8/11/2019 L,Oyd Quantum Biology of Natural Selection

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164 nature physics | VOL 5 | M ARCH 2009 | www.nature.com/naturephysics

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A quantum of natural selectionSeth Lloyd

The modern evolutionary synthesis, which marries Darwins theory of natural selection with Mendelsgenetics, was developed around the same time as quantum mechanics. Is there any connectionbetween the two?

In the hundred and fy years since thepublication o On the Origin of Species1,much has been made o what CharlesDarwin knew and did not know in

ormulating the theory o natural selection.Perhaps the most surprising aspect o thebook is that Darwin was able to construct

his theory o how physical traits werehanded down and naturally selected, despiteoperating with a demonstrably incorrecttheory o genetics. Although Gregor Mendelspioneering work on the genetics o peaplants 2 was published in 1866, Darwin neverappreciated the signicance o this work.He had ormulated his theory o naturalselection using the pre-Mendelian blendingtheory o genetics, in which traits o themother and ather were blended togetherin the offspring. Darwin was aware o thelimitations o the blending theory theoffspring o a blue parrot and a yellow parrot

are either blue or yellow, not green but itsimper ections were insufficient to undercuthis power ul argument or natural selection3,4.

Darwin was not the only nineteenth-century scientist to under-appreciateMendels work. Mendels laws o geneticinheritance were virtually ignored until1900, when Hugo de Vries, Carl Corrensand Erich von schermak rediscoveredthem and publicized their signicance 5. Inthe rst decades o the twentieth century,geneticists and statisticians began combiningMendelian genetics with Darwins idea onatural selection to create the modern

synthesis, the theory o evolution throughnatural selection o genetic variation 4,5. But1900 was also the year that Max Planckdiscovered the quantum-mechanicalnature o light6. Te same years that sawthe creation o the modern evolutionarysynthesis also saw the rapid developmento quantum-mechanical theories o atomsand molecules, ollowing the mathematical

ormulation o quantum mechanics byErwin Schrdinger and Werner Heisenberg.Which brings us to the central question thatI wish to consider here: what, i anything,does quantum mechanics have to do withnatural selection?

Te answer, in short, is: quite a lot.Despite the act that quantum mechanicsrules the world at length scales many orderso magnitude below the size o Darwinsnches or Mendels pea plants, quantummechanics has a pro ound effect on thenaturally selected world. In act, Mendels

work, which demonstrates the atomicor unsplittable nature o inherited traits,already hints at the answer to why quantummechanics is important to li e.

F v d g l g f o Planck discovered that light, which hadpreviously been thought to consist o waves,possessed an intrinsically discrete quality:light came in chunks. Planck called thesechunks quanta. Soon, Niels Bohr and othersshowed that atoms possessed a similarlychunky quality: the electrons in atomscould only take on a discrete set o states.

Atoms were also quantum mechanical.Tis chunky quality implies that nature,at bottom, is not continuous, but discreteinstead. Quantum mechanics makes naturedigital. It is this digital character o all thingsat their smallest scale that nature discoveredand used to construct the genetic basis orli e. Quantum mechanics effectively givesa package o digital gifs to nature, whichin turn uses these gifs crucially in thedevelopment o li e.

Te rst gif that the digital quality oquantum mechanics gives to nature is thegif o stability: the quantum-mechanical

hydrogen atom is stable. I the hydrogenatom obeyed the laws o classical mechanics,by contrast, the electron would spiral intothe proton in a tiny raction o a second,ending the atoms li e in a burst o radiation.Radiation is exciting stuff, but not the stuffo which li e is made.

Te second digital gif is countability:the orms o matter that can exist at amicroscopic scale do not vary continuously.Tere is only a small, countable set o stableelementary particles. Tese elementaryparticles can combine into stable atomsin only a countable number o ways. Teatoms can combine to orm only a countable

set o chemical compounds. And so on:at each length scale, the countable set odifferent entities can combine to orm alarger, but still countable, set at the nextlarger scale.

Te third digital gif that quantummechanics gives nature is in ormation. Te

undamental unit o in ormation is the bit;it represents the distinction between twopossible states. Te word bit is also usedto re er to a physical system, such as anelectron spin, that can take on one o twodistinguishable states. Te key eature oin ormation is that a small number o bitscan have a large number o possible states:n bits can take on 2 n states. So, or example,300 bits can take on 2 300 1090 states, where1090 happens to be the number o elementaryparticles in the Universe (more precisely, thenumber within the particle horizon o theUniverse). Te discrete quality o quantum

mechanics, together with the ability tostring together many atoms in a molecule,implies that physical systems are naturallycapable o registering a large number o bitso in ormation. Perhaps the most explicitexample o such natural in ormation isDNA. Te entire human genome registersaround six billion (6 10 9) bits (which inturn contain between 20,000 and 25,000distinct Mendelian genes). Nowadays, sixbillion bits doesnt sound like so much but this number o possible congurations

or our DNA is two raised to the six billionthpower, 2610

9

, a more than astronomically

large number. It is this huge potential varietyo genetic combinations that gives naturalselection its power.

Te ourth digital gif rom quantummechanics is in ormation processing. Asnoted above, an electron spin can be takento register a bit o in ormation: or example,spin clockwise along some axis (spin up)can be taken to represent a logical 0, andspin anticlockwise along that axis (spindown) can be taken to represent a logical1. When the spin ips, or example byabsorbing a photon, then spin clockwisegoes to spin anticlockwise, and vice versa,spin up goes to spin down, and 0 goes to 1.

2009 Macmillan Publishers Limited. All rights reserved

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