Why Reproduce Sexually At All?

Living creatures reproduce in many different ways, of which the kind of sexual reproduction we're used to is only one.  It's not immediately obvious why sexual reproduction evolved in the first place, or why it's as common in nature as it is.  What makes it such a good idea? 

The short answer is, evolutionary specialists are not really sure yet.  This answer, of course, delights those who are perfectly certain themselves, without having much interest in the problem at all.  But the problem is fascinating!  And one of the most intriguing parts of it is that there probably isn't just one answer, there will probably turn out to be a whole system of interlocking answers, because evolution is so unconstrainedly complicated. 

I'll try to give a partial answer here, to help clarify a bit why the question is complicated. 

Selection in action

The "winners" in the evolutionary design competition are those whose life-forms survive and reproduce most successfully, right?  Seems brutal and effective, like something a corporate efficiency consultant would come up with.  If nature were a big company, we'd call in the engineers and have them design a good, simple product that would most efficiently survive and reproduce.  And wouldn't it be great to be able to cut costs by simply killing off the engineers whose designs were inferior? 

We'd want to make full use of the process of natural selection, to keep our product adapting all the time, but the key to winning the corporate race, as we all know, is getting there first, getting biggest, outproducing everyone else.  So what we want is the fastest, most efficient form of reproduction possible, right?  A few minutes with a calculator show us that the laws of exponential growth turn even a small advantage in reproductive speed into a crushing population victory in ten or twenty generations, all other things being equal. 

So our engineers come up with critters that clone themselves.  That's obviously the best design.  Every single individual can work full-time on reproducing, and good survival designs are selected in their entirety (a successful parent passes a perfect copy its design to every one of its children).  Turns out, we've just invented simple bacteria!  And just as predicted, the product is a great success. 

Then, at the party celebrating our product launch, somebody's grandmother tells a bunch of the engineers that there's a much better reproductive solution, in which only half the population actually produces offspring, while the other half just lives to carry genetic information.  When they reproduce, each individual finds a suitable member of the other half of the population and they mix their genetic design information in some arbitrary way (often exposing themselves to predation while doing so).

The engineers are much amused at this idiocy.  Twice as many resources would be used up to achieve the same reproductive rate.  Even worse, good designs would constantly be diluted, since genetic information from both parents is arbitrarily mixed in each child.  Even the stupidest engineer can grasp that such an arrangement would be an inefficient and greatly inferior design.  All other things being equal, self-cloning is clearly the way to go. 

So how come sexual reproduction is the norm in real life, then?  Well, to start with, in real life all other things never stay equal.

Yes, we're all engaged from time to time in a race to fill an ecological niche, at which point reproductive speed is definitely crucial, but what happens once we succeed?  What happens once the niche is full?  At that point, the whole game changes radically, entering a kind of fluctuating equilibrium state that is life's most common environment.  It is precisely in this sort of equilibrium that sexual reproduction is a superb design, because now competition for resources really heats up, and rapid reproduction is no longer nearly as important as rapid adaptability.

A beach metaphor for ecological equilibrium

One good way to visualize the kind of equilibrium in which most creatures evolve is to think of an ocean beach, with waves breaking regularly on the sand, with water constantly rushing forward and then receding.  Imagine that the exposed sand represents available resources, the waves are seasonal changes, the tides are longer cycles, and storms are unpredictable but inevitable disasters.  This is the kind of ecological dynamic that most genetic designs have actually evolved to handle. 

When times are good, the population explodes to take advantage of all available resources.  The expansive part of the cycle favors rapid, even indiscriminate breeders, while survival is relatively easy.  Self-cloning would be ideal at this point, and a number of creatures (aphids, for example), clone themselves during the good times while reverting to sexual reproduction later on. 

Quite quickly, though, the population overruns available resources, and the game changes to a survival competition between individuals of the species, in which the less successful lose out and die.  The reproductive challenge shifts from producing as many offspring as possible to producing as many surviving offspring as possible.

Then, inevitably, bad times follow good, and available resources shrink.  Now the competition to stay alive becomes intense as the population drops, and the goal of reproduction is to produce any surviving offspring at all.  This is a period characterized by ruthless battles to determine who lives and who dies. 

Finally, there are the intermittent, unpredictable but absolutely inevitable periods of catastrophe, when for one reason or another the whole niche collapses and there are very few resources at all available for a time.  These are key periods from an evolutionary standpoint, because these are the points at which many fair-weather designs go under.  All kinds of life forms that have worked wonderfully well in a given niche while things were stable now fail completely and vanish in the course of the inevitable catastrophe. 

Genetic designs that have stood the test of millions of years have had to pass through many such "bottlenecks" of periodic disaster.  They've done so by building in safeguards and maintaining a lot of flexibility, and this is where sex is a big advantage. 

Sex permits flexible genetic design patterns

When we speak of "sex" in the broad arena of biological reproduction, we're usually referring to three aspects of the kind of sex we humans are used to:

• Multiple Inheritance — where more than one adult contributes to the genetic design of an offspring.  This strategy of combining genetic material provides for designs that can adapt rapidly to a wide range of conditions.
• Role differentiation — where the "parents" of offspring play different roles in the process of combining genetic material.  Normally, we distinguish two roles, which we label "male" and "female." 
• Physical differentiation — where individuals have distinct, non-interchangeable physical forms (e.g. different genitalia)  depending on what reproductive role they play.  Physical differentiation is by no means a given — there are plenty of life forms of which any individual can play either the male or the female role or both.  Another term used to describe physical gender differences is sexual dimorphism, but it usually applies more restrictively to secondary sexual characteristics rather than simply to differences in genital configuration.  Examples of such characteristics are differences weight and size, and features specific to one gender or the other that are not directly associated with reproduction, such as horns in deer, or manes in lions. 

Sex-role differentiation and sexual dimorphism fall in the category of engineering details used to make the re-combination of genetic material work in complex organisms — multiple inheritance is what gives sexual reproduction its huge advantage over self-cloning at every level of complexity. 

As we remarked above, self-cloning has the apparent advantage that it selects for the very most effective design.  Each winner's design is passed on in its entirety to every descendant of the winner, which results in faster propagation of winning designs than is the case when only part of a design goes to each offspring. 

But this apparent advantage turns into a huge drawback when things keep fluctuating.  The life-form's design keeps getting honed for precisely one set of conditions, and when those change, the optimized design is often too specific to be able to survive under the new rules.  At that point, more generalized, less optimized traits may well all have been eliminated, and the whole population may die. 

When genetic material is combined, on the other hand, a great deal of genetic diversity can remain in a population for a long time.  Then, when conditions change and the dominant design starts to die off, there are still many alternative designs latent in the population that can start being selected for right away.  Even if most of the population dies at that point, the underlying design complex reconfigures itself and survives.

So that's one explanation of why we have sex — to stay diverse and adaptable so at least a few of us survive when disaster strikes. 

For a discussion of the many (many!) issues, problems and controversies not included in that brief explanation, good places to start are John Gribbin and Jeremy Cherfas' The Mating Game: In Search of the Meaning of Sex (2nd edition), Bruce Bagemihl's Biological Exuberance: Animal Homosexuality and Natural Diversity, and Joan Roughgarden's recent Evolution's Rainbow: Diversity, Gender and Sexuality in Nature and People.