Blute Blog

Blute's blog about evolutionary theory: biological, sociocultural and gene-culture.

Tangled Trees?

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David Quammen book, The Tangled Tree: A Radical New History of Life was reviewed in Nature Aug. 2 by John Archibald (not incidentally the author of a book on symbiosis). It was also reviewed in the New York Times Book Review on Aug 14 by Erika Check Hayden under the title of “Uprooted”. On Aug 19, Quammen himself published an article in the New York Times Magazine titled “The Scientist Who Scrambled Darwin’s Tree of Life”.

Quammen’s book is largely about Carl Woese, the great pioneer of molecular phylogenetics, who in 1977 with coauthor Fox, made clear from their study of a component of ribosomes (protein factories in cells) that there are three not two domains of life – the Archaea in addition to Bacteria and organisms with nucleated cells like plants, animals, fungi and some unicellular organisms (Eukarya). Woese was apparently not a fan of Darwin – he commented to a prospective co-author of a book to be titled Beyond God and Darwin, “Jan, you accord Darwin so much more substance than the bastard deserves”.

Anyway, the theme of the reviews and article (and I assume of the book which I have not, or at least not yet read), is that life is better described as a “network”, a “tangled web”, or a “topiary” rather than Darwin’s tree because of the existence of horizontal i.e. lateral gene transfer (LGT ). LGT can take place by transformation (taking up DNA from the environment), transduction (a piece of host DNA carried to another organism by a virus), and conjugation (bacterial “sex” which is unidirectional). Unfortunately Hayden casually equates LGT with “swapping genes” which is exactly what it is not i.e. bidirectional rather than unidirectional. Not does its existence invalidate Darwin’s tree metaphor for evolution as these authors generally seem to think. Quammen’s article is illustrated with outrageous illustrations of what looks like an early hominid with a chicken’s head, one with a fish’s head, a human with what looks like a frog’s body between head and shoulders on top and lower legs and feet on the bottom. Perhaps we should not blame Quammen for these illustrations but the point is that because laterally transferred DNA is normally just a tiny part of the genomes of eukaryotes, the best metaphor I have seen is “trees with some cobwebs”. (In 2005 Liza Gross in PloS attributes this metaphor to Fan Ge and coauthors.)

Yet, once anisogamy (micro and macro gametes) have evolved, then sex again becomes unidirectional, i.e. from males or male functions to females or female functions. But that is unidirectional within a species – indeed the ability to interbreed after Mayr is what is most commonly said to define species boundaries. Of course Woese was right that we know very little of what went on in the early history of life. However, only if hybridization (sex between members of different species) were rampant in the Eukarya could Darwin’s trees truly be said to be entangled and despite the occasional case there is no evidence of that.

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Written by Marion Blute

September 6, 2018 at 7:50 pm

Posted in Uncategorized

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2 Responses

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  1. Eucaryotes like us are often menageries where > 90% of the DNA of the organisms that compose us are inside bacteria, mostly in our guts. Factor in the likely viral origins of transposons, retrotransposons and their junky residues, and “horizontal” gene transfer starts to look as though it may be more than just “cobwebs” – IMO.

    tmtyler

    October 2, 2018 at 1:35 am

    • Fair enough up to a point, but I would not assume that most of those symbionts are horizontally acquired rather than inherited from the mother and beyond her from distant ancestors.

      Marion Blute

      October 2, 2018 at 12:43 pm


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