From species to hybrids to horizontal gene transfer; from tree to web of life.

David Quammen, Blurring Life’s Boundaries, Anthropocene, 2019.

From near the end of the article:

Genes going sideways among animals? That was definitely supposed to be impossible. It wasn’t.

HGT started showing up among insects as well. The most dramatic case was one species of fruit fly, which had accepted almost the entire genome of a bacterium known as Wolbachia—more than a million letters of genetic code—into its own nuclear genome. Again, this was supposed to be impossible.

By 1999, discoveries had progressed to a point such that Ford Doolittle, a highly respected researcher and theorist based in Halifax, Nova Scotia, published an overview paper in Science that put HGT at the center of a new discussion: whether it’s even possible to classify organisms into some “natural order” by placing them on a schematic tree of life. Doolittle illustrated—literally—the difficulties with his own hand-drawn figure of what he called “a reticulated tree.” To his surprise, the editors of Science published it along with his paper.

And more recent research has found evidence of bacterial DNA transferred horizontally into the genomes of human tumors. What that dizzying revelation means is still unclear, but there’s at least some chance that such insertions might play a role in causing cancer. Putting horizontal gene transfer on the list of suspected human carcinogens brings it out of the realm of microbial arcana.

The cumulative effect of these discoveries has been to challenge three concepts that we have long considered categorically solid: the concepts of species, of individual, and of the tree of life. Now we can understand better. The boundaries between one species and another are not nearly so clear and impervious as we thought. The living individual, including the human individual, is a singular thing, yes, but at the same time a mosaic of life forms and genes of varied origin. And the tree of life, as I’ve said earlier, is not a tree. That is, life’s history doesn’t conform to the pattern of any arboreal plant you’ll ever find in a forest. Again, it’s more tangled.

H/t 3QD.