What a Wonderful World (5 page)

But DNA explains not only the mechanism of inheritance but the mechanism of variation too. If an offspring is to inherit traits from its parents, their DNA must be copied. With a whopping 3 billion letters to reproduce faithfully in the case of human DNA, the amazing thing is how good the copying process is.
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But it is not perfect. A mistake is made about once every 1 billion base pairs. Sometimes a letter is not copied correctly. Or a sequence of DNA is deleted or duplicated. There are a myriad possible transcription errors. In addition, changes in genes can be caused by cancer-causing chemicals, viruses, ultraviolet light and nuclear radiation.

The upshot is that over time
genes gradually change
.

There is a lot of redundancy built into DNA to minimise copying mistakes, so many of the individual changes make little difference – the protein encoded by the gene still works. Some changes are harmful, causing inherited diseases such as cystic fibrosis. But, very occasionally, a change in DNA turns out to make a beneficial change to an organism – for instance, conferring on it an increased resistance to malaria. Of course, the ultimate arbiter of what is
beneficial
to an organism is its environment. A change in a gene that results in a thick, warm coat is beneficial to an animal living in a world plunging into an ice age but not to one living in a tropical world.

It is worth pointing out that changes, or mutations, occur in the DNA of all organisms. But, whereas simple organisms such as bacteria merely create copies, or clones, of themselves when they reproduce, other creatures have sex, producing offspring
with half their genes from each parent. Such a composite of different traits passed down the maternal and paternal line greatly boosts the novel gene combinations available for natural selection.
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Mutations explain the existence of
species
– groups of animals, which, broadly speaking, cannot interbreed. Species can arise in many ways. For instance, a geographical barrier such as a river or mountain range might split a population in two. Or, as in the case of the Galápagos, an ocean might divide creatures from their cousins on the mainland. Separated in this way and subjected to different survival pressures, the DNA of each group accumulates different mutations, so the populations gradually diverge. Eventually, the two groups can no longer interbreed.

There could be many reasons for this. It could be that a mixture of their genes simply does not lead to a working organism, in much the same way that putting a motorbike engine in a Rolls-Royce does not create a viable car. Or it could be that members of one group hang out on a particular type of fruit, waiting for a mate, whereas members of another group prefer another type of fruit entirely; though they could easily mate, they miss each other like ships in the night. In the case of insects, which have complex genitalia, two groups might no longer interbreed because one develops sex organs that, like a skeleton key and a Yale lock, physically do not fit each other.

Whatever the reasons for groups of creatures diverging from each other, natural selection has populated the world with a myriad distinct species, each with as little ability to breed with each other as humans and oak trees.

Darwin’s theory explains so many aspects of the world. For instance, it explains why life on Earth is so staggeringly diverse, boasting more than 5 million living species. It also explains why we share around 99 per cent of our DNA with chimpanzees – and even a third with mushrooms. This is exactly what would be expected if we evolved from a common ancestor. Since changes in genes accumulate over time, the DNA differences reflect the fact that the common ancestor of humans and chimpanzees lived relatively recently whereas the common ancestor of humans and mushrooms lived in the very remote past.

Arguably, the most remarkable DNA sequence on Earth is GTG CCA GCA GCC GCG GTA ATT CCA GCT CCA ATA GCG TAT ATT AAA GTT GCT GCA GTT AAA AAG.
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It is present in every single living organism – even in organisms not technically classed as alive such as giant mimiviruses. The reason the sequence is so widespread is that it existed in the common ancestor of all life. Carrying out a crucial process, it has remained unchanged for 3 billion years: the oldest fossil in your body.

Darwin’s theory also explains why our antibiotics become less and less effective with time. Initially, they may kill the overwhelming majority of bacteria infecting a person. However, genetic variation within a population of bacteria ensures that some, inevitably, will survive to reproduce. Each successive generation will, therefore, contain a higher proportion of antibiotic-resistant bacteria, until eventually the antibiotic is next to useless. ‘Evolution is … an infinitely long and tedious biologic game, with only the winners staying at the table,’ says Lewis Thomas.
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Most of all, however, Darwin’s theory explains the illusion of design – why organisms are so perfectly suited to their environments. The reason a finch on an island in the Galápagos has a beak perfect for cracking open the nuts it lives on is because its ancestors prospered, leaving more offspring than did finches with less effective beaks. The shape of a beak turns out to be controlled by a single gene, slight variants of which express different proteins in the growing jaw of a finch embryo.

The remarkable thing is that such an exquisite match between organism and environment is achieved without a designer. But, then, the natural process identified by Darwin is not random. ‘Mutation is random,’ says Richard Dawkins. ‘But natural selection is the very
opposite
of random.’
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It preferentially culls all the variants except those that confer on their host the ability to survive to reproduction. Incrementally, generation by generation, it accumulates advantageous changes, slowly but surely assembling machines far more exquisite and complex than any designed by humans. ‘The whole trend of life, the whole process of building up more and more diverse and complex structures, which we call evolution, is the very opposite of that which we might expect from the laws of chance,’ wrote American biologist Gilbert Newton Lewis.
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But evolution by natural selection has its limits. The only organisms that can arise are those that are the result of a long string of advantageous changes. ‘Evolution walks backwards into the future,’ says British biologist Steve Jones. ‘It doesn’t know what’s coming.’
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This has led some people to claim that Darwin’s theory cannot explain the existence of complex organs such as the eye, which consists of multiple components. Until all components are in place – a lens, a light-detecting surface, and so on –
goes the argument, no advantage is conferred on an organism. What use is 50 per cent of an eye? Or 5 per cent of one?

However, it turns out that all the steps along the road to the eye were indeed advantageous. Examples of primitive eyes can be seen throughout the animal kingdom. Some creatures have only a patch of light-sensitive cells for sensing which way is up and which down. Others, like the pit viper, have light-sensitive – actually, heat-sensitive – cells at the bottom of a pit in their skin, so their ‘sight’ has a directional capability. From this, it is a short step to close over the pit with a transparent protein, creating a lens that can focus an image of an object.

In addition to having no foresight, evolution by natural selection does not necessarily result in more complex forms. It
can
, but it does not always do so. After the advent of the first cell, there really was nowhere to go but
up
in terms of size and complexity. But, as soon as larger creatures evolved, it was possible to evolve back down to simpler forms. This can be seen in the case of parasites, which live off their more complex hosts.

Darwin’s theory of evolution by natural selection – Dawkins’s ‘greatest idea in the history of science ’ – has passed every test. ‘It could so easily be disproved if just a single fossil turned up in the wrong date order,’ wrote Dawkins.
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All it would take would be the discovery of a rabbit in the pre-Cambrian period 500 million years ago. As yet, this has not happened.

In his 1986 book,
The Blind Watchmaker
, British biologist Richard Dawkins paints a beautiful and evocative image. ‘It is raining DNA outside,’ he writes. ‘On the bank of the Oxford canal at the bottom of my garden is a large willow tree, and it is pumping downy seeds into the air … spreading … DNA whose coded characters spell out specific instructions for building willow trees that will shed a new generation of downy seeds … It is raining instructions out there; it’s raining programs; it’s raining tree-growing, fluff-spreading, algorithms.’

Of course, the DNA in each of those countless fluffed-up seeds will remain nothing more than an inert coil of chemicals – a non-running computer program – unless it collides and merges with another chunk of DNA, kick-starting the creation of a new willow tree. Sex, as Dawkins so eloquently points out, is everywhere. It is what makes the world go round. Pretty much all creatures – from ants to antirrhinums, pine trees to pangolins, sunflowers to sail fish – indulge in it. Yet, to steal the words of Winston Churchill, sex is ‘a riddle, wrapped in a mystery, inside an enigma’.
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The central mystery of sex is not hard to appreciate. Once upon a time, in a primeval pond on the newborn Earth, there arose molecules that could copy themselves.
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Those that were most successful became the most numerous; those that were least successful were outcompeted for the necessary chemical building
blocks and so disappeared. Eventually – and this undoubtedly took a mind-cringingly large number of steps, a vast amount of
pre-evolution
– a single type of molecule became pre-eminent because of its ability to build molecular machines that could exploit energy resources to promote its own reproduction. This was DNA – a necklace of genes, many of which coded for individual pieces of protein nanomachinery. ‘All of today’s DNA, strung through all the cells of the Earth, is simply an extension and elaboration of [the] first molecule,’ said American biologist Lewis Thomas.
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Over billions of years, natural selection, which has seen some gene sequences outcompete others for resources and so propagate into the future while others have fallen by the wayside, has created the most amazingly elaborate vehicles for promoting genes. But that is essentially all they are. Whether fungi or fur seals,
E coli
or elephants, hydras or humans, they are
vehicles for propagating genes
. ‘A hen is only an egg’s way of making another egg,’ as Samuel Butler put it.
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And the most successful vehicles are those that get their genes into the next generation. Not just some of them.
All of them
.

The straightforward way for an organism to do this is simply to make a copy, or clone, of itself. Asexual reproduction is the strategy used by most simple organisms such as bacteria plus a few more complex organisms such blackberries. However, mysteriously, the large majority of multicellular organisms use an alternative reproductive strategy. They combine
half
their genes with
half
of the genes of another organism. This is of course sexual reproduction.

The obvious disadvantage of sexual reproduction is that, instead of passing 100 per cent of an organism’s genes into the
next generation, it transfers a mere 50 per cent. ‘Sexual reproduction is analogous to a roulette game in which the player throws away half his chips at each spin,’ says Dawkins.

Common sense says that a creature reproducing sexually can compete with one reproducing asexually only if it produces
twice as many offspring
. But this is very costly in terms of energy. And, in a world of cut-throat competition for food resources, energy efficiency is imperative for survival. But the cost of producing extra offspring is not the only extra cost of sex. It takes energy, after all, to find a partner with which to merge genes. Think of the willow tree in Dawkins’s garden, which must create and release such a tremendous quantity of feathery seeds into the air of Oxford. ‘The reproduction of mankind is a great marvel and mystery,’ wrote Martin Luther. ‘Had God consulted me in the matter, I should have advised him to continue the generation of the species by fashioning them of clay.’
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The striking feature of the world, however, is that sex is ubiquitous. Not only do birds and bees do it, so do pretty much all plants, reptiles, mammals and birds. Clearly, it must have a huge evolutionary advantage not only to have survived but so evidently to have thrived. But what is that advantage? Remarkably, it is not obvious. Not obvious at all.

One hint of the possible advantage of sex comes from the variety of its resultant offspring. The offspring of an organism that reproduces asexually are not
exact copies
of that organism because the copying of DNA is never perfect. However, the variety produced by occasional copying errors, or mutations, pales into total insignificance compared with the variety created by sexual reproduction. If the genes of an organism are likened to a deck of playing cards, each offspring of an asexual organism
inherits the same deck of cards, possibly with one card substituted by a wild card. However, each offspring of sexual reproduction inherits half the cards from two separate decks shuffled together. And, for each individual offspring, the two decks are shuffled together
differently
.

What this means is that the offspring of sexual reproduction are
very
different from their parents.
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Sexual reproduction generates maximum novelty in the next generation. Conceivably, at times when the environment is stressed, such as when the climate is changing rapidly, sexual reproduction can throw up such a large range of organisms that some at least will have novel traits necessary for survival. By contrast, asexual organisms, terminally stuck in a rut, will die out. Is this enough of an advantage, though, to explain the survival of sex? Biologists are not entirely clear.

Another possible reason for the survival of sex is that it provides the means to combine in a single organism advantageous gene mutations
from two organisms
. Think of two asexual organisms each of which acquires a mutation in one of its genes that aids its survival. The two mutated genes are doomed to remain forever separate, isolated in each line. Sex, however, changes everything. It means that two good genes from two separate organisms can end up side by side on the same strand of DNA, compounding the survival chances of any offspring. This sounds like a big advantage. Unfortunately, of course, sex not only concentrates good genes in a single organism but also bad genes in a single organism. No one is quite clear whether the advantage sufficiently outweighs the disadvantage.

So what, then, is the overwhelming – yet mysteriously elusive – advantage of sex? One idea that has gained popularity – though not universal acceptance – is that sex wrong-foots potentially
deadly parasites. Such creatures are the bane of all complex organisms. More than 2 billion people worldwide are infected by parasites, ranging from malarial protozoa to intestinal worms. Evolution by natural selection acts on parasites in the same way it does on all organisms. But a parasite ’s environment is
its host
. Consequently, its success at exploiting the resources of its environment comes at the cost of depleting the resources of the host. Parasites drain their host of life and may eventually even kill it. And this can all happen very rapidly since parasites are generally small and fleet of foot, capable of reproducing many times over during the lifetime of their host.

How can a population of host creatures possibly survive so relentless and effective an assault? The answer is by continually replacing its members by new members that are utterly novel and to which the parasite is not perfectly adapted. This is exactly what sex accomplishes, claimed American biologist Lee Van Valen in 1973.
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Yes, parasites can change rapidly. But a host population can survive if it can change
even more rapidly
, said Van Valen. In
Through the Looking-Glass
, Lewis Carroll’s 1871 sequel to
Alice in Wonderland
, Alice is running alongside the Red Queen but is completely baffled that she appears to be making no discernible progress.

For this reason, Van Valen’s parasite explanation for sex has become known as the Red Queen Hypothesis.
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In 2011, biologists in the US tested the idea in a controlled laboratory environment.
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They genetically manipulated the mating system of the roundworm
Caenorhabditis elegans
so that different populations could reproduce either asexually, by fertilising their own eggs, or sexually, by mating with male worms. They then infected
C. elegans
with the pathogenic bacteria
Serratia marcescens
. The bacteria rapidly drove extinct the self-fertilising population of
C. elegans
. However, this was not the case for the sexually reproducing population. It was able to outpace its co-evolving parasites – continually
running faster
– appearing to confirm the Red Queen Hypothesis. Sex is a weapon against parasites.

Sex involves the combination and shuffling of genes from two organisms to create an entirely novel organism. The devil, however, is in the detail. And the detail is both subtle and complex.

To appreciate it, it is first necessary to know some background. If the DNA in a single one of your cells was arranged into one straight piece, it would stretch right the way from your head to your toe. Packing all that DNA into a tiny cell, invisible to the human eye, is therefore a biological challenge. A cell achieves this impressive feat by packaging the DNA into shorter stretches known as chromosomes, so called because they were first revealed with the aid of coloured, or chromatic, dyes.
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Human cells have 46 chromosomes – two sets of essentially the same chromosomes.

Dogs have 78 chromosomes; horses 64; and cats and pigs 38. The number of chromosomes appears to bear little relation to an organism’s complexity. The Adder’s-tongue fern,
Ophioglossum vulgatum
, for instance, has a whopping 1,440 chromosomes, the largest number of any living thing.
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Back to humans. Recall that, every day, your body creates about 300 billion new cells – more than there are stars in our Milky Way Galaxy.
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In this process, known as mitosis, a cell first creates a copy of all 46 of its chromosomes. That makes a total of 92. Then, the cell splits into two ‘daughter’ cells, each with 46 chromosomes, exactly like the original.

Sex is the opposite process. Instead of splitting one cell into two, two cells – one from each parent – are fused into one. However, if the final cell is to have the correct complement of 46 chromosomes, the cells from each parent – known as sex cells, or gametes – must each contain only 23 chromosomes, or
half the normal number
.

The creation of sex cells by both males and females, therefore, requires a process quite distinct from mitosis. In meiosis, as in mitosis, a cell first makes a copy of all 46 chromosomes, to make a total of 92. But then it splits
not once but twice
. The end result is four gametes, each of which contains 23 chromosomes.

Incidentally, some shuffling of the genes is carried out during meiosis, so that each of the gametes is genetically different from its parent. This shuffling might once upon a time have been an accident – a result of the complex manoeuvring of chromosomes during meiosis – but it might have become frozen because creating offspring with the maximum amount of genetic novelty has survival value. And this shuffling of genes to create variety is
even before
sex cells fuse to generate yet more variety.

The gametes from each parent could, of course, be the same size. And this is true for some organisms. But very often one is far bigger than the other because it contains the fuel and protein machinery to drive development once fusion occurs. For biologists, the essential difference between the sexes lies in the gametes. Organisms that produce large gametes that cannot move about – known as eggs, or ova – are female – while organisms that produce small gametes that can move about – known as sperm – are male. All other things that are usually associated with the difference between the sexes – penises, vaginas, breasts and beards – are ultimately just consequences of the differences between sperm and egg.

Biologists believe that the first sexually reproducing organisms produced gametes of the same size. This is an interesting observation. It means that
sex came before sexes
.

Now, finally, we come to the fusion of two gametes – one from each parent – which is the central act of sex. Here, the two gametes, each with 23 chromosomes, combine to make a single cell, known as a zygote. Subsequently, the zygote will split, again and again, by normal mitosis, to make the 100 trillion or so cells that compose an adult human being.

Clearly, the zygote contains 23 chromosomes from the mother and 23 chromosomes from the father.
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At a gross level, therefore, each one of your cells has two copies of
exactly the same genes
. After all, men and women are genetically more similar than, for instance, men and chimpanzees – and, recall, chimpanzees share 98 to 99 per cent of their DNA with humans.
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