"The Network of Life: A New View of Evolution
By David P. Mindell
Princeton, 272 pages, $27.95
In "The Network of Life," David Mindell attempts to lay out what his subtitle calls "a new view of evolution." There's a lot going on in evolutionary biology, and our understanding of evolution continues to, well, evolve. Mr. Mindell's book is a valuable contribution for anyone who wants to keep abreast of current developments, though it doesn't quite deliver the "new view" it promises.
Rather, "The Network of Life" offers a modification of the more conventional, "vertical" perspective on evolution, in which living things evolve by passing genes down through succeeding generations. Mr. Mindell, a senior researcher at UC Berkeley's Museum of Vertebrate Zoology, offers an account of "horizontal" evolutionary processes that are not an alternative to the established view but a tweak and an addition.
Vertical descent from common ancestry is fundamental to our understanding of evolution. Charles Darwin analogized the panoply of life to a tree with a thick, basilar trunk branching out into numerous limbs, all connected through shared descent. Here is the master himself: "The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth."
Phylogeny -- the tracing of evolutionary relationships -- relies on the tree metaphor, though Mr. Mindell is correct to draw attention to the simultaneous reality of horizontal networks. What animates "The Network of Life" is not a radical revision of our current understanding of biological interconnection or the mechanism of evolutionary change. The book instead offers an updated, more sophisticated appreciation of how some living things, some of the time, exchange genes with members of the same generation.
The most familiar example of horizontal gene exchange occurs when individuals of closely related species interbreed, producing hybrids and thereby introducing genes from one lineage into another.
"Researchers are finding hybridization among species to be much more common than previously thought," Mr. Mindell writes. "Recent estimates are that roughly 25 percent of the world's flowering plant species and 10 percent of animals have arisen through hybridization."
Introgression -- the mixing of genes between species -- has been revealed in human ancestry by the presence, in modern populations, of Neanderthal and Denisovan DNA.
Mr. Mindell points to other cases of introgression, including between coyotes and gray wolves and between brown and polar bears. "All hybridization phenomena, including introgression," he writes, "qualify as horizontal evolution, because genetic material is exchanged between different species, rather than between parents and offspring, the path of vertical evolution. They denote networking rather than branching."
Birds of a feather may flock together, but they do not literally give genes to one other -- the actual exchange manifests in offspring. Thus inter-species hybridization still has a vertical component.
A notable exception is recombination, a process that is widespread in bacteria, archaea and certain viruses. Among these populations, individuals will occasionally connect, exchange genetic material and then go their separate ways: the equivalent of a one-night stand, with important consequences for human health.
It is in part because of their penchant for such networking that the viruses that cause AIDS, influenza and Covid-19 are so quick to evolve and thus so difficult to combat.
As "The Network of Life" ably demonstrates, horizontal evolution has shaped ancient processes that have set the stage for life as we know it. Mr. Mindell pays special attention to endosymbiosis, in which one tiny organism comes to reside inside another, sometimes creating a merger. "Some of the most consequential innovations in life's 3.8-billion-year history," he writes, "stem from a joining of previously distinct lineages by endosymbiosis." The process gave rise to mitochondria, the "energy powerhouses" of our cells, and to chloroplasts, the intracellular denizens that enable plants to conduct photosynthesis.
Mr. Mindell recognizes that the horizontal perspective does not supersede its vertical counterpart: "Horizontal evolution is often most reliably identified against a backdrop of vertical evolution. The two are complementary in revealing the complex patterns of relationships among life-forms." He argues that, compared with the conventional narrative, life should be envisioned "as a vast tangled system of streams, variously dividing, joining, meandering, and dividing again, as it carries and integrates species and their genes through time, with succeeding generations linked by currents and networks of heritability."
Mr. Mindell is an excellent guide to this complex picture. Considering "both vertical and horizontal evolution together," he writes, "is like gazing at a world map on a school classroom wall with the country and state borders clearly drawn, and then superimposing another map showing the highways and the airplane, train, and ferry routes that cross the borders and seas." This, he argues, "is a richer, more connected map."
"The Network of Life" may be a bit overstated at times, but the book offers good, solid science and a clear illustration of an emerging perspective that is beginning to pay dividends in our understanding of evolution." [1]
1. Natural Mixer. Barash, David P. Wall Street Journal, Eastern edition; New York, N.Y.. 28 June 2024: A.13.
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