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Evolution and Our Sexual Behavior

Looking at our sexual experience from an evolutionary perspective can be incredibly valuable — we have so much to learn from our fellow creatures.  Seeking evolutionary rationales for biological forms and behaviors has proven extraordinarily fruitful from Darwin's time onwards.

Does that mean our behavior is simplistically deterministic?  Not at all. If you love the natural world, you know how deeply and creatively chaotic it is;  the closer you look, the more complex and profoundly beautiful it seems.  Looking out at the rest of the natural world helps us understand ourselves much better, but that doesn't mean we necessarily understand ourselves or nature all that well.

Evolutionary causality is particularly intricate and hard to tease out.  We're talking about long-term interactions between the complex biological system that makes up a life form and the complex ecological system in which it lives and which it also influences.  When we ask "Why?" we're used to getting an answer that begins, "The reason is..." When it comes to evolution, though, answers almost always have to start, "Some of the more important reasons seem to be..." 

Evolution, politics and religion

Unfortunately, biological causality has long been regarded as having useful or dangerous implications for political or religious agendas.  Because people engaging in propaganda have little or no reason to test their hypotheses or scrutinize the validity of their assumptions, a great number of uninformed and downright silly notions have therefore been put forward in the name of "science" in this area. 

Ever since Darwin's day, Christian fundamentalists have opposed the idea that the universe is billions of years old because that conflicts with the creation myth in the biblical book of Genesis.  For the same general reason, "Creation Scientists" and proponents of "Intelligent Design" continue to this day to propound a raft of uninformed and largely specious arguments designed to show that evolution couldn't have produced life as we know it. 

Racists and colonialists, on the other hand, quickly jumped on Darwin's evolutionary theories as a handy way to justify their own social prejudices and inequitable treatment of others.  "Social Darwinism" claimed to demonstrate that wealth and privilege are natural rewards conferred by the "law of the jungle" on superior individuals, tribes and cultures. 

Not to be left behind, male gender politicians have been invoking "biology" in support of demonstrably unfair and unfounded gender distinctions for several centuries as well.  Even today, reputable researchers in the field of evolutionary behavior can let their own gender prejudices slip into their conclusions with little or no examination.  Two notable and enjoyable counterbalances to such tendencies are Natalie Angier's Woman: an Intimate Geography and Joan Roughgarden's recent Evolution's Rainbow: Diversity, Gender and Sexuality in Nature and People

And yet, all the fuzzy-headed axe-grinding is easier to understand when you realize how subtle and complex evolution really is.  It's an incredibly difficult combination of biology, ecology and mathematics, and it's certainly not an area where simple, "obvious" conclusions often turn out to work as expected.  Serious evolutionary theory must be as sophisticated and elegant as nature, which is sophisticated and elegant indeed. 

The bottom line, as is almost always true in science, is that we understand a lot more than we used to about evolution, but only a tiny portion of all there is to understand.  Below, I'll sketch in some what I currently think I comprehend.

Selection: the core of evolutionary theory

The key concepts of evolutionary theory are deceptively simple: 

  • Each life-form carries its own genetic "blueprint" that it can pass on to its descendants in whole or in part.  These genetic designs are subtly variable between individuals, both through random mutation and through reproductive recombinations. 
  • The natural environment with its challenging conditions and limited resources acts as an effective filter to "select" and propagate the "best" of these designs (ones whose life-forms most successfully survive and reproduce) and to reject others that don't work as well. 
  • Across thousands of generations, the constant design improvements propagated by relentless selection are additive, and often result in changes so radical that it's hard to recognize where they originated.  This is the magic of evolution. 

One subtlety that has recently received some attention is the possibility that our DNA itself evolved to mutate more efficiently.  Entirely random mutations don't often enough result in useful changes to explain the speed with which evolution sometimes seems to occur.  Scientists have recently been finding evidence that back when we were bacteria, our DNA started organizing itself so as to be more likely to mutate beneficially!  

A quick glance at evolutionary feedback mechanisms

Darwin and his successors have distinguished several different ways in which the trials of life and challenges of reproduction can shape the evolution of genetic design: 

Natural selection — favors traits that help individuals survive.  If you have such a trait, you're less likely to die, and thus more likely to produce offspring who also have that trait, who in turn are less likely to die, and so on. 

Reproductive selection — favors traits that help an individual breed successfully.  If you have a trait that makes you seem sexy, for example, you have more opportunities to mate, resulting in more offspring who have that trait, who in turn have more opportunities, and so on. 

Although Joan Roughgarden has recently declared Darwin's theory of sexual selection to be fundamentally flawed and no longer useful, she's mainly attacking some of Darwin's simplistic generalizations about what constitutes reproductive advantage.  Her work enhances our understanding of reproductive selection by analyzing important social factors that provide access to the resources needed for successful procreation in social species (see below). 

Whatever name you call it by, reproductive selection often plays a clear and important role in determining which genetic designs are propagated and which are not, and probably had an important part in guiding the development of our intelligence. 

Kin or group selection — favors traits that help your family or social group survive.  Such a trait is selected for indirectly, not because it helps an individual survive and pass on its gene, but because it helps others who carry the same gene to survive and pass it on. Although group selection appears to be rare in nature and probably hasn't affected human evolution at all, kin selection is fairly common and does appear to have been important for us as well.

Spandrels, competition and design refinement

Stephen Jay Gould, an eminent but often controversial evolutionary theorist, used the term "spandrel" to describe accidental aspects of genetic design, neutral traits that just randomly evolved alongside of advantageous ones.  Gould argued that if such traits weren't particularly disadvantageous, they wouldn't be selected out, and so might just came along for the ride without any particular reason.  He argued that a lot of traits could fall into this accidental category. 

The idea of spandrels led biologists with a mathematical bent to try to estimate just how fine nature's evolutionary filter is.  How easy is it for an accidental trait to propagate?  How much "noise" is likely in the process of selection?  How significant does a disadvantage have to be in order to be selected out of a genetic design over time? 

The answer, of course, is complicated, like everything about evolution, but it seems that under "normal" circumstances (a highly competitive ecological niche), and a "normal" time frame (thousands to millions of generations), the selective process is a very fine-grained filter indeed.  Under conditions of intense competition, even small advantages and disadvantages are selected for and against, and there's little room for accidental traits to propagate. 

However, the whole idea of "normal" circumstances turns out to be in itself a gross oversimplification.  Selection, after all, takes place as the specific result of a species interacting with its environment, so it's not surprising that characteristics both of the species and of its environment should affect the mechanisms and outcome of the selection process.  Although statistical probabilities applied over time do smooth away some circumstantial differences, others prove to be intrinsic to what gets selected for and how.  Our complexities page contemplates some of these issues in greater depth. 

Joan Roughgarden's Theory of Social Selection

In Evolution's Rainbow, Roughgarden calls for Darwin's theory of sexual selection to be jettisoned entirely.  She clearly states her belief "that this theory has promoted social injustice and that overall we'd be better off both scientifically and ethically if we jettisoned it."  (page 164)

Sexual selection theory has long been used to perpetuate ethically dubious gender stereotypes that demean women and anyone else who doesn't identify as a gender-normative heterosexual male.  (page 172)

Her passion has a strong basis in her personal experience, as she explains:

I've been clear about where I'm coming from.  I'm a transgendered woman; I have standing, as lawyers say, to sue for damage against this theory: it denies me my place in nature, squeezes me into a stereotype I can't possibly live with — I've tried.  For me, discrediting sexual selection is not an academic exercise.  (page 175)

She then goes on to propose that "animal species with distinct males and females interact socially to acquire opportunities for reproduction," and she calls this approach her "Theory of Social Selection."  One compelling benefit of this approach, in her view, is that it reduces what she regards as an unwarranted emphasis on competition between individuals, an emphasis she believes was "discarded over fifty years ago as the central metaphor of mathematical natural selection."  (page 162) 

It seems to me that Roughgarden has been led (like so many of the scientists she opposes) into seeking conclusions that are ideologically comfortable.  Those earlier scientists deeply feared homosexuality, so they tried to ignore it or downplay its importance wherever possible.  Roughgarden, I believe, equally ignores unpleasant aspects of evolutionary competition, and reduces Darwin's theory of sexual selection to his admitedly crude and over-simplified generalizations about male and female behavior.  While it is true, as she points out, that competition over resources is not the only factor that operates in natural selection, it very clearly plays a central role in many instances, and many "co-operative" symbiotic relationships in nature, such as between predators and prey, seem to have evolved in order to ensure sustainable resource usage. 

Roughgarden believes in particular that the widespread existence of gay sex in so many species invalidates Darwin's insights:

The final nail in the coffin of Darwin's sexual selection theory is the discovery of extensive same-sex sexuality in nature.  (page 127). 

Yet the essence of Darwin's theory of sexual selection is simply that sexual choices are a key determinant of what traits are selected for, and this is not something she disputes.  What she is doing is adding nuance (important nuance, at that) to our understanding of how and why sexual choices further reproductive success. 

Are we Post-Darwinian yet?

So is Darwin really washed up?  Are scientists gradually refuting his theories?  Certainly many earlier ideas about how evolution works are increasingly shown to be simplistic and mistaken, and quite a number of evolutionary biologists have indeed begun to wonder whether Darwin was right after all. 

This, of course, delights the Christian fundamentalists, because obviously if Darwin was wrong, the Bible must be right.  Oddly enough, biologists don't seem to agree.  Instead, they've proposed several alternative theories that have sparked some interest. 

One is James Lovelock's "Gaia hypothesis," that the earth itself developed as a single large organism, of which we and all other living things are organic parts.  We've all really evolved so as to serve as the analog of its cells or organs. 

Another is the economic "exuberance" theory of Georges Bataille, favored by Bruce Bagemihl.  This theory holds that a significant part of life's forms and behaviors evolved not to cope with scarcity or competition, but to dissipate some of the vast store of surplus energy available to us. 

These ideas have more to recommend them than such brief summaries may suggest, but in my opinion, they don't constitute any viable "new paradigm."  Just as Roughgarden's analysis extends and revises Darwin's theory of sexual selection rather than refuting it, so the primary value of these other theories comes from adding perspective and complexity to our understanding of Darwinian evolution rather than from offering any viable replacement of it. 

Darwin's central, brilliant contribution remains as valid today as when he first published Origin of Species: the idea that survival coupled with reproductive success form a kind of filter that has been the underlying mechanism for change and design refinement in all forms of life we know. 

Even though we manifestly don't (and probably can't) understand every factor that may have caused one design to be more successful than another, and even though success may often have "accidental" components not directly related to "fitness," nonetheless it's extremely useful to keep in mind that the reason life takes the forms we observe is always because those forms have successfully survived and bred over many generations. 

So no, we're probably not entering into a post-Darwinian age, because as we continue to unravel and understand more about the nature and complexity of the evolutionary filter than Darwin did, using a vastly larger body of available data, still his core concept of the underlying mechanism of change continues to prove valid. 

Simplicity, Complexity and Chaos

The mathematics of chaos theory has delightful aesthetic applications, and is increasingly being applied to biological systems.  As in the case of evolution, however, what we understand in chaos theory is very clearly dwarfed by what we don't (see Gregory Chaitin's Understanding Randomness, Springer-Verlag 2001, for an extreme statement of this point). 

One of the things that I believe has caused Darwin's simple insight to be questioned is the mass of evidence showing that evolution is not simple. 

At the same time, it's widely understood in biology, and more recently also in mathematics (see, for example, Wolfram's exploration of automata in his controversial A New Kind of Science, 2002) that complex, chaotic systems arise out of interactions between simple building blocks. 

Darwin gave us the tools to understand what the building blocks are, but figuring out the resulting systems is a challenge we're very unlikely ever to fully meet. 

Evolution and Sex

What we currently understand about the evolution of sex, and in particular about the evolution of human sex, is still dwarfed by how much we don't know and can only yet guess about.  As I remarked earlier, almost everything about evolution turns out, the more closely you examine it, to be far more complex and subtle than it first appears.  That's why propaganda aimed primarily at making political or religious points so seldom turns out to have any scientific value in this area. 

One thing that should be clear though, is that sex and evolution are intimately, inextricably connected — sex is the way we reproduce, and in an evolutionary context, reproduction is the single most important action of any individual. 

In the next few pages, I'll suggest how sex can be understood in the context of evolution, to give you a flavor of how much fascinating information is available on the subject, while trying also to keep in perspective how much uncertainty still exists in the scientific community about how we evolved as we have. 


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