Classic evolutionary theory holds that species separate over time. But it’s fuzzier than that – now we know they also merge
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.
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.’
After I finished graduate school, I fell off the academic path and became a science writer. Three years ago, I started writing a book about the future of corals and discovered the research of the Australian scientist John E NVeron. Veron, nicknamed ‘Charlie’ after Darwin by a gradeschool teacher who noted his predilection for nature, is an icon of the field of coral taxonomy, the science of identifying and describing species. Reading his definitive work Corals of the World (2000), co-authored with Mary Stafford-Smith, my questions about evolutionary-tree inadequacies came flooding back.
In 1972, Veron was the first full-time researcher on the Great Barrier Reef off the northeast coast of Australia, and two years later he earned the distinction of being the first full-time employee of the Australian Institute of Marine Science (AIMS). Veron had completed his doctorate, with award-winning work on colour change in insects, but he knew almost nothing about coral. Yet, he set out to tackle the project that AIMS hired him to do: describe all the corals of the Great Barrier Reef.
The Great Barrier Reef is the most massive biologically built structure on our planet. Composed of around 3,000 smaller reefs, it covers an area greater than Italy. Cataloguing its species was no less monumental. It took nearly a decade of diving, visiting museums across Europe, and studying the work of others for Veron to inventory the more than 400 coral species of the Great Barrier Reef.
Then Veron visited the other side of Australia. There, on Ningaloo Reef, the corals he saw seemed more or less identifiable at first. But, as he looked at them longer, he wasn’t so sure.
Edited by Pam Weintraub
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