quammen, we should add a couple of faint dashed lines showing horizontal genetic transmission between its spreading branches.