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Essay: Sexual Selection, Adaptation and the Human Brain

Posted By Robert DePaolo On December 19, 2011 @ 6:15 pm In Psychology | No Comments

by Robert DePaolo


This article discusses human evolution in terms of cue and trait detection skills facilitated by expansion of the human brain. A comparison is drawn regarding the co-impact of natural and sexual selection. The latter is considered to have a more potent, yet culturally influenced, happenstance effect on human evolution.

While natural selection is typically presumed to be the main driver of organic evolution, one could argue that in the course of an organism’s phylogenic development, nature, which changes slowly and often in punctuated fashion, has less to do with organic change than the simple interactions between males and females. Darwin of course alluded to this phenomenon in his discussions on sexual selection (1871). On the other hand the concept of fitness, which provides a cornerstone of his theory is not necessarily part of the sexual selection process, particularly for humans whose sensory discrimination, language and cognitive-creative capabilities might or might not coincide with fitness in a completely natural context – indeed might in some instances reverse nature’s tendency toward optimal organism-environmental congruence.

Evolution is a tricky concept; once tautological and vague. In and of itself the word implies a change or mutation into some new form. Yet the formal definition of the word has to do largely with the capacity among organisms to reproduce, as suggested in the work of Eberhard, (1996). The ambiguity of the term has to do with the fact that while adaptation and fitness are oft-used criteria, they are not encompassed in these narrow definitions. For example an organism can possess traits that are, or could be adaptive vis a vis the natural environment but unless he or she passed their genes on to the next generation, the potentially adaptive advantage of these traits would be irrelevant.

Thus the central question revolves around the likelihood of any given organism mating with members of the opposite sex – a point emphasized by Freud in discussing his evolution-rooted psychoanalytic methodology.
That implies that sexual selection could be considered on at least equal footing with natural selection in terms of what comprises “fitness.” Indeed it is tempting to substitute attractiveness for fitness as a survival determinant, for two reasons. First, the notion of environmental change leading to a winnowing process lacks what might be called chronological precision. For instance an organism (say a lion) might mutate toward a white coat and larger canines. If an ice age occurred around that time, both a light coat and larger teeth might provide an adaptive advantage; the former because it would better enable the cat to process vitamin C, the latter because larger teeth would help bring down prey who would tend be larger as a result of their own cold adaptations. Yet unless an ice age and these mutations occur within some similar time frame, or unless this mutated lion could somehow manage to pass on its new, rainy day genes, those traits would be inconsequential.

Interestingly, within the domain of sexual selection such mutations can actually be maladaptive. Females (who do most of the selecting) would likely be repelled by the odd looking lion, thus reducing his chances to pass on his genes and traits. Females, prescient as they can be, are not wired sexually toward future concerns. They think, act and emote in terms of the present, and in terms of what they perceive to be high quality traits as per current species norms. In that sense their tendency to veer away from novel versions of their species could serve to counteract any potential adaptive advantage white coated genes might have.

If, as discussed above, evolution is heavily dependent on reproductive rates and opportunities, and if sexual selection is based largely on species trait conformity, i.e. on traits and behavioral standards esthetically recognizable to females, then the only way for odd types to make a mark in subsequent generations would be to produce offspring at a much higher rate to make up for infrequent mating opportunities. In other words, the weird looking lion would have to find some way for the females with whom he mates to have more cubs than is the norm. Unless his sperm evolved into a more powerful, prodigious cellular entity that wouldn’t happen, especially since, regardless of sperm count only one sperm can fertilize one egg at a time in ontogeny.

In some sense that might be why complex mammalian social systems (which correlate with greater brain mass) (Lindenfors 2005) evolved. It might be why, despite his rather flimsy physique and bodily structure (small teeth, slow running speed and highly visible upright walking posture) homo sapiens was able to endure. Large brains can of course lead to sophisticated tool making and various other inventions that enhance group survival.

However, once again, for the large brain genes to be passed on could have required highly encephalized males and females participate fervently in the mating game.
How might that have occurred in the course of human evolution? First, consider that the first bipeds (presumably the

Australopithecines) would have initially mated in small numbers. It is entirely possible (and probable) that the genomes for themselves and the quadruped apes living around them were sexually compatible, so that the biped genes could have been introduced into the general population. It is even possible that bipeds had the advantage over quadrupeds, for example, their freed up hands might have made for better food gathering, perhaps offered a more gentle grooming touch, or an alluring hand clasp (“take my hand – come with me)” that proved persuasive. On the other hand there are quadruped apes in Africa but at present no bipedal apes and that contradicts the notion that bipeds out-reproduced quadrupeds. The first bipeds were different and quadruped females would surely have noticed that. Was this a repellant, an attraction or was the change irrelevant with regard to mating preferences? Regardless of the answer to that question, somehow the bipeds managed to mate and propagate, despite being initially fewer in number. The question is why?

One possibility is that bipeds were able to establish dominance, thus ascend the hierarchy and thereby enhance their mating opportunities; bearing in mind that in the world of the great apes females are extremely submissive so that their freedom to choose mates is nothing like in human social groups.

Thus despite the odd looking legs and posture of a biped, his apparent height – augmented by upright posture and use of hands – might have enabled him to dominate and attain alpha status. The dominance dynamic could have led to a proliferation of bipedal apes, that is, until their social groups expanded beyond a certain population threshold because something happens when the group gets so large and diverse that sexual selection becomes a complicated matter.

Social hierarchies are a way of establishing sexual fitness and attraction (Sadalla, Kenrick et. al 1987) but complex societies allow more members to have functional responsibilities and skills. As a result dominance becomes a more diverse process. More and varied males can be deemed attractive and capable of protecting females and offspring. However selecting mates amidst such societal complexity requires enhanced sensory, language and cognitive discrimination capacities.

The Evolution of Mr. Right…

As mentioned above, primate sexual politics (for species other than our own) tends to be fairly cut and dry. Typically the question of who gets to mate is resolved by one member of the troop – the alpha male. However, it doesn’t always work out that way. For example lower ranking male chimps occasionally, surreptitiously, find ways to pass on their genes. There is a certain risk involved in being caught by the alpha male but since sex is such a compelling activity, this happens anyway. In such instances the chimp female might be presumed to act in defiance of natural selection. On the other hand she might be displaying wisdom. Alpha males might have the strongest genes but genetic diversity is also crucial to the long term survival any species. Thus the idea of mate-variety might be a temptation to which she occasionally succumbs.

There is another factor to consider. Not all lower ranking members of the troop would be bold enough to take up with the alpha’s female. Thus she might surmise that this cunning male possesses an advantageous trait – a relatively high level of testosterone. By the same token, the fact that his sexual passion can override his fear of retribution also points to high levels of testosterone.

Since there is a strong correlation between testosterone levels and male dominance she is in a position where she can play with the house money by introducing both genetic variety and testosterone-fueled boldness into the gene pool.

Other factors come into play; for example mate availability at the time of estrus, duration of estrus (the shorter the duration the more frenetic the mating practices) and others. Thus even in chimp social groups the correlation between fitness and reproductive opportunities can be a bit loose. That in itself warrants discussion – particularly regarding the relationship between brain volume and fitness.


The volume, structure and function of the human brain might have altered, even diminished the importance of human evolution (Bronowski 1973). The process did not begin with humans. The advent of brains per se did that by creating insular mechanisms enabling highly encephalized organisms to bypass some of the pressures of the natural environment.

To further clarify: brain expansion leads to increased inter-neuronal connections, and consequently more complex firing differentials and neurochemical pattern redundancy. Electrochemical activity in the brain occurs on two levels. One is in response to external stimuli, as manifest in the “sodium pump”/action potential mechanism (Marmor, 1971), Barnett & Larkman, 2007). The other takes the form of ongoing, cross-synpatic patterns of activity in the brain which seem to have monitoring rather than reactive functions – much like the division of labor among genes concerned with protein/anatomical construction and other genes concerned with regulation of those functions (Pribram 1971). The latter type of activity sweeps the brain for purposes of providing distinguishing, recognizing and stabilizing functions for various faculties. Both processes can be described mathematically as wave patterns or algorithms.

Having such a dualistic mechanism allows the brain to respond to external input, in line with natural selection, but also in terms of internal memories and regulatory mental/emotional, i.e. feed forward processes which are submitted as a gauge, or proactive response so that preparatory arousal levels can be established and the individual can anticipate events that have not yet occurred. The reactive and monitoring functions enable the individual to differentiate between familiar and unfamiliar inputs and, through information-seeking behaviors arising from mismatches between input and internal schemes, lead to a curiosity drive, i.e. an a priori motive to learn about the world around them.

In simple terms the expansion of the human brain enabled our species to pre-empt direct experience and develop a capacity for imaginative adaptation.
In that context it might be said that to an extent the human brain obviates the need for trait variations. That doesn’t mean mutations would cease – they occur at a fairly regular frequency even in humans (Nachman & Crowell (2000). It does mean that natural selection might in the future have less of an impact on human evolution than some have presumed.

Internal Adaptation…

The human brain obviously has enormous information storage and retrieval capacities due to its volume as well as its structure. In information terms, having so many neuronal connections increases the probability of noise interference or oscillation levels (Wright, Robinson et. al 2001). That would pertain in particular to the cross-synaptic activities of the brain, which due to their sheer volume would have to deal with interference patterns constantly. The noise arising from the neuronal vastness of the human brain would lead – does lead – to a fairly permanent need for info-resolution. Thus humans are seldom “content.” They seem to require new experiences, explorations, forms of entertainment and art. In the absence of external conflict, duress or eustress (a creative manifestation of conflict) we conjure it up from within. We need to laugh, to experiment, to resolve internal confusion.

That tendency toward ongoing information seeking and closure has significant ramifications for human evolution.

That is particularly true with regard to human sexual selection. To explain why, one need only ask what it means to be an adaptive or “fit” human. Arguably that question has so many answers as to be unanswerable. For example having intelligence is important – but what kind? With a brain as large as ours cognitive abilities can be manifest in various ways. In a pinch, should a female choose a gifted poet, rock musician or mechanic? Does it depend on extant socio-environmental factors? Our species’ survival over the long run has correlated with tool advancement and if mechanics make better tools than guitar players or writers, then why wouldn’t the “grease monkey” as per his auto-repair gig produce as much female hysteria as the rock star? It might seem an absurd example but in a way exemplifies the odd relationship between our large brain and our trait selection practices.

Another example regards physical prowess. Stronger males are usually more capable of protecting mates and offspring. So why is it that fragile males with puerile features are often more attractive to females than their more robust counterparts? Is it because some human females develop maternal feelings toward the fragile male – as for example in the early Sinatra/Montgomery Clift genre? Is it because strong males who can harm intruders can also harm offspring, thus making them somewhat threatening to human females, who want to know both what dad’s genetic and behavioral influence will be on the family? Or is it because human evolution has been driven by a process known as neoteny, a prolonged youthful look extending past maturation which females have come to perceive as a distinctly human trait (Jones 2011).

Still another criterion for mating choices is discussed under the rubric of symmetry theory. The theory, as reflected in the work of (Moller 1992) suggests that symmetrical features serve as an attraction for the opposite sex.

Because anatomical symmetry can provide clues as to the overall health of any individual within a species that makes sense; that is until one considers all the “disproportionate” men and women out there who not only find mates but turn out to be just as healthy and able to sire and bear children as their “more perfect” competitors.. Even more interesting is the possibility that as “bio-natural” as it might seem, sexual selection is a function of culture and cognition and is therefore malleable. If that were true, then it could be said that unlike natural selection sexual selection would be heavily influenced by learning, which would contradict to an extent not only Darwin’s theory but also the ideas developed within the field of evolutionary psychology. It makes human evolution seem somewhat arbitrary. Indeed the fact that sexual choices by human males and females often seem adaptively inconsequential, or even at odds with the concept of ecology-based fitness would seem to render uncertain the evolutionary future of mankind.

Human emotion offers still another potential counter-evolutionary influence. This is most clearly seen in the human tendencies toward compassion and altruism. Like elephants and chimps, humans protect and nurture the weak and disabled. We do not ordinarily condone their bearing children but we do help them live long enough and in productive enough circumstances to keep nature from weeding them out. In a meta-biological (rather than strictly humane) context, the question could be asked as to why.

This is an interesting topic that entails a bit of a twist. Many evolutionary psychologists describe human behaviors generally considered negligent, cavalier, or perhaps even cruel as instead being “natural.” At the same time they argue that altruism is highly adaptive. In a strictly biological sense both arguments have merit – a point made by Freud long ago in his discussion of a modulating mechanism of mind he called the ego.

Reciprocal altruism enhances group solidarity and could readily enlist the cooperative efforts of the group in hunting, gathering and child rearing practices. On the other hand altruism has also led to the premature release of prisoners and the coddling of unproductive but capable individuals in society and the siring of children by psychopaths. Thus the question of whether altruism is invariably adaptive is perhaps best answered through a glance at human neurology.

The main reason we are altruistic is because of our brains. Due to our enhanced memory capacities we can come to know individuals (and typologies) much more specifically than other creatures. The need for resolution resulting from the high noise levels in our large brains requires closure, ie. some way to assemble experiences and feelings into a gestalt – or “good fit” while tossing out the irregular aspects of experience. That leads us to develop typologies and to label and conceptualize our fellow humans in ways that lead to personality, moral and character traits. Such evaluative cognitive habits constitute a type of judgment and provide a bridge leading to moral thought. Also, due to our brain’s capacity to hold memories and to exert control over fine motor faculties we are able to symbolize, write down and talk about our experiences so that future generations need not go through a trial and error process from scratch. All of that provides for intellectual and informational continuity that in itself obviates the need for mutations. In effect historically transmitted information enables even (ostensibly) less adaptive members of our species to thrive.

The speed and volume by which humans learn to distinguish and conceptualize traits and typologies (which directly influences rank order), exceeds by far the slow pace of natural selection. Since sexual attraction is driven by such image distinctions it too can override to an extent the alternate forces of nature. Since the emotion and homeostatic circuits in the mid brain are vastly connected to the memory, sensory, and monitoring circuits, it would appear all our emotions are subject to the same distinguishing, memorizing, social organizational faculties that emanate from higher centers in the cortex.

That is why human sexual selection can be a rather whimsical process. At times mates select one another based on hard logic, for example… how attractive they feel vis a vis rivals and potential mates. It is often couched in appraisals such as.. She’s gorgeous – what chance do I have? … or perhaps… Not many eligible bachelors in this small town, guess I’ll marry the first one who makes me laugh…or even…He’s kind of repulsive but he’s also rich, and since the grandeur of his mansion makes up for his craggly face and sagging physique I think I’ll take the plunge. In other instances the appraisal is based on emotion, for example… He seems so vulnerable, needs someone to take care of him. Humans select mates for almost as many reasons as there are humans. Thus the same hyper-discriminatory capacities produced by a brain as large as ours also leads us to engage in a wide variety of sexual choices, which ultimately has a strong but arguably vague impact on human evolution.

In the final analysis the impact of hyper-discriminatory cognition on human evolution might be more consequential than natural selection. While nature tends to select the strongest, healthiest and smartest vis a vis environmental conditions, humans select sexually at a far more rapid pace and with much greater variety. In other words in a race for adaptability, attractiveness will always win out over fitness, making natural selection much less relevant for our species.

That might be one reason the human gene pool features few differences among peoples in various parts of the world (Storey, Madeoy et al (2007 ). In that context a correlation of sorts becomes apparent. We have the largest brain to body ratio in the phylum and perhaps as a result have evolved over time less than any other species. Homo sapiens is, despite his penchant for espousing class distinctions, group allegiances, jingoism and ideologically and racially – fomented warfare, a more insulated, singular species than perhaps any animal that has ever walked the planet.

Does that signify that large brains are all we need, and as a corollary; is it possible to override natural selection? If so, does this portend for a good or bad future? It is difficult to even hazard a guess. We might be end up protecting our fellows and choosing mates dispersed all along the normal curve for every imaginable trait, personality characteristic and behavior pattern (not to mention the influence of our own interpretation of those traits) to such an extent as to ameliorate the fitness paradigm. Or we might become increasingly more informed, and able to control the environment, thus prepare for and modulate ecological changes that have been the undoing of other organisms.

Ultimately the most adaptive human trait might be cognitive; specifically a capacity for proportionate thinking. That skill would enable us to decide which altruistic standards to accept or reject, how to balance our sense of dependency with the need to be independent, how much weight to put on physical features, cognitive ability, social empathy and other characteristics. Since multi-apportioning thought is extremely difficult – and arguably beyond the unforced and natural capabilities of modern humans – finding the golden mean of human fitness from among the factors of direct experience, cultural learning and the natural world might require further expansion or structural changes within the human brain to create a faculty devoted to the integration of social, emotional and biological algorithms.
If that did not happen and instead humans continued to mate with one another based on social trends and personal whims, our future might turn out to be ironic, In such circumstances it would mean the one species capable of discovering the laws of evolution is unable to abide by them.


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