The web of life
Classic evolutionary theory holds that species separate over time. But it’s fuzzier than that – now we know they also merge
The coral reef surrounding Sand Cay No 6, Great Barrier Reef, Queensland, Australia. Photo by David Doubilet
Juli Berwald is an ocean scientist and science writer whose work has been published in The Guardian, National Geographic and Nature, among others. She is the author of the books Spineless (2017) and Life on the Rocks (2022). She lives in Austin, Texas.
Edited by Pam Weintraub
3,600 words
I remember standing at the front of a biology classroom at the University of Southern California sometime in the 1990s and placing an acetate film on an overhead projector. The words cast onto the white screen read something like:
Species: a group of organisms that interbreed to produce fertile offspring.
More than a century earlier, Charles Darwin’s On the Origin of Species (1859) was published. Its central hypothesis held that, because populations contain variety, some members were born with characteristics, or adaptations, that made them more fit – better able to produce offspring. Others were less fit and they, along with their adaptations, were winnowed away as they added fewer and fewer offspring to future generations. This variation coupled with the winnowing was the fuel that drove changes in populations, eventually leading to populations that could no longer interbreed with each other and produce fertile offspring. Thus, new species evolved.
Darwin’s revolutionary idea was well summarised by the German biologist and artist Ernst Haeckel in the graphic form of a tree. Every one of its twig-tips symbolised a different species. The crook between two twigs represented an ancestral species that diverged into two (or more) modern ones. While many branches were pruned away, others grew ever longer, diverging into the future.
Ernst Haeckel’s illustration of the Tree of Life in The Evolution of Man (1879). Courtesy the Wellcome Collection
In that southern California classroom, I told my students that once a species diverged from its ancestor – when it became unable to interbreed and form fertile offspring – those branches were separate, forever isolated. But, even as I spoke the words, I knew something wasn’t exactly right.
I was studying phytoplankton at the time. Single-celled creatures such as phytoplankton reproduce by cell division, which makes the question of what’s an offspring tricky. When you clone yourself, which one is the ancestor?
Graduate students down the hall in a microbiology lab regularly used viruses to transfer genes from one species to another. And gene shuffling wasn’t just happening by manipulation. I’d heard seminars about how different species of bacteria naturally perform a kind of sexual reproduction called conjugation, transferring genes from one to another. How did that kind of gene-hopping fit into the concept of a branching tree?
What I didn’t know then was that, even as I ambivalently placed the overhead film on the projector, the concept of the tree of life had begun to wilt. Four decades on, it’s morphed entirely.
‘That whole abstraction of evolution as being a tree, we always knew was a little inadequate,’ Rasmus Nielsen, a geneticist at the University of California at Berkeley and co-author of the book An Introduction to Population Genetics (2013), told me by video call. ‘But now we know it’s really inadequate.’
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