Evolution: a different view (Introduction)

by David Turell , Saturday, September 24, 2016, 19:19 (2771 days ago) @ David Turell
edited by David Turell, Saturday, September 24, 2016, 19:26

Another essay from Andreas Wagner which explains the enormous protein landscape and how evolution seems to navigate it through large relationship networks. One should read the whole essay, as it is very informative and very large, and I can present only a small portion: - https://aeon.co/essays/without-a-library-of-platonic-forms-evolution-couldn-t-work?utm_... - "The glass lizard itself comprises billions of cells. Each cell contains thousands of different kinds of proteins - long string-like molecules made of 20 different kinds of amino acid. And each of these proteins has a unique ability. It might catalyse a chemical reaction, or prevent a cell from collapsing, or sense nutrients, or receive signals from other cells, and so on. Each of these abilities was an innovation - a qualitatively new and useful feature that can make the difference between life and death - when it first arose, millions of years ago. - "How do random DNA changes lead to innovation? Darwin's concept of natural selection, although crucial to understand evolution, doesn't help much. The thing is, selection can only spread innovations that already exist. The botanist Hugo de Vries said it best in 1905: ‘Natural selection can explain the survival of the fittest, but it cannot explain the arrival of the fittest.' (Half a century earlier, Darwin had already admitted that calling variations random is just another way of admitting that we don't know their origins.) - *** - "A protein is a volume in a library just like this, written in a 20-letter alphabet of amino acids. And while protein texts might not be as long as Tolstoy's War and Peace, their total number is still astonishing. For example, a library of every possible amino acid string that is 500 letters long would contain more than 10600 texts - a one with 600 trailing zeros. That vastly outnumbers the atoms in the visible universe. - *** - "..evolution can't simply look up the chemicals it needs in a giant catalogue. No, it has to inch its way painstakingly along the stacks. Imagine a crowd of browsers - each one representing an entire familial line - who must blindly explore the library, step by random step. This sounds like a party game, but there's a grisly twist. A mutation that compromises an essential protein such as haemoglobin is punishable by death. On that ill-fated volume, the bloodline ends. - *** - "And imagine that you could start this journey in not just one but 100 different ways, each one tracing one of a myriad alternative paths through the library, each encoding only synonymous texts that differ in most letters. Nature's libraries are just like that, permeated with sprawling networks of synonymous texts - I call them genotype networks - each encoding a molecule and its biochemical function. - *** - "But when you cannot make a beeline, because each step takes you in a random direction prescribed by a DNA mutation, it turns out that sprawling genotype networks are just what you need to survive. - "Random DNA changes in some members of a population could disable an essential protein such as haemoglobin and lead to death, but because genotype networks exist, other mutations can create a synonymous text that preserves the protein's function and saves the organism. This cycle of mutation and natural selection repeats in the survivors' descendants. Some of them die, but others live and get to take one step further. Step by step, the population of survivors spreads out through the library in a process that unfolds over many generations. - "Relatives of the lizard's oxygen transport protein illustrate how far this exploration can go. They are all descendants of a single long-forgotten ancestral protein that existed more than a billion years ago. By now, they occur not only all over the animal kingdom but even in plants. They have travelled far and wide throughout the library. And still they express the same chemical function. They all bind oxygen. - "Their amino acid text, however, has diverged beyond recognition. Today's haemoglobin proteins share as little as four per cent of the letters among their roughly 100 amino acids. - *** - "The remarkable thing is, having so many different ways to say the same thing means that there are many more possible slips of the tongue. And with each slip of the tongue comes the possibility of saying something different. Just as the word GOLD emerges from a single letter change in MOLD, some neighbours of a text express new meanings. And as the browsers work their way through each synonym for some original text, different innovations become accessible. By creating safe paths through the library, genotype networks create the very possibility of innovation. - *** - "So nature's libraries and their sprawling networks go a long way towards explaining life's capacity to evolve.....They exist in a world of concepts, the kind of abstract concepts that mathematicians explore." - Comment: He is suggesting chance mutation drives evolution because slightly changed molecules have similar function. He is ignoring the necessity of planning the coordinating of the millions of molecules that make up a single cell. He is also ignoring the highly complex world of giant enzymes in using, as an example, hemoglobin with only 100 amino acids.