On the Nature of Natural Selection

If you hear “natural selection” and your mind jumps to “survival of the fittest,” this post is for you. I’ll strip the theory of evolution by natural selection down to the studs, in order to rebuild it without misconceptions.

Acknowledgements: This is my introductory chapter from The Cambridge Handbook of Evolutionary Perspectives on Sexual Psychology, so bear in mind that it was written for an audience of research psychologists. I’m including it here as a reference, and to showcase my reasoning skills. Many thanks to Todd Shackelford for inviting me to contribute a chapter to the handbook, and to Courtney Crosby, Patrick Durkee, and David Buss for their feedback.

Give me Order, [Darwin] says, and time, and I will give you Design. Let me start with regularity – the mere purposeless, mindless, pointless regularity of physics – and I will show you a process that eventually will yield products that exhibit not just regularity, but purposive design.

–Daniel Dennett, Darwin’s Dangerous Idea, p. 65

“Why do humans have sex?” The answer seems so self-evident that the question might never occur to most humans. The obvious answer is: We have sex because it feels good. Many other answers at different levels of analysis can be provided: We have sex because we are motivated to have sex. We have sex because sexual stimulation is positively reinforcing. We have sex because sexually arousing stimuli often trigger hormones that produce sexual arousal and desire. We have sex because we have psychological mechanisms that motivate us to respond to events with sexual content with specific behaviors.

But each of these questions produces another “why” question of its own. Why does sex feel good? Why are we motivated to have sex? Why are we reinforced by sexual behaviors? Why are certain stimuli sexually arousing? Why are certain events coded as sexual, and why do they trigger particular thoughts and behaviors? Prior to the mid-nineteenth century, for anyone who took them seriously, these questions would have resulted in an infinite regress of “why” questions akin to the experience of explaining the topic to a three-year-old (Dennett, 1996). Frameworks that ignore or do not provide answers to these questions can be productive in understanding how things work, but will ultimately be shallow without the answers to why. It is hard to understand a phenomenon fully knowing only its mechanisms, without knowing its purpose. Frameworks like this limit science’s capability to explore psychological phenomena by limiting the kinds of questions that can be asked.  

Charles Darwin’s theory of evolution by natural selection made it possible to address these deeper “why” questions. When it was published, On the Origin of Species by Means of Natural Selection overturned two fundamental assumptions of the time: that species had ideal essences that made them qualitatively distinct from one another, and that design could only be produced by a mind (Dennett, 1996). Darwin’s explanation of how function and purposive design could be produced by a mindless process gave us the opportunity to meaningfully ask and answer those “why” questions that would have previously resulted in infinite regress, or been cut short by an appeal to the unknowable goals of an omnipotent deity. In response to a “why,” we could now find an ultimate “because” – ultimate in the sense of being further removed in the causal chain of events. For any apparent design in nature, we could now consider evolutionary function.

This idea has echoed through all domains of natural science, psychology included. In the late 1980s and early 1990s, following several precursors, the paradigm of evolutionary psychology emerged, centered around explaining human psychology through the lens of the mind as a product of evolution by natural selection (Buss, 1995). A key feature of an evolutionary psychological approach is a focus on psychological mechanisms: the information-processing programs of the mind. Mechanisms are not visible – all one can observe are inputs to the mind and its behavioral or physiological outputs. The mind is effectively a black box, and understanding the nature of the mechanisms contained within it is essential to the enterprise of psychology.

A guiding heuristic for evolutionary psychology is that “form follows function” (Cosmides & Tooby, 1994, p. 328): If you can ascertain or reasonably hypothesize the ultimate function of a mechanism, this will vastly simplify the job of understanding how the mechanism operates. Steven Pinker presents a demonstration of this principle, where he brings out an olive pitter and

holds it up for the crowd to see. “What is it?” he asks. “To figure it out, you’ve got to do a little reverse engineering. You notice the rings and the lever and you think, well, maybe it’s supposed to take seeds out of something. So you try cherries but they’re too small. Then you try olives, and all of a sudden you know why canned olives have a little X on the end: that’s where this little blade cuts them.”

(Soderstrom, 1998)

The point of this demonstration is that observing the design of the device narrows the list of potential functions, each of which can then be tested empirically. Therefore, evolutionary psychology proposes that the ideal approach to understanding the mind in its present form is to focus on the likely psychological problems that must have been solved by our ancestors – solutions without which they would not have become our ancestors. This approach provides a systematic empirical framework with logically derivable hypotheses and predictions about the function, and therefore the structure, of our psychological mechanisms (Buss, 1995).

To make fruitful predictions about the nature of human psychological mechanisms, we must comprehend the process by which they evolved. This chapter and those that follow it in Part I of this handbook will provide the groundwork for developing and assessing productive hypotheses about the evolutionary function and structure of sexual psychological mechanisms. The cornerstone of this framework is the phenomenon of natural selection.

The Process of Natural Selection

Natural selection, in the broadest sense, is the process by which entities that are better at making themselves more numerous than others will become more numerous than others. In this sense, natural selection can be described as a tautology: something that is true by necessity or by virtue of its logical form (Dawkins, 2016a; Williams, 1972). In other words, it happens by definition. As stated, natural selection is tautological because the negation of the earlier statement – “entities that are better at making themselves more numerous than others will not become more numerous than others” – is logically impossible. It is a complex process, but given certain starting conditions, the results could not be otherwise.

This might sound surprising to those less familiar with evolutionary theory. How can the most important idea in the history of biological thought be a tautology? The term “natural selection” often conjures a grand notion of some abstract, goal-directed agent – Mother Nature herself – consciously evaluating populations and selecting the organisms she deems worthy. This misleading perception is not helped by the fact that evolutionary scientists often find it expedient to use phrases like “selected for/against” to describe the process of some entities becoming or remaining more numerous than others over time. It can be difficult to accept or understand the simplicity and inevitability of a force like natural selection when faced with the astounding diversity and functional complexity observed in its products.

The confusion often comes down to a conflation of the phenomenon of natural selection, on the one hand, and the theory of evolution by natural selection, on the other. The latter is the theory initially proposed by Charles Darwin in 1859 and expanded by many scientists in the subsequent century. It is the best accepted scientific explanation of the origin of biodiversity and biological adaptation; it is a mechanism of systematic change in populations over time. It is a complex theory that has spawned many subtheories and niche fields of investigation. Natural selection itself was identified by Darwin as the key mechanism operating within the theory – but natural selection itself is not a theory. It’s a simple fact, not only of nature, but of any population of reproducing entities that satisfies three preconditions: variation, inheritance, and nonrandom differential reproduction (Godfrey-Smith, 2009; Heams, Huneman, Lecointre, & Silberstein, 2015). To begin with, a population must possess

  1. variation. Diversity in the population’s traits is a necessary precondition for certain entities to become more numerous than others. If all individuals in a population were identical, it would not be meaningful to say that the descendants of some individuals would occupy a greater proportion of future generations than others. For natural selection to occur, entities must differ.

But difference is not enough: There must also be

  1. inheritance of variation. If a population varies on a trait, but there is no rhyme or reason to where the variants arise, there will also be no rhyme or reason to how numerous any given trait becomes in the future. In other words, variation would not only start randomly, it would continue to arise randomly. For natural selection to occur, entities must resemble those that directly gave rise to them, more so than unrelated individuals.

The final precondition of natural selection can be divided into several components, but can most succinctly be defined as

  1. nonrandom differential reproduction. Different individuals reproduce at different rates as a function of the variation in their traits. If differential reproduction occurred but it was not related to differences in traits, then change over time would be random. While some variants might become more common than others on the preconditions of variation and inheritance alone, nonrandom differential reproduction means that this change that occurs over generations is systematic and results in something functional – although the “function” for each variant is always to get more of itself into future generations.

Natural selection will persist as long as these three elements are present – they might even be thought of as constituting natural selection itself. Though they are sometimes framed as two, four, or five components, these are just the same ideas lumped or split differently. Because Darwin’s (and Wallace’s) original theory was substantially inspired by Thomas Malthus’ work on population dynamics related to resource scarcity (Dennett, 1996), an additional requirement of limited resources has historically been included in many articulations of natural selection. But this stipulation is not necessary for the process of selection in its most abstract form (Heams et al., 2015). As long as there is variation, inheritance, and nonrandom differential reproduction, it is a logical consequence that some entities will become more numerous than others over the course of generations.

Daniel Dennett has called natural selection an algorithm: a process that, given certain inputs, is logically guaranteed to produce certain outputs (Dennett, 1996). The algorithm itself is only guaranteed to produce one thing: whatever is best at promoting copies of itself in future generations, given the current environmental conditions. One way to remedy the misguided intuition of natural selection as an intentional, goal-driven process is to describe it instead as “automatic selection.” It happens inevitably, without thought or foresight. The rest of this chapter and several of those that follow will be dedicated to explaining how such a mindless process can result in the functionally complex and diverse life-forms we observe in nature.

One important feature of natural selection – a feature common to all algorithms – is that it is substrate-neutral (Dennett, 1996). We are used to thinking of its operation on genes composed of deoxyribonucleic acid, but it can occur in any situation involving the three components listed above, from organisms on Earth to alien organisms to computer simulations – potentially even transmitted ideas (Dawkins, 2016b). However, since this handbook concerns sexual psychology, the remainder of this chapter will focus on natural selection of sexually reproducing organisms: reproduction that involves the union and recombination of genetic material from two organisms.

Before explaining the concept of natural selection as it applies to organisms, several key biological terms must be defined. A crucial piece of information unavailable to Darwin when he articulated his theory of evolution by natural selection was the concept of genetic inheritance. Genes go by many definitions, but for the purposes of this chapter, it may be best to use Richard Dawkins’ definition of a gene as “any portion of chromosomal material that potentially lasts for enough generations to serve as a unit of natural selection” (Dawkins, 2016b, p. 36). Different variants of genes are called alleles. Biological evolution simply means change in allele frequencies within populations over successive generations (Forbes & Krimmel, 2010), though “evolution” is often used interchangeably with the theory of evolution by natural selection.

An organism’s genotype – its complete set of genes that can be transmitted to offspring – is technically invisible to selection. Genes are inherited, but genes themselves are not traits. A gene can only replicate itself by interacting with the environment to produce something else, whether another copy of the gene, or a protein, or some other indirect method of interacting with the outside world. In cellular organisms, expressed traits are aspects of an organism’s phenotype: its set of observable traits that are produced as a function of the interaction between the organism’s genes and its environment. With this terminology established, it can be clarified that natural selection within biology is defined as differential survival and reproduction of organisms due to differences in phenotypic traits (Larson, 2016).

To take a pertinent example, let’s imagine a phenotypic trait that we’ll call sexual desire. Broadly speaking, sexual desire is a motivational state constituted and supported by physiological and psychological mechanisms that predispose an organism to seek out and sexually engage other individuals (among other things). Given what we have discussed so far about natural selection, it should be apparent why such a trait would be selected for over time. Assuming the propensity to experience sexual desire (1) varies and (2) can be inherited, it will tend to produce (3) nonrandom differential reproduction: Individuals who experience sexual desire will tend to produce more offspring by virtue of engaging in more sexual activity. This is especially true in conjunction with the trait of sexual attraction to individuals with whom the organism can produce sexually viable offspring. Conversely, sexually reproducing individuals who don’t experience sexual desire will tend to reproduce less. With this psychological example in mind, let us consider the process of natural selection in more detail.

Fitness, Reproductive Success, and Selection Pressures

A central concept within evolutionary theory closely tied to the phenomenon of natural selection is “fitness.” The concept of “survival of the fittest” may be one of the first things that comes into people’s minds on the topic of natural selection. The layperson likely hears this phrase and imagines that organisms that are more physically fit – faster, stronger, more physiologically resilient – are more likely to survive, and are therefore favored by natural selection. But this colloquial notion of fitness doesn’t always map onto the evolutionary concept of fitness.

It may be easier to understand the evolutionary sense of “fitness” if we tease apart two subtle meanings of the word “fit.” The first is the colloquial sense of being in good health and physical condition. This sense implicitly holds the subject up to some absolute standard: Being “fit” is often assigned positive moral value and has human-specific references to traits like speed, strength, and agility. On the other hand, the second sense of the word “fit” is that of complementing a given condition or circumstance. Rather than assuming some absolute standard, this is a relative concept that calls to mind the fitting together of two puzzle pieces. There are no desirable or undesirable puzzle pieces, only those that fit together and those that do not. It is this sense that more closely resembles evolutionary fitness.

Genes that are favored by natural selection are those that produce traits that fit with the environment in which they operate. A gene’s environment is an extremely broad construct, ranging from an entire ecosystem to the neighboring genes in a single organism’s genotype (Dawkins, 2016b). Basically, anything that isn’t the gene is part of that gene’s environment. An allele fits with its environment if and only if it is able to make more copies of itself in future generations within that environment, relative to competing alleles. And sometimes, “fitting with” the environment can mean inducing a change in that environment that allows the organism to reproduce more effectively (Dawkins, 2016a). The point to recognize is that fitness is a thoroughly relative concept. It may be best to think of it as “fit-ness”: the quality of fitting with the environment in a way that tends to cause replication or reproduction.

The definition of fitness in evolutionary biology is not without controversy (Godfrey-Smith, 2009). In the broadest sense, fitness refers to the relative likelihood that something will be represented in future generations. Many use the term fitness with reference to a single organism, in which case it can become difficult to distinguish from reproductive success – the number of fertile offspring produced by a given organism. The term fitness is also frequently used with reference to specific phenotypic traits, and others argue that fitness can only be a property of alleles. Yet all these concepts tend to cluster together. Under most circumstances, heritable traits that improve an organism’s reproductive success also improve the future representation of the alleles within that organism’s genotype, because more offspring usually means more copies of the parent’s alleles. The cases in which these concepts come apart are intriguing but largely constitute a diversion from an introductory treatment of the subject. Regardless of the level of analysis at which fitness is applied, it is not a unitary property that can be measured directly in living organisms: It is a probabilistic property, usually one that can only be assessed at the population level.

Reproduction has been central to this account so far, which may clash with another misleading implication of the “survival of the fittest” mantra: the idea that survival is of paramount importance. The truth is that survival serves reproduction. An organism’s survival ultimately means nothing to natural selection without reproductive success. Without reproduction, there is no opportunity for systematic changes in the representation of traits or genes within a population over time. Thus, natural selection only “cares” about survival insofar as it benefits reproductive success – or the replicative success of different genes.

In the long run, the “function” of an organism’s existence is ultimately to reproduce, and the organism must survive only long enough to do so effectively. How long is “long enough” depends entirely on the organism in question. The relative importance of survival duration varies tremendously between species. Some can spend their whole lives reproducing and largely ignore their countless offspring. Others – notably, species like humans with dependent offspring – spend an inordinate amount of developmental time in preparation for relatively infrequent reproductive events, producing small numbers of offspring. Regardless of these differences, when push comes to shove, reproduction will always take precedence over prolonged survival. This perspective makes sexual psychology particularly salient for sexually reproducing species.

Returning to the example of sexual desire, one might assume that such a trait would fit well with any environment because it would tend to increase its own representation in future generations. But organisms are not capable of infinite production of perfectly viable and complete offspring because they survive and reproduce using limited resources. While an allele that tends to increase sexual desire will tend to make its bearers more fit in most circumstances, trade-offs will eventually have to be made. If an organism spends all its time having sex and none on metabolic processes, its life span and thus its reproductive success will tend to be cut short by untimely death, or perhaps not even surviving to reproductive age.

The short-term benefit of increases in sexual desire, and the long-term cost of increased sexual desire to the exclusion of bodily maintenance and other crucial functions, constitute opposing selection pressures – features of the environment that make some variants in the population more effective at surviving and reproducing than others. There are countless environmental considerations in evaluating the fitness of a trait like increased sexual desire. For instance, other traits in the organism’s phenotype constitute features of the environment with respect to the sexual-desire-heightening allele in question. If the trait of high sexual desire is accompanied by a variant of sexual attraction that focuses the organism’s sexual efforts on a different species, that organism will likely not go on to become an ancestor. Thus the “heightened sexual- desire” allele would have low fitness in conjunction with the “misplaced- sexual-attraction” allele.

This example is helpful to illustrate the relativity of fitness and the action of selection pressures, but it is an oversimplification. As explained earlier, sexual desire is by no means a simple phenotypic trait: it is a complex coordination of many systems within an organism. The process by which such complex systems arise cannot be explained without a brief description of adaptation.


This chapter has been devoted thus far to breaking down typical agentic intuitions about natural selection and showing its automatic, algorithmic nature. But this can obfuscate the evidence of design and functionality in nature (Dennett, 1996). This is where the concept of adaptation comes in. Adaptation has two meanings in evolutionary biology: it can refer to the process by which organisms become better fit to their environment; and it can also refer to a phenotypic trait resulting from this process. In this second sense, an adaptation can more specifically be defined as a phenotypic trait that came about because it increased the chances of survival or reproductive success of its possessors during the course of its evolution (Larson, 2016). Much more will be said about adaptation in later chapters of this handbook, but a cursory overview is warranted here.

We tend to think of adaptation as functional in the sense of pertaining to goals of the organism in possession of the adaptation. Of course, being a product of a process like natural selection, adaptation itself is thoroughly relative and automatic: “Natural selection would produce or maintain adaptation as a matter of definition. Whatever gene is favorably selected is better adapted than its unfavored alternatives” (Williams, 1972, p. 32). In other words, whatever trait reliably makes some individuals’ genes more numerous in future generations is defined as adaptive. This is certainly true in the abstract, and it’s important to keep in mind. But there is a good reason that people perceive design and function in organisms produced by natural selection. Because traits are selected based on their fit with the environment,

we recognize adaptedness as an informational match between organism and environment. An animal that is well adapted to its environment can be regarded as embodying information about its environment, in the way that a key embodies information about the lock that it is built to undo.

(Dawkins, 2016a, p. 264, emphasis added)

It is this informational match between organism and environment that gives organisms their apparent design and function. The traits that become more common in future environments are reflective of their fit with past environments, and often current ones as well.

Some features of the environment are constantly in flux, so past environments may no longer be representative of those that organisms are currently navigating. For instance, the specifics of what constitutes a viable sexual partner can and do change frequently. As species evolve, they usually change in perceptible features, so members of each sex in each successive generation must in some way adjust to the visual, auditory, and olfactory cues that indicate a suitable potential mate. These shifts could occur through selection – those whose preferences best match partners with whom they can produce the most offspring will produce the most offspring by definition – or through flexible mate preference learning mechanisms (e.g., Vakirtzis, 2011), or both.

On the other hand, some features of the environment remain constant for hundreds of thousands or millions of years. These features allow for complex adaptations to evolve. For instance, anisogamy, the sex difference in gamete size and other gametic traits, including motility and quantity, has been a fact of life for many sexually reproducing lineages for hundreds of millions of years, and results in some behavioral sex differences that are consistent across taxa (Lehtonen, Parker, & Schärer, 2016). This constancy of specific environmental features – environmental with reference to any given gene – allows successive generations to go through an iterative filtering process, with small mutations being either amplified or pruned from the population. Over the course of countless generations, this process results in increasingly functional traits through the accumulation of minuscule changes. Selection recurrently fine-tunes simple behavioral predispositions over time into increasingly complex and functional traits – ultimately resulting in intricately designed psychological and physiological traits like the sexual desire systems we have today.

Though we must be careful to avoid conferring forethought upon natural selection, there is a real sense in which adaptations are designs, even if not designed with intent (Dennett, 2017). They are an arrangement of features produced according to functional criteria (Oxford English Dictionary, n.d.), the functional criteria being their systematic interactions with “stable and recurring features of the environment” (Tooby & Cosmides, 1990, p. 384). These designs are manifested in phenotype, but they are the result of millions of years of accumulation of infinitesimal alterations within organisms’ genotypes. This odd tension between genotype and phenotype has important consequences for understanding the nature of the biological products of natural selection, so it deserves its own explanation.

Genotype and Phenotype

Genes are not traits. They can only produce traits through interaction with the environment. For the most part, a gene has one role: to direct protein synthesis. Enzymes turn DNA into RNA, and RNA usually into proteins. Even the most basic aspect of gene expression is contingent on the presence of certain environmental factors – enzymes, amino acids, and other crucial chemical building blocks of protein synthesis and gene regulation. This picture has been complicated by our increasing understanding of epigenetics – the study of heritable changes in gene expression and activity – but the idea that genotype produces phenotype and not the other way around remains the central dogma of molecular biology (Cobb, 2017). Because of this, natural selection cannot “see” genes. Genes by themselves cannot have any impact on the world, let alone replicate themselves effectively.

Because genes are invisible to natural selection, they can only be selected by proxy through phenotype. To have high fitness, an allele must have phenotypic effects throughout the bodies in which it resides that result in relative reproductive success over the course of generations (Williams, 1972). It is crucial to understand the interplay between genotype and phenotype because they correspond to two distinct aspects of natural selection. Genes are the main mode of inheritance, but the phenotype is where we observe variation in traits that can impact fitness. This relationship between genotype and phenotype can sometimes defy intuition, and it leads to misconceptions.

One common false assumption is the direct correspondence between individual genes and specific phenotypic characteristics: a gene “for” a given trait. Earlier sections discussed an allele “for” increased sexual desire, but this is an oversimplification. It is easy for humans to categorize and think in terms of discrete physical or behavioral traits, just as it is easy to think in terms of discrete chunks of DNA, but discrete traits and genes are often not compatible, and neither is accurate in the first place. To see why, a good first step is to break down the blueprint analogy of genotype and phenotype.

The genotype is often referred to as a “blueprint” for creating an organism’s phenotype. This is a misleading analogy for two key reasons: The process of creating a house from a blueprint (1) is theoretically reversible, and for the most part (2) has a one-to-one correspondence between components. A skilled foreman should be able to inspect a house and reconstruct a rough idea of a blueprint that describes it. But this would be impossible with an organism because there is no one-to-one correspondence between aspects of the genotype and phenotype. A door drawn in a blueprint corresponds to a door constructed in a house, but there is no gene corresponding to an ear, an iris, or even a single liver cell.

A more apt analogy is the relationship between a recipe and a cake (Dawkins, 2016a). Like a recipe, genes are a set of instructions that, if followed properly under a set of “normal” conditions, will result in a certain phenotype. There is no single step or listed ingredient that corresponds to the frosting, just as there is no single gene that corresponds to the iris. All the ingredients are enmeshed and chemically altered by the processes of mixing and baking, to the point that none would be identifiable within the cake itself. This can go a little way toward clarifying what people mean when they say they have discovered a gene “for” something. Taken literally, saying that someone has identified “the gene for sexual desire,” for instance, would be like saying “the ingredient for cake batter.” What people often mean by these statements is that they’ve identified a segment of DNA without which sexual desire does not develop properly in that particular organism and environment, much like omitting a line of a particular recipe may result in a completely different (likely inedible) product.

Nature and Nurture

It is impossible to discuss the relationship between genotype and phenotype without touching on “the nature–nurture question,” because it directly bears on the process of natural selection in sexually reproducing multicellular organisms. This oft-debated dichotomy can be discussed at varying degrees of sophistication. At the bottom rung is the simplistic worldview that assumes that any given trait is either “genetic” or “environmental” – or the even more simplistic idea that all traits are “genetic,” or that all traits are “environmental.” But readers of this handbook likely recognize this as a false dichotomy.

A slightly more sophisticated approach, much more common among educated nongeneticists, is to ask to what degree genetic and environmental factors contribute to a given trait. However, even this level of sophistication doesn’t reveal the depth of falsity in the nature–nurture dichotomy. Returning to our cake analogy, if the genes are the ingredients and instructions listed in the recipe, then we can think of environmental factors as processes and material components: the oven, the cake pan, heat, the chemical interaction of ingredients, the altitude, and so on. The category error in the nature– nurture dichotomy is revealed when one sees that asking “how much of this behavior is genetic and how much is environmental?” is analogous to asking “how much of the cake’s frosting is stirring and how much is sugar?” They are not only different factors: they are different types of factors, and they cannot be compared quantitatively.

Ultimately, the real question can only be “How does a gene achieve different phenotypic variations under different environmental conditions?” No aspect of genetic expression can be teased apart from environmental factors, or vice versa. This is why “stable and recurring features of the environment” are an indispensable component of Tooby and Cosmides’ (1990) definition of adaptation. When the environment strays too far from the environmental selection pressures that produced the adaptation in the first place, the adaptation is no longer “embodying information” about its present environment and is therefore no longer adaptive. In fact, the genes that produced the adaptation in the ancestral environment may produce something new in a novel environment.

To illustrate this, we can ask: What would a gene “for” sexual desire look like? Does the gene have some direct influence on the immediate sexual behavior of the organism? Is the gene an abstract force that forms the entire motivation of an organism to have sex? Of course not. A gene “for” sexual desire would most likely be one of countless genes involved in the production and regulation of sex hormones or other neurotransmitters, development of areas of the nervous system related to the processing of sexual stimuli, or any number of other crucial physiological functions on which our mechanisms of sexual desire depend (Pfaus, 2009). And any one of these genetic factors may have a massive or minimal impact, depending on an incalculable number of other environmental factors, including those within the organism’s body and without. A gene that causes the production of a sex hormone that motivates adaptive sexual behavior would be considered fit, but it would be useless in an environment without a gene to produce receptors for that hormone inside the organism. Genes cannot act in a vacuum: “It is simply meaningless to speak of an absolute, context-free, phenotypic effect of a given gene” (Dawkins, 2016a, p. 60).

Conversely, the environment can have no systematic impact on an organism without some “instruction” as to how to react to the environment (Tooby & Cosmides, 1992). If you attempt to seduce a rock, little of note will occur. If you attempt to seduce a pigeon, it will likely fly away. If you attempt to seduce a person, they may reciprocate your interest, politely assert their disinterest, or sucker punch you, depending on the nature and context of your sexual advance. It is logically evident that some features inherent to the organism’s nervous system are necessary to transform inbound features of the environment into any behavioral response, let alone a sensical or even adaptive one. Relative to the number of potential actions that an organism could take, the list of useful ones is infinitesimal (Tooby & Cosmides, 1992). And learned responses, while indispensable in the lives of most animals, depend on evolved mechanisms for learning to occur in the first place. Thus, not only can genes not act in a vacuum, but environments cannot systematically act or be acted upon by a vacuous organism.

Genetic Determinism

Still, the rhetoric of traits being “more” or “less” genetic or environmental persists even in academic writing. These nuances of the relationship between genes and the environment are difficult to grasp and poorly taught. Due to the popularity of the “how much is genetic?” approach to traits, many misconceptions about behavioral development have flourished, and these make it difficult to understand precisely how natural selection bears on behavioral adaptations. One of the most dangerous and pernicious misconceptions is genetic determinism: the idea that behavior is controlled directly and wholly by genes. In its most extreme form, it is relatively uncommon as a consciously espoused belief. But softer versions of it often slip into our everyday thought.

For example, a frequent implicit assumption is that if something is “more genetic,” it is more difficult to change. This can be shown to be false from two angles. First, many traits that have established genetic bases also have massive plasticity built into them. Language capacity is an excellent example. Evidence suggests that variation in human linguistic capability has important genetic bases (Kang and Drayna, 2011; Newbury and Monaco, 2010; Centanni, Green, Iuzzini-Seigel, Bartlett, & Hogan, 2015). But language itself is an exceptionally flexible adaptation: The language spoken by each human varies wildly, and can be changed with varying degrees of success across the life span. Second, many environmental effects on psychology and physiology are difficult if not impossible to change. Many environmental factors that affect psychological development, such as ingesting toxins during pregnancy or lead poisoning in early childhood, can lead to irreversible cognitive deficits (Mattison, 2010).

The idea that we have more control over environmental factors than genetic ones is also somewhat of an illusion. Since the actions of our genes are causally contingent on the existence of specific environmental factors, with enough scientific knowledge we should be able to control the effects of many of our genes through manipulation of specific environmental variables. This might sound like a sci-fi pipe dream compared to environmental interventions, but in reality we typically only pay attention to environmental factors that our psychology is designed to pick up on: nutrition, parenting, socialization, and so on. There are practically infinite environmental factors that might be relevant but that we have yet to consider because they would never occur to us – and discovering these would take a similarly ambitious scientific program to that of genetic engineering.

Implicit genetic determinism within psychology is sometimes transformed from contrasting phenomena as more or less genetic to more or less evolved, but this is no less illogical. These points cannot be overstated:

  • everything about us is 100 percent genetic because the environment cannot act systematically on an organism without genes to determine how to react;
  • everything about us is 100 percent environmental because genes cannot act in a vacuum; and
  • everything about us is 100 percent evolved because we are evolved creatures.

Genetic, biological, evolved, and innate are not found at the opposite end of a spectrum from environmental, social, cultural, or learned: The idea that “more of one means less of the other” is incoherent.

Why is all of this important to understanding natural selection as it applies to organisms on Earth? Because behavior itself is not hereditary: Ultimately, it is only the genes (and potentially epigenetic markers) that are inherited, so behavior itself cannot be an adaptation. While cultural transmission can be thought of as a mechanism of behavioral inheritance, it still crucially depends on the evolved psychological mechanisms of the receiver (Tooby & Cosmides, 1992). Everything depends on the inheritance of genes that orchestrate the development of mechanisms that produce behavior. Further, genes passed down from parent to offspring cannot directly produce behavior. They depend at every step on an immeasurable number of environmental factors. Environmental conditions that diverge significantly from those under which the mechanism evolved can and do produce significant divergences in the psychology of organisms in the present. Thus, the relationship between genotype and phenotype is crucial to understanding how natural selection has shaped our genotypes and phenotypes, and how our evolved psychology interacts with its modern environment.

Blind Selection

Humans are not gifted when it comes to thinking about gradually changing traits within populations over vast stretches of time. It’s much easier to interact with the world on a human timescale by categorizing things into discrete classes of entities with defined essences, thinking about actions as the goal-directed behavior of intentional agents, and seeing ourselves as the arbiters of truth and reality. There are many features of evolutionary processes that are difficult to grasp, and they lead to many common cognitive pitfalls.

Arguably the most basic and pernicious false assumption is the teleological mindset. It’s easy to think of adaptations as something that organisms strive for over the course of generations, or that natural selection “equips” organisms with the best tools to solve a problem. In line with the pervasive survival-of-the fittest attitude, we tend to think of functions as serving the goals of the organism: The function of the anteater’s snout and tongue is to obtain food, and the function of the peacock’s tail is to attract peahens. It can be frustrating to think in terms of “function” when the function from the perspective of natural selection can be a far cry from those with which humans typically operate.

But “[s]election has nothing to do with what is necessary or unnecessary, or what is adequate or inadequate, for continued survival. It deals only with an immediate better-vs.-worse within a system of alternatives, and therefore competing entities” (Williams, 1972, p. 36). Natural selection can only deal with the current environment and current variation. It cannot “see” future environments or mutations. Biotic adaptations can only evolve bit-by-bit, small change after small change, over the course of many generations. Since natural selection works on what’s there at the time, it can’t give organisms optimal adaptations. At best, natural selection can gradually produce a phenotypic local optimum (Wright, 1932): Within a given phenotypic range, there is a “best” solution surrounded by small design changes that would only make the solution worse. But there is a vanishingly small chance that this is the best possible solution, because the hypothetical “design space” in which one might travel to alter the adaptation is essentially infinite.

There are countless specific constraints that preclude optimality in adaptation (Dawkins, 2016b). Evolutionary mismatches between phenotype and environment, for instance, are the product of time lags: rapid environmental shifts that make organisms who were adapted to the previous environment less successful at surviving or reproducing in the new one. Pornography and related technological advances in artificial sexual stimulation cause a time lag for our sexual attraction adaptations. In terms of producing offspring, our efforts are wasted when directed at virtual objects, but our attraction mechanisms have not evolved the capability to clearly distinguish such intense illusions from reality. Another category of constraints on optimality is interference from other organisms. No matter the sophistication of one organism’s sexual psychology, they may be thwarted in their goals by a sexual rival, or by a sexually deceptive potential mate. These and many other features of the natural world conspire to preclude optimal design even further and introduce the potential for flaws in otherwise functional designs.

A final myth founded on thinking teleologically about natural selection is that evolution is consistently progressing toward “better” forms. The flaw in this assessment is that “better” is always relative to the current environment. As long as the environment changes, natural selection’s definition of “good” will change. While our criteria may sometimes align with that of natural selection, this alignment will never be perfect because the fitness of current entities is a moving target that can never be definitively established. We may be able to choose our own criteria on which to evaluate organisms’ traits, such as speed or intelligence – or perhaps some moral good that we expect to evolve because we assume that whatever is natural must good (the naturalistic fallacy). But our criteria will ultimately be arbitrary compared with natural selection’s fundamental criterion: the capability to produce more self-copies in future generations.


There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

–Charles Darwin, On the Origin of Species

It would be hard to overstate the impact of the theory of evolution by natural selection on our understanding of the world. It allowed the consilience of all fields of natural science, from physics to psychology. More importantly, it allowed us to answer questions about biological phenomena that we had not considered possible to answer. It allowed us to shift our focus to function in order to better predict the structure of psychological mechanisms. This simple, inevitable process of variation, inheritance, and nonrandom differential reproduction has given us a window into the potential functionality that can arise from the interaction between organisms and their environments.

Though I have been careful so far to emphasize the blindness of natural selection as a process, this should not be construed to imply that the entire process is random, chaotic, or nonsensical. With such conclusions, it would be hardly possible to describe organisms as having designs or adaptations for anything since these imply order through their functionality. An important clarification is the contrast between the stochasticity of mutations and the order imposed automatically through eons of natural selection. Natural selection is a blind force, but it is still a force of order.

Astronomer Fred Hoyle once famously argued against the possibility of organic life arising from inorganic matter with a thought experiment:

A junkyard contains all the bits and pieces of a Boeing 747, dismembered and in disarray. A whirlwind happens to blow through the yard. What is the chance that after its passage a fully assembled 747, ready to fly, will be found standing there? So small as to be negligible, even if a tornado were to blow through enough junkyards to fill the whole Universe.

(Hoyle, 1983, p.19)

Hoyle’s mistake is assuming that the random component of evolution – mutation, which he analogizes to a tornado – is responsible for functional biological organization. But his analogy leaves out the very subject of this chapter: There is no selection process involved in the junkyard. There is no opportunity for different post-tornado junk heaps to replicate; for them to be tested in flight; and then to survive another round of tornadoes, only to be tested in flight once again, in an epic iterative process (Le Paige, 2008). Complex structures arise through the accumulation of small advantageous mutations, each often no more complex than a nucleotide substitution or repetition, but sometimes with surprisingly substantial downstream phenotypic results.

It is difficult to wrap our minds around the concept that something as complex as the human eye could arise from this blind process: after all, our minds were built to operate on the span of decades, at most. But evolution takes place on a timescale incomprehensible to humans: It is “a process that depends on amplifying things that almost never happen” (Dennett, 2017, “A bird’s eye view,” para. 2). It is a process of astonishing simplicity producing even more astonishing complexity over vast stretches of time.


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