Evolution as a web (Introduction)

by David Turell , Tuesday, April 05, 2022, 19:24 (751 days ago)

The latest interrelationships show a web, not a bush:

https://aeon.co/essays/why-evolution-is-not-a-tree-of-life-but-a-fuzzy-network

"The hypothesis of reticulate evolution is that species are not as isolated from each other as Haeckel’s branching trees propose. Instead, species both diverge and merge together. The tree of life doesn’t look like a tree so much as the reticulated pattern of a python’s skin.

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"Veron argues that today’s corals are a product of Darwin’s classical natural selection when currents are slack, and of hybridisation when they are strong. Species separate and merge, and more so over long expanses of time and space. That’s why it was so hard for Veron to nail down what a ‘species’ is across an area as big as the Pacific Ocean.

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"Since Veron’s eureka moment, genetic tools have become more widespread and sophisticated. Does data support the hypothesis that species don’t just separate, they also merge? The answer is a resounding yes.

"It’s not just rare freaks or accidents, it’s happening all the time. And in quite divergent species too,’ said Nielsen. Roving genes have been found in every branch of the tree of life where geneticists have looked. Today, the technical terms for the process of genes moving between populations are introgression or admixture.

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"‘I think that process of splitting up and merging back together again, and getting a bit of DNA from here to there, that’s happening all the time, in all of the tree of life,’ Nielsen said. ‘And it’s really changing how we’re thinking about it, that it really is a network of life, not a tree of life.’

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"‘The reality is that a single tree to represent the entire species isn’t possible,’ Kirkpatrick told me. ‘Different pieces of DNA have different gene trees, and you just can’t represent all of that in a single diagram.’

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"...Cui, Schumer and their colleagues analysed 160 different genes in 24 different species, and built trees for each gene. Then, they overlaid all 160 gene-trees on top of each other. The branches of the gene trees flowed together and apart. Where they separated, genes meandered from one species to another.

"‘Look at that. They’re all fuzzy because they don’t agree because of hybridisation,’ Kirkpatrick said. ‘So that’s a visualisation of, I think, what [Veron] was struggling with.’

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Re-envisioning evolutionary trees as fuzzy networks raises another question: if genes rove so much, why do we see species at all? Why don’t species just lump together like so many paint pigments into a genomic version of brown?

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"Bateson surmised that if multiple genes could work together to influence a trait, it was just as likely they could not work together too. Such genetic incompatibilities would compromise hybrid health.

"While the idea is easy enough to think about theoretically, it’s tough to find practically. That’s largely because the evidence – an unviable individual – won’t have much of a presence. But recently geneticists have begun to discover gene incompatibilities that look like they keep species from merging together.

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"What we do know is that the tree of life is much more complicated and twisty than we believed it to be just a generation ago.

"I imagine having the chance to speak to my students again. I would place Haeckel’s tree on the screen and tell them that it is an anachronism. I would explain how species are not isolated on the ends of branches, that there are many more interconnections and opportunities for innovation. I would say that only by recognising its inherent fuzziness can we begin to understand evolution more clearly."

Comment: a very new view. Hybridization and gene transfer are very active processes. Humans are an interconnected part of the web at its endpoint. Let's hope dhw doesn't try to slice it up.