13 Things That Don't Make Sense (19 page)

This seems to be the only answer left: that there is no one simple explanation for sex. Because none of the big, obvious explanations
have panned out, the trend among researchers is now to look for a combination of smaller effects that give sex an advantage.
One example is the way that sexual reproduction changes the genetic architecture. Experiments with artificial gene networks
(more computer simulations) have shown that sexual reproduction gives rise to genomes that are “robust”; mutations don’t have
a strong effect on them. What is even more interesting, though, is the fact that sex also produces genomes that are more likely
to be split into modules, self-contained entities whose genes have no effect outside the module. In sexual reproduction, the
combinations of modules are shuffled rather than the genes, which reduces the risk of pleiotropic problems where one gene
adversely affects another somewhere else on the genome. With a modular genome, the genes inside each prefabricated module
are already tried and tested together and—if the creature has survived to reproduce—self-evidently do not produce enormously
adverse effects (at least, not before reproductive age). Since the genes do not affect anything outside their own module,
no amount of modular shuffling can produce further adverse effects, but there is still the possibility of advantageous recombination.
Which means ongoing survival for the organism.

If it is true, it is still only part of the puzzle. In general, the random genetic drift due to chance variation offers the
best hope of explaining the apparent advantage of sex. Research has shown that if populations aren’t too large or small, and
if the variations don’t interact too much (that is, if pleiotropy is limited), sexual reproduction, more than asexual reproduction,
can use genetic drift to enhance survival. But that’s hardly a conclusive argument; biologists are still effectively offering
up an argument that lacks strong supportive evidence. They just cannot answer the question of how we pay the twofold cost
of sex.

TO
Charles Darwin, the reason for the prevalence of sexual reproduction was “hidden in darkness.” More than a century later,
in 1976, Maynard Smith said the problem with sex was so intransigent it made him feel “some essential feature of the system
is being overlooked.” Three decades later, the problem is still here. It must be the longest-lasting scientific anomaly of
them all. So, is it a Kuhnian anomaly?

It certainly has some of the hallmarks. In our efforts to combine a whole raft of small effects, it seems that our explanations
for the origin of sex begin to look like something Kuhn called “a scandal”: the Ptolemaic epicycles. These described the motions
of the planets and stars as observed by the Greeks. The basic premise was that these objects revolved around the Earth. As
observations got better and better, however, the astronomers had to repeatedly tweak their models of exactly how that revolving
happened, adding layer upon layer of complication. It involved a gargantuan effort to keep the theory together—astronomy in
those days largely consisted of anomaly-proofing the Ptolemaic system.

Early in the sixteenth century, an astronomer called Nicolaus Copernicus recognized that Ptolemaic astronomers had created
a monster, and set about working out a better system. When he published
De Revolutionibus
, it all suddenly became clear. The motions of stars and planets made sense—and worked out ever so simply—if everything was
in fact revolving around the Sun.

Is our theory of sex unwittingly Ptolemaic? And if so, can we see from whence its Copernican revolution might come?

Perhaps Maynard Smith’s missing “essential feature” is the connection between sex and death (the subject of the previous chapter).
If death—or at least cell senescence—is the root of sexual reproduction, the twofold cost of sex can plausibly be offset (perhaps
more than offset) by the gain that comes with death: the ATP-generating machinery at the heart of every cell. Without it,
we eukaryotes wouldn’t have been able to take over the world. Let us run with this for a moment and see where it leads.

If sexual reproduction is a spandrel, a by-product of death, perhaps we can downgrade the primary assumption of biology: that
the natural world is a fierce competition to pass on your own genes at everyone else’s expense, using the best partner available
(if a partner is necessary). Perhaps this drive is less intense than generally thought, and mitigated by other considerations,
such as individual survival. If sex evolved as a result of the evolution of death in eukaryotes, survival must surely beat
sex in the hierarchy of impulses. And we know that in most (but not all) sexually reproducing creatures, the desire to exist
is stronger than the desire to reproduce.

Now let us imagine organisms living, as they ordinarily do, together in a group. (We are, necessarily, considering the higher
animals here, but these are the creatures in which sexual reproduction is most firmly established.) They have a proclivity
for sexual behavior and some impulse to reproduce, but also an awareness of the power of the group: their individual survival
(the root of sex in our narrative) is linked in with the well-being of the group. What will happen?

There will be sexual behavior. As we well know, whatever the reason it has evolved, it has evolved to be a pleasurable bonding
activity, at least in the higher animals. There will inevitably be reproduction. But there will also be consideration and
effort directed at maintaining the integrity of the group so as to preserve the individual. John Maynard Smith once suggested
that if the male partner contributes a significant amount to a sexual partnership, providing resources and working so hard
that the female can produce twice as many offspring as an asexual female, the cost of sex disappears. Is it possible that
a group dynamic such as that described above could more than offset the cost?

It is a difficult question to answer, but we can certainly make some interesting observations. Sexual creatures do often live
in groups, and while it makes sense that each organism puts its own “best interests” at the top of its priority list, you
can only work out what those best interests are when you take the whole group into consideration. It is not in a smaller male’s
best interest to try copulating with the sole female in the group, for example; if other males are much bigger, he could die
in the attempt.

In some ways the issue parallels a well-known mathematical phenomenon known as
the stable marriage problem
. Imagine a party where a roomful of people are looking to hook up with a partner of the opposite sex. If all the men will
only settle for the best-looking woman—and vice versa—almost everybody is going to end up unhappy. In 1962 two mathematicians
worked out how, given a little compromise from everyone, you could actually make everyone happy. David Gale and Lloyd Shapley
showed that if everyone compiles a ranking, in order of desirability, of potential partners, it is possible to arrange things
into a stable equilibrium state. In this equilibrium, people are partnered in such a way that it is impossible to find a man
and a woman from different couples who would both rather be married to each other than stay with their current partner. It’s
not the ideal for most individuals, but it is a satisfactory outcome for the group.

This is just one application of
game theory
, a mathematical tool used to track how the benefits and costs of decisions and actions will shape group behavior. Invented
by the Hungarian mathematician John von Neumann, game theory has the central goal of finding an optimal solution to a problem,
one where everybody involved in a situation is as happy as possible. Once this equilibrium is established, no one involved
has any incentive to change it. The theory has proved a vital tool in a vast number of arenas: it helped establish the fragile
peace of the Cold War; it has been brought to bear on economics and international relations; it explains how societies establish
their social norms. In some ways, everything humans and animals do can be treated as a game. And that—according to Joan Roughgarden,
at least—includes sexual reproduction.

Roughgarden is a professor of evolutionary biology at Stanford University and specializes in issues of sexual selection. In
February 2006 she provoked an almighty row in the pages of the journal
Science
when, writing with two colleagues, she called for the wholesale replacement of Darwin’s theory of sexual selection with a
theory of
social
selection. The choice of sexual partner, she said, has to do not so much with reproduction, the propagation of genes, as with
group bonding. And game theory, she said, shows why.

In her paper Roughgarden lays out a new theory to explain reproductive choices. Choosing the “best genes” is not involved
in determining reproductive behavior, she says. Instead there is a kind of bartering system: opportunities to reproduce can
be exchanged for services like attracting females, keeping a territory clean, or fighting off competitors.

Though many biologists have been critical of Roughgarden’s ideas and approach, the theory does allow an organism to regain
ground lost through sexual reproduction. She argues, for instance, that game theory shows social selection will increase the
numbers of young raised to maturity. If group members are involved in performing the various functions necessary to group
cohesion and survival, and these contributions mean that, in time, everybody gets a chance to reproduce because they are making
a contribution, reproduction will be a more successful affair, pushing the numbers up.

It certainly provides a stark alternative to the traditional standpoint of biology—a standpoint that does have shortcomings.
If you take the standard view of sexual selection, choosing a mate is meant to be a straightforward affair. It is based on
the display of “good genes,” usually manifest in the adornments and athleticism of the male of the species. For the most part
the females choose (their eggs are limited; sperm is cheap and plentiful), and males slug it out for the chance to be chosen.
However, recent studies showed that all that talk about females choosing males with the biggest antlers, or loudest roar,
or, as in the case of peacocks, the most elegant tail feathers in order to get the “best genes” is just far too simplistic
to describe what happens in the real world.

John Maynard Smith appreciated this. He took red deer as an example of where things go wrong for sexual selection theory.
The powerful males get busy rutting in an exhausting, drawn-out, and impressive display of antler bashing. Often, though,
the females aren’t impressed and slope off to have sex with the less macho males of the herd. In a stroke of typical genius,
Maynard Smith labeled them the
sneaky fuckers
.

Is it even sneaky? Perhaps it just makes good evolutionary sense. There isn’t strong evidence that females really are impressed
by the antler bashing or link it with the good genes that they supposedly seek for their offspring. And are there really so
few good genes out there that the females are willing to focus all their attention on just one or two males? After all, if
the theory holds together, all the males are the progeny of strong, fit males from the previous generation. It’s hard to imagine
that there is such a marked difference that females would be so discerning. The issue, known as the
Lek Paradox
, is well known to biologists. Although there are some explanations for why female choosing should persist, it still stands
as a point of contention in standard sexual selection theory.

There are more examples of problems with the standard theory. Two Australian researchers, Mark Blows and Rob Brooks, found
that the kinds of selection done by fruit flies, for example, goes almost in the opposite direction to the one that sexual
selection theory would predict. And the same researchers’ studies with guppies showed the females are often lazy, not making
an effort to choose their mate carefully, but just mating randomly. Others choose, but apparently on the basis of past experiences
rather than genetic traits. There are those who put in some effort and scrutinize the males, but it is by no means the norm.
As the biologist Steven Rose pointed out, although it seems like a compelling idea, the empirical evidence for sexual selection
based on impressive male traits is weak—and that is even true among peacocks, the “classic” example. What’s more, there is
evidence suggesting that the key to reproductive success lies somewhere other than a display of brute strength.

In the summer of 1994 Elisabet Forsgren spent a couple of months playing matchmaker at the Klubban Biological Station on the
west coast of Sweden. She was studying sand gobies, fish that swim around the shallows of European shores, that she had caught
in a shallow sandy bay and put into tanks at the station. The fish dined on fresh mussels that Forsgren provided; in return,
they showed her just how complicated sexual selection can be.

First, Forsgren let two males fight each other for the best egg-laying site. The winner was usually the slightly larger fish.
Then she gave them a brood of eggs to guard from a marauding crab. The smaller fish turned out to be a better guardian. Finally,
she let a female choose between them. The female—who knew none of what had gone on—nearly always went for the male that was
a better guardian rather than the larger, dominant male.