Book ReviewFlorian Fisch
Arrival of the Fittest: Solving Evolution’s Greatest Puzzle.
Paperback: 304 pages
Publisher: Oneworld Publications (2 July 2015)
€12.20 (Paperback), €10.98 (Kindle Edition),.
Solving evolution’s greatest puzzle
Andreas Wagner shows how evolution depends on the sheer number of possible genotypes. Can this bioinformatician live up to Richard Dawkins as a writer?
The harlequin ladybird beetle (Harmonia axyridis) occurs in numerous colours and with between none and 22 spots. Photo: Entomart
How did the bewildering diversity of life on earth arise? “Random mutation and natural selection” is the standard answer that every college graduate would give. Apart from creationists, few would contradict this simple formula of evolutionary biology.
Andreas Wagner from the University of Zurich, however, raises serious objections. The evolutionary biologist shares at least one concern with creationists: How could there have been enough time for organisms to try out all the possibilities? In the prologue of his book, Arrival of the Fittest, he discusses the number of possible protein sequences: There are 10130 possible proteins made of just 100 amino acids, that is 1040 as many arrangements as there are hydrogen atoms in the whole universe. That’s in fact a “hyperastronomical” number!
Wagner – far from being a creationist – does not deny the amazing force of natural selection, “But given the staggering odds, selection is not enough. We need a principle that accelerates innovation.” Random mutation, for him, needs explaining. And that is, at least partially, what he does in his book.
To illustrate his case, he uses the analogy of the universal library that contains all the books that could possibly be written. In it, of course, almost everything is complete gibberish but, “if you wandered through the universal library of books long enough, you would find books that surprise you. They contain novel thoughts, ideas and inventions.” Interested readers should start to spot the parallels with evolution about now.
In this boring universal library, each book sits between two neighbours. In the strange library of all possible proteins composed of 100 amino acids, each protein has 1,900 neighbours. So, one neighbour stands for one possible substitution of one amino acid, 19 possibilities at each of the 100 positions. In his bioinformatics lab, Wagner’s collaborators make algorithms to walk through such strange hyperastronomical libraries – substitution by substitution from neighbour to neighbour, as evolution does.
Even if only one protein in 10,000 is able to fold, there are still 10126 left to perform the magic of life. Four in six billion, or as many as 1093 bind ATP. There really is no shortage of possible ATP binders. Different functions yield similar numbers. And the sequences for the same function vary enormously. Take globin-folds that, in extreme cases, share only 20 percent of their sequence. So, there are unimaginably numerous ways to arrive at the same function or fold in the library. And the overwhelming majority of proteins have never been tried by an organism, many became extinct and only very few exist today.
Wagner’s libraries are not only hyperastronomical in size but hyperdimensional in structure (1,900 dimensions for 100 amino acid proteins instead of one dimension for books). In this weird space, Wagner’s collaborators showed that there are paths – substitution by substitution or neighbour by neighbour – from one end of the library to the other without a change in function. In contrast, it is possible to reach quasi full diversity – most possible functions – in a few steps from each position.
Wagner concludes, “The astonishing fact that evolution needs to explore only 101030th of a library to secure the arrival of the fittest goes a long way to explain how blind search produces life’s immense diversity.” And proteins are only a subset: Wagner begins with metabolism, where each genotype is characterised as a set of possible biochemical conversions (fructose to glucose, for example). But there are also regulation networks (like hox genes) and RNA.
Complexity allows for flexibility. For Wagner, the characteristics of these libraries resolves the debate between neutralists, who think that most genetic variants are irrelevant, to fitness and selectionists, for whom this would stop the evolutionary process. The paths with the same function within “genotype networks” (as Wagner calls them), “are crucial to find each change, and natural selection is crucial to preserve it.”
Wagner tries hard to follow the example of Richard Dawkins as a science writer. Although the result is a surprisingly enjoyable read – especially given the dry matter at hand – it will never match its model. It lacks the civilised aggressiveness for defending a unique idea: the selfish gene against the muddled thought of group selection.
In contrast to Dawkins, Wagner clearly overstates his case, by claiming to “present the missing piece in Darwin’s theory” on the cover. On the other hand, he clearly enables biologists to understand the paths that evolution takes to reach stunning diversity and adaptedness.
Letzte Änderungen: 21.06.2016