13 Things That Don't Make Sense (18 page)

All because of sex. Which may well have evolved as a response to cell damage by oxygen radicals. Which, in turn, can be traced
back to the mechanisms behind the production of the very energy that makes life worth living. Where there’s life, it seems,
death is close behind, but nobody has a full explanation for it. And then, somewhere in there, the sexual shuffling of genes
has found a role.

The archaea and bacteria get by without sex and don’t senesce. But when the first eukaryotes, our genetic ancestors, put these
organisms to work to produce energy, it was with mixed results. They happily used the energy, which has enabled us to become
all that we are, but it put the mechanisms of their eventual demise—death programs, if you will (and Hayflick certainly won’t)—right
into the heart of their cells. Only through sexual shuffling of genes could the cells mitigate against it.

If we haven’t reached the true origin of death, is this at least the root of sexual reproduction? Was it just a repair mechanism
designed for self perpetuation that gained a life of its own and took an unexpected path? If this is the story, the survival
of sexual reproduction as we see it today makes it an evolutionary spandrel, something that has arisen in the natural world
as a by-product of another adaptation. And that might explain why it is that, as with death, we can’t make sense of sex.

10

SEX

There are better ways to reproduce

I
n 1996 the arch-Darwinian Richard Dawkins published
Climbing Mount Improbable
, an outstanding exposition of the theory of natural selection. During his discussion of genetic mutation, and how it leads
to advantage in the environment, he is forced to talk about the origin of sexual reproduction. “There are many theories of
why sex exists,” he says, “and none of them is knock-down convincing.” Dawkins goes on to declare that he may at some time
in the future summon up the courage to write a book about the evolution of sex.

He hasn’t done it yet. In his 2004 book
The Ancestor’s Tale
, he again admits defeat over the origin of sex. “To do justice to all the theories would take a book—it has already taken
several … Yet no definitive verdict has emerged.” In the end, he settles for discussing a consequence of sexual reproduction,
rather than explaining its origin. The question of what is so good about sex is one that “better scientists than I have spent
book after book failing to answer,” Dawkins admits.

Dawkins is not alone in his frustrated silence at the prevalence of sexual reproduction. That chief among evolutionary biologists,
the late John May-nard Smith, referred to an “evolutionary scandal” surrounding sex. Thanks to sex, said George Williams,
there is “a kind of crisis at hand in evolutionary biology.” In his book
What Evolution Is
, the biologist Ernst Mayr added his contribution. “Since 1880 the evolutionists have argued over the selective advantage
of sexual reproduction,” he says. “So far, no clear-cut winner has emerged from this controversy.” Bringing things right up
to date, a 2007
Nature
review paper declared that “the explanation for why sex is so common as a reproductive strategy continues to resist understanding.”
You may never have thought too hard about it, but sex is a mystery.

The central enigma is simply that asexual reproduction, where an organism produces a copy of itself, is a much more efficient
way to pass your genes down to the next generation. It does happen; many species, notably a number of reptiles and fish, perform
limited amounts of asexual reproduction, copying themselves rather than collecting genetic material from a male (it is a female
endeavor, producing only females). London Zoo houses a Komodo dragon that produced offspring without any male assistance in
2006, for example.

The puzzle is, why hasn’t asexual reproduction taken over? Involve another organism by using sexual reproduction, and only
half your genes get passed on. What’s more, if a sexual and an asexual population are living side by side, every one of the
asexuals is producing offspring while only half the sexual organisms are. Sex is a recipe for extinction; the asexuals will
quickly take over the environment. So sex has what Maynard Smith called a “twofold cost”: why would anything get involved
in reproduction that is, genetically speaking, only half as effective as it could be—while also halving the speed of reproduction?

And that is just the genetics; we haven’t yet mentioned the effort of competing for a mate, the inefficiencies inherent in
the physical mixing of egg and sperm, and the problem of vulnerability to predators during the act of sexual reproduction.
There’s also the chance that the good gene combinations, the ones that evolution has selected for, will get pulled apart during
the process of recombination and not get passed on. Almost every way a theorist looks at it, sexual reproduction is a disaster.

Countering this theoretical take, though, is the fact that, when you look around, sex obviously isn’t a disaster; it is one
of the most ubiquitous phenomena on the planet.

There is a quick and logical solution to this paradox. Evolution by natural selection is all about advantageous mutation;
thus sex can only be so common because it confers a survival advantage. That advantage must come through the main outcome
of sexual reproduction: offspring that are slightly different from the parent. And that difference must be valuable enough
that it overcomes the enormous cost of using sexual rather than asexual reproduction.

Most observations of asexual reproduction show that it is an evolutionary dead end, a fast track to extinction. It comes and
goes—lasting maybe a few tens of thousands of years—but it almost never persists in a species. It sometimes occurs in response
to environmental stress, but it is not a universal strategy for most of the creatures that are capable of it. According to
the orthodoxy, that’s because any species that doesn’t shuffle its genes can’t survive natural mutations and shifting environmental
conditions; in a variable environment there are obvious advantages to producing offspring that have different capabilities
and tolerances.

In 2000, however, Harvard University’s David Mark Welch and Matthew Meselson turned this argument upside down. They had been
studying bdelloid rotifers, microscopic aquatic creatures that make great fish food. You can find rotifers almost everywhere
there’s water: in ponds, lakes, and roadside puddles, even in damp soil and mosses and lichens. What you won’t find is a male
bdelloid rotifer. These creatures reproduce without sex—and they have done so for longer than seems possible. Welch and Meselson’s
analysis showed they haven’t needed males for eons; the 360 species of bdelloid rotifer have survived intact, using only asexual
reproduction, for 70 million years.

It was this dogged survival, flouting biologists’ best theories, that Maynard Smith called an “evolutionary scandal.” It makes
a mockery of the one argument in favor of sex: the idea that organisms need to shuffle their genes in order to survive in
the long term. So although biologists see rotifers as the anomaly, it is really the rest of the natural world that needs explaining.
Theory is all very well, but where is the evidence of the advantage of sexual reproduction? Just how catastrophic must the
shifts in the environment be to make it worth paying the twofold cost of sex? To answer that, we have to look at what sex
can do.

FIRST,
let’s consider the issue of the bad—biologists call them deleterious—mutations that accumulate through asexual reproduction.
If an organism is just reproducing itself, any chance mutations in its DNA, caused by radiation damage, say, will be passed
on. Thus, over the generations, the mutations will accumulate (the phenomenon is known as
Muller’s ratchet
, after the discoverer of genome mutation through exposure to X-rays). The result is an organism that is always losing fitness.
In sexual reproduction, on the other hand, there is always a chance that mutation-free blocks of genetic material will be
transferred to the next generation.

It’s a good, even obvious, theory, but the devil is in the details. The evidence in its favor is not nearly as positive as
you might imagine.

Biologists gather such evidence—for and against—via some rather bizarre routes. William Rice and Adam Chippindale of the University
of California, Santa Barbara, for example, converted a fruit fly from sexual reproduction to a cloning machine for their experiments.
Aurora Nedelcu and her colleagues at the University of New Brunswick subject asexual algae to stress by heating to make their
sexual reproduction turn on. (In the wild, it’s the water temperature that operates this switch.) Matthew Goddard of the University
of Auckland, New Zealand, performs genetic engineering on yeast cells, which can normally reproduce sexually and asexually,
to switch off their sexual reproduction. Kellar Autumn of Lewis and Clark College, Portland, Oregon, made geckos run on treadmills,
comparing the performance of those born through asexual reproduction and those born through sexual reproduction.

All these techniques—and there are more—are employed to test theories and see how sexual and asexual populations fare in different
conditions. The answers, unfortunately, have not been as clearly confirming of the theories as anyone would like.

Autumn’s asexual geckos, for example, were better athletes than the sexually reproductive ones, running farther and faster.
But a previous study, carried out using a different species, found the converse was true. A series of experiments on water
fleas found that asexual reproduction produced four times more deleterious mutations than sexual reproduction. But a study
on nematode worms revealed absolutely no difference in the number of deleterious mutations in asexual versus sexual populations.
Computer simulations of evolving genomes showed that the size of the population matters here too: small populations do better
with sex, but larger populations of sexually reproducing species accumulated more deleterious mutations.

What about the idea that sexual populations can adapt to a changing environment more quickly because they are shuffling their
genes? Again, the evidence is mixed. A 1997 study with yeast found no advantage for sexual species of yeast when adapting
to a new environment. Another study, though, showed that sex can win out when the environment takes a turn for the worse,
but the populations remain evenly matched if the environment improves. Yet another study, which took place in 2005, put a
sexual and an asexual yeast strain into a test tube with minimal nutrients. The asexual strain won. When the same mix was
smeared on a mouse brain, something supposed to mimic a highly varied environment, the sexual population won out. That result,
though, contrasts with the findings of two Canadian researchers. In 1987 Graham Bell and Austin Burt showed that sexual reproduction
didn’t
give the kind of genetic diversity that would profit an organism’s offspring in a varied environment.

There is evidence, then, that sexual reproduction can increase the rate of adaptation in some situations, but it is hardly
earth-shattering—and it is certainly not significant enough to account for the high cost of sex.

Further problems with sex arise when we look deeper into the mutations that are supposed to give sex an advantage. First,
only a subsection of the virus family—RNA viruses—and the more evolved eukaryotes, such as humans, have high enough mutation
rates to make it worth having sex to purge the deleterious mutations. Then there’s the issue of
epistasis
, the interaction of genes. Multiple deleterious mutations in a genome can compound or diminish each other’s effects, but
the various studies that have been carried out into the effects of epistasis show no overall effects that would give sexual
reproduction the edge.

Another possibility—and one that has been given a lot of credence—is William Hamilton’s contention that sex is all about parasites.

Hamilton, who died in 2000, was an extraordinary figure. Not only for his academic prowess—one obituary called him “a good
candidate for the title of most distinguished Darwinian since Darwin”—but also for his fearless personal exploits. He trekked
through Rwanda at the height of the civil war, looking for ants (and was captured as a spy); he once jumped into the Amazon
and used a thumb to plug a hole in his sinking boat; in Brazil he was knifed when he refused to yield in a street robbery.
It was malaria, caught on an expedition into the Congo’s jungle, that finally killed him.

Hamilton’s imaginative approach to biology led him to coin a phrase that now resounds in the field:
the Red Queen hypothesis
. It was named after the character in Lewis Carroll’s
Through the Looking Glass
; the Queen tells Alice, “here, you see, it takes all the running you can do, to keep in the same place.” Hamilton used the
idea as an illustration of the evolutionary arms race between an organism and its parasites. You evolve to get rid of your
parasites; then they too evolve to use you as a host again. Sexual reproduction evolved as the best weapon in this never-ending
struggle, Hamilton suggested.

Evidence in favor of this idea comes from various groups of researchers looking into the effects of parasites on yeast, beetles,
sheep, and snails, among other creatures. Most show more successful reproduction and lower infestation by pathogens if their
genes are reshuffled through sex rather than being replicated through asexual reproduction. With a variety of genetic makeups,
it seems, there is a better chance that someone will live long enough to reproduce.

There is also evidence against the Red Queen hypothesis, however. Water fleas have shown no advantage over parasites when
they use sexual reproduction. And the pesky rotifers don’t fit within this paradigm, either. Why should they have managed
to resist their pathogens for so many millions of years without sexual reproduction? There is evidence that, for rotifers,
their advantage lies in genes that have adapted to help the organism survive in diverse conditions.

In 2004 Sarah Otto and Scott Nuismer struck another blow against the Red Queen. Their computer simulations of genetic interactions
between a range of organisms in a large and varied environment—something like the real world, in other words—led to less sex,
not more. So although the Red Queen hypothesis works in certain situations, it by no means accounts for the ubiquity of sexual
reproduction. The only way it might work is if it is just part of a wide range of phenomena that, taken together, make sex
a good reproductive choice. The Red Queen, they suggested in a paper published in
Science
, “might be less impotent with the right partner.”