Cumulative Selection & Evolutionary Psychology

Darwinian evolution is the idea that all the varieties of life on earth are related, having descended by a process of gradual and cumulative modification from a single common ancestor. An important driving process of this modification is natural selection, described in my earlier post entitled An Evolutionary View of Emotions. Note that I have emphasised the words gradual and cumulative. A common criticism of Darwinian evolution is that the probability of any complex biological structure, whether it be an eye, a wing, or the brain assembling itself by chance (without the guidance of some supernatural creator) is so incredibly small as to be, for all practical purposes, impossible. It has been equated to the probability of a hurricane sweeping through a junkyard and assembling a Boeing 747 out of the scrap metal (Dawkins, 2006).

This criticism betrays a strong misconception about the evolutionary process. Complex biological structures DO NOT suddenly spring into being by chance. The notion that an eyeless organism would give birth to an offspring with a fully-functioning pair of eyes is absolutely ludicrous. The eye, like any other structure that has evolved by Darwinian evolution has done so by a gradual and cumulative process of selection. When previous designs are modified in very slight ways that provide an organism with some type of advantage, however slight, the modified designs are preserved and becoming more prevalent in future generations. In the often cut-throat natural world, even very slight advantages to an organism can prove telling in the evolutionary struggle. While the probability of a complex biological structure suddenly springing into being as a functioning whole may be effectively zero, the probabilities of the much smaller individual modifications that will accumulate over time, leading to the fully functioning structure are much, much higher. A metaphor for natural selection used by the British zoologist Richard Dawkins is that of a mountain – Mount Improbable:

"Mount Improbable rears up from the plain, lofting its peaks dizzily to the rarefied sky. The towering, vertical cliffs of Mount Improbable can never, it seems, be climbed. Dwarfed like insects, thwarted mountaineers crawl and scrabble along the foot, gazing hopelessly at the sheer, unattainable heights. They shake their tiny, baffled heads and declare the brooding summit forever unscalable." (Dawkins, 1996, p. 64)

The peaks of Mount Improbable stand for evolution’s most complex achievements such as the eye, or the human brain. However, as Dawkins continues:

"Our mountaineers are too ambitious. So intent are they on the perpendicular drama of the cliffs, they do not think to look around the other side of the mountain. There they would find not vertical cliffs and echoing canyons, but gently inclined grassy meadows, graded steadily and easily towards the distant uplands. Occasionally the gradual ascent is punctuated by a small, rocky crag, but you can usually find a detour that is not too steep for a fit hill-walker in stout shoes and with time to spare. The sheer height of the peak doesn’t matter as long as you don’t try to scale it in a single bound. Locate the mildly sloping path and, if you have unlimited time, the ascent is only as formidable as the next step." (Dawkins, 1996, p. 64)

Another criticism leveled at Darwinian evolution at this point is that an eye is a very complex structure. For example, it has a focusing lens and a light sensitive screen positioned just at the focal plane of the lens, and an iris that contracts and expands to let in just the right amount of light. If some or all of the parts arranged in a seemingly delicate balance were not in place, the eye would not be able to function properly and therefore would not be of any advantage to its bearer. So how could a complex biological structure such as an eye evolve by cumulative modification? A more concise way of putting this argument is “What good is half an eye?”

As I noted before, in the cut-throat natural world even very slight advantages to an organism can prove telling in evolution. In the case of the eye, an eyeless ancestor could have been born with a bit of skin that happened to be a bit more sensitive to light than normal. All skin is slightly sensitive to light anyway (Evans & Zarate, 1999). This slight modification would have enabled its bearer to detect the shadow of a predator slightly quicker than its siblings and therefore improve its chances of escape and survival ever so slightly, allowing natural selection to occur (Evans & Zarate, 1999). Further slight modifications continued to accumulate until eventually we had a complete eye. This short essay is meant simply as an introduction to the idea of cumulative selection. For those readers that are interested in reading further, I refer you to Richard Dawkins’ wonderful book, Climbing Mount Improbable, which goes into much greater detail about the evolution of the eye. I also include a link here to a video of a lecture given by Dawkins on the topic of cumulative selection.

The evolution of complex biological structures by natural selection is a gradual process which may take millions of years. Part of the reason for this is that natural selection can only act upon the phenotypic variation that is already present in a population. An important source of this variation is mutation.

As noted in my earlier post entitled An Evolutionary View of Emotions, one of the conditions that need to be met in order for natural selection to occur is that phenotypic (physiological, anatomical or behavioural) variation must be able to be fairly reliably passed from parent to offspring. In Charles Darwin’s day people were aware that children tend to resemble their parents in certain characteristics, but the actual mechanism by which these characteristics were passed from parent to offspring was unknown. Nowadays we know that genes are the agents by which this variation is fairly reliably passed from parent to offspring. A gene contains instructions vital to the development of a particular characteristic. These instructions are in the form of a universal code of bases called DNA that is present in all living organisms. This code needs to be copied and then passed onto the offspring during reproduction. Occasionally random copying errors are made. These copying errors are mutations. Most mutations are either inconsequential or are harmful to the individual’s chances of surviving and reproducing. If a mutation is harmful, then typically the gene containing that mutation is not passed on to offspring and is therefore not present in future generations. But very occasionally a mutation will cause a change in the phenotype of an individual that will prove beneficial (however slightly). Because mutations are uncommon, beneficial mutations much more so, the evolutionary process is typically very slow.

Evolutionary psychologists argue that the human brain is the product of a long period of evolution in much the same way as an eye or wing. Fitness-enhancing modifications to the brain’s information processing mechanisms have been preserved and accumulated over the course of millions of years (Buller, 2005). As noted in an earlier post, an organism’s fitness is largely dependent on how well adapted its characteristics are to the demands or pressures of the environment it inhabits. These “selection pressures” can be thought of as adaptive problems. So the brain must have been “designed” by selection to produce behavioural solutions to adaptive problems faced by human populations.

The slow nature of the evolutionary process leads us to one of the key theoretical assumptions of the paradigm of evolutionary psychology associated with Tooby, Cosmides, Buss and others. Because cultural and technological evolution occurs much faster than biological evolution, our minds are adapted to a very different environment than the one we inhabit today. Evidence of settled agrarian (agriculture-based) cultures is only evident from around 11,000 years ago. This is a blink of an eye in evolutionary time. For about 95% of the last two million years of our species evolution, a period of time referred to as the Pleistocene, early humans – our ancestors – would have been living a nomadic hunter-gatherer lifestyle in tribal groups in the savannah plains of southern and eastern Africa (Fletcher, 2002). Fossil evidence indicates that the ancestors of humans originated in Africa (Campbell et al., 2008). Evolutionary psychologists argue that natural selection has shaped our minds to the demands of this life-style with no major changes to our psychology in response to the significant cultural and technological advances since the Pleistocene because 11,000 years is nowhere near long enough for significant evolutionary change to take place. To put it another way – “our modern skulls house a Stone Age mind!” (Buller, 2005)

I have already noted one apparent piece of evidence for this theoretical assumption in an earlier post on this blog (An Evolutionary View of Emotions) – the strong tendency for humans to develop a fear of snakes. A fear of snakes remains by far one of the most common fears that people the world over exhibit, even in places where the only contact that people have with snakes is in very safe, controlled environments such as zoos and wildlife parks. Evolutionary psychologists argue that this is because poisonous snakes were a major threat to our hunter-gatherer ancestors in the African savannah, and the tendency to develop a fear of snakes was the evolved “solution” to the adaptive problem of avoiding this potential danger. A continued pre-disposition towards snake phobias, even in places where we are unlikely to face a threat from poisonous snakes, is a result of this Pleistocene “evolutionary strategy” to avoid potentially dangerous snakes.

From an evolutionary perspective (that is, thinking in terms of what will improve the odds of survival), wouldn’t it make more sense if people that lived in large urban areas were born with a biological predisposition to develop a fear of motor vehicles? After all, in these areas being hit by a car is a more likely way of getting yourself killed than being bitten by a poisonous snake. Yet, for humans in all walks of life, it is much easier to develop a fear of snakes or spiders than cars. Evolutionary psychologists would argue that because motor vehicles were only invented a little over 100 years ago – roughly four generations ago – such a biological predisposition would not have had anywhere near enough time to evolve by natural selection.

A common argument against evolutionary psychology is that if the aim of life is to (survive and) reproduce, why do so many people use contraception during sex. This argument betrays a common misunderstanding about evolutionary psychology. Natural selection has NOT resulted in a conscious motive to have children. Evolution has instead made sex desirable and rewarding because it is through this behaviour that we reproduce. People generally enjoy sex, but don’t necessarily want children. Also, reliable contraception is a development of the last few decades. As a result, we are now generally able to separate the act of sex from reproduction fairly reliably, unlike our Pleistocene ancestors. This hasn’t had any effect on our evolution-installed desire for sex, with or without a condom. As would be predicted from an evolutionary psychology perspective, the very recently acquired ability to separate the act of sex from reproduction has not changed our underlying psychology.

According to an evolutionary hypothesis, referred to as Parental Investment theory, in any species, the sex that invests more in offspring (generally the female) will evolve to be more choosy about mating, while the sex that invests less in offspring will evolve to be more competitive with other members of their own sex for sexual access to the valuable, high investing opposite sex (Buss, 1995). In the case of humans, like all other mammal species, the female is the high-investing sex. The minimum obligatory investment for a female during reproduction is nine months of pregnancy. Estimates tend to vary (depending on who you ask!) about the minimum obligatory investment by males, but post-coital investment is not required by males. Research supports Parental Investment theory (see Fletcher, 2002). In one famous experiment, the experimenters had attractive confederates approached members of the opposite sex on the University of Hawaii campus and asked them if they wanted to have sex. More than seventy percent of males said yes, but not one of the females did! What’s more, most of the males that turned down the offer apologized to the female confederate and explained themselves by saying they were married or already involved with someone else, whereas most of the females responded with outrage (Fletcher, 2002).

References

Buller, D. J. (2005). Adapting minds: Evolutionary psychology and the persistent quest for human nature. Cambridge, MA: MIT Press.

Buss, D. M. (1995). Evolutionary psychology: A new paradigm for psychological science. Psychological Inquiry, 6, 1-30.

Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2008). Biology (8th ed.). San Francisco: Pearson.

Dawkins, R. (1996). Climbing mount improbable. London: Penguin.

Dawkins, R. (2006). The god delusion. London: Bantam Press.

Evans, D., & Zarate, O. (1999). Introducing evolutionary psychology. Cambridge, UK: Icon Books.

Fletcher, G. J. O. (2002). The new science of intimate relationships. Oxford: Blackwell.