Genome complexity: a book extolls horizontal transfers (Introduction)

by David Turell , Friday, June 28, 2024, 18:40 (147 days ago) @ David Turell

Demonstrates importance:

https://www.wsj.com/public/resources/documents/jklT7uqz6jGqeF5D3CZe-WSJNewsPaper-6-28-2...

"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 opulations, 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.

***

"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 owerhouses” 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 abackdrop of vertical evolution. The two are complementary in revealing the complex patterns of relationships among lifeforms.” 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.'”

Comment: we have seen all of this through the years. These mechanisms are part of the engine of evolution in its advances into increasing complexity.