There are experts in evolutionary science who can explain this far better than I can. Still, I want to set aside space for it in my own voice, because the basic logic is not hard to understand, the evidence is overwhelming, and the emotional resistance to it often has very little to do with evidence and a great deal to do with identity, belonging, and sacred narrative—topics I have already been discussing.
What follows is a tour through the core mechanism of natural selection, a few common misunderstandings, the idea of speciation, and then several side corridors that matter for this essay: cultural evolution, sexual selection, and the uncomfortable fact that even religiosity itself is shaped not only by culture, but also by temperament, inheritance, and biology.
Selection
Natural selection is the central guiding principle of evolutionary theory. The logic is profoundly simple. It requires only that we accept three basic facts:1) Organisms vary (physically, physiologically, behaviorally).
2) Some variation is heritable (traits are influenced by DNA, even though environment matters enormously too).
3) Some traits affect reproductive success—not in a morally loaded sense of “deserving,” but in the literal sense that some variants leave more surviving offspring than others.
If a heritable trait increases the probability of leaving more surviving offspring in a particular environment, then over generations the population will contain more of that trait. If a trait reduces reproductive success, it tends to diminish. That's it--natural selection is differential survival and reproduction acting on inherited variation, repeated over time.
You can see the basic logic everywhere, from selective breeding in crops and animals ("artificial selection"), to antibiotic resistance in microbes, to the obvious family resemblance in both physical and psychological traits among human relatives. None of this requires the belief that genes are the only cause of traits. It requires only the admission that heredity is a major contributor. A point worth emphasizing, because it is often misunderstood, is that natural selection is not about “improvement” in any moral or progressive sense. It is simply a filter that favors whatever works well enough in a local environment at a given time.
Mutation
DNA replication is highly accurate, but not perfect. Across generations there are small changes—mutations—introduced into genetic material. “Mutation” here does not mean “bad.” It simply means “change.” Most mutations are neutral, many are harmful, and a few are beneficial in a given environment. Sexual reproduction adds still more variation by shuffling existing variants into new combinations, so evolution does not work only with brand-new mutations but also with new mixes of old genetic material.Mutations do not happen because the organism needs them. A bacterium under stress does not somehow produce the exact helpful mutation it would most like to have. At the deepest level, whether a particular copying error happens in a particular cell at a particular moment is still a fundamentally accidental event. But that does not mean every part of the genome is equally likely to change. Some kinds of DNA changes happen more often than others, and some parts of the genome have a higher probability of experiencing mutations than others. Mutation is fundamentally random in origin, but statistically biased in pattern, and those biases affect what raw material natural selection gets to work with.
When small genetic changes happen to produce a trait that improves survival or reproduction in a particular environment—say, a slightly different beak shape that lets a bird get food more efficiently—those variants will, on average, become more common. They become more common for a very simple reason: the creatures carrying the useful variant survive better and leave more offspring. Over many generations, the accumulation of such changes can produce substantial transformations, including changes in complex organs and behaviors. Darwin’s finches in the Galápagos remain a famous entry point into this idea because long-term field work and later genetic work showed how ecological pressures can shape beak traits across populations.
The Time Scale of Evolution
One reason evolution feels counterintuitive is that large organisms reproduce slowly relative to a human life. Big evolutionary changes can take thousands or millions of years, just as major geological or astronomical processes do. We do not watch a canyon form in a single afternoon, and we do not watch a star age in real time, but the evidence for those processes is still decisive. Evolution is similar: long processes are inferred from converging lines of evidence. Fossils are one line of evidence—imperfect, but immensely powerful. Fossilization is rare and biased toward certain environments and tissues, so the record will always be incomplete. Still, the overall pattern—order in time, branching diversification, and transitional forms—fits evolutionary predictions very well. Comparative anatomy, fossils, biogeography, embryology, and genetics all point to the same branching story.Common Ancestry and Cousins
A cliché misunderstanding goes like this: “Evolution says humans descended from chimpanzees.” That is not what evolutionary biology claims. Humans and chimpanzees are not ancestor and descendant; they are cousins. And “cousin” is not just a metaphor here. It is literally the right genealogical idea: two living lineages sharing an older common ancestor. In ordinary family language, we talk about first cousins or second cousins. Chimpanzees are obviously not that kind of close cousin; if one insisted on stretching the family language into absurd distances, they would be something like our three-hundred-thousandth cousins. Current genetic evidence places the human–chimpanzee split on the order of about 6 million years ago.The same logic generalizes outward. Every living thing on Earth is, in the broad genealogical sense, our cousin. For many people, that is not depressing at all. It is a source of awe—a kind of cosmic kinship. If a reader wants a visual sense of this, it is worth looking up one of the modern phylogenetic tree charts online, such as OneZoom. The exact dates of particular branching points are still being refined, but the branching structure itself is already known very well. And the scale is staggering. Humans and mice last shared a common ancestor around 90 million years ago. Mammals and birds last shared a common ancestor on the order of 310 million years ago. Humans and fish share a common ancestor on the order of 450 million years ago. The exact numbers continue to be refined, but we are talking about tens to hundreds of millions of years, not a few thousand. (OneZoom) (Tree of Life Explorer)
Speciation
Over the short term, evolution often looks like shifting trait frequencies within a species. Over the long term, divergence can accumulate until populations become reproductively isolated—meaning they can no longer interbreed successfully under natural conditions. That is speciation.Speciation is not always a sharp on/off switch. In nature it often behaves more like a continuum. The Ensatina salamander example is a real case, and it is useful because it makes that idea vivid. Around California’s Central Valley, populations of these salamanders spread in a rough loop. Neighboring populations along the loop can still interbreed with the populations beside them. But by the time the two far ends of the loop meet again in Southern California, they have changed enough that they no longer interbreed successfully with each other, or do so only rarely. A simple analogy would be a circle of people, each speaking a dialect just a little different from the dialect of the person next to them. Each person can understand the neighbors on either side, but the person across the circle sounds incomprehensible. Ring species work something like that, except the differences are genetic and reproductive rather than linguistic. That is the point: species boundaries can emerge gradually, not by magic and not all at once.
One thing I want to state explicitly, because people often get confused here: even if a behavior or trait has evolved, that does not mean it is morally right, or that we should accept it. Evolution describes how traits spread. It does not tell us what we ought to value.
Compromise, Not Perfect Design
Evolution also does not produce perfect design. It modifies what already exists. That is why living bodies so often look less like clean engineering and more like a history of workable compromise. Humans are full of such compromises. In adult humans, the passage for food and the passage for air share anatomy in the throat, which is one reason choking is even possible. Our spines and backs also show the costs of walking upright. The human spine has an S-shape that helps us balance over our hips and walk efficiently on two legs, but it also turns a structure inherited from four-legged ancestors into a vertically loaded column. The result is chronic stress on the lower back, high rates of disc degeneration, herniated discs, sciatica, and persistent back pain. Human childbirth is unusually difficult because pelvic form, fetal growth, and the evolution of large brains create a tight compromise. It is not as though the body is simply poorly designed. The point is that evolution works with inherited materials and trade-offs, not with fresh blueprints.The vertebrate eye is another striking example. The retina is effectively wired backward: light has to pass through layers of neural tissue before it reaches the photoreceptors. Where the optic nerve exits the eye there is a literal blind spot, because that patch contains no photoreceptors. And because light must travel through the retinal layers first, the vertebrate eye needs compensatory tricks just to reduce scattering and preserve image quality. Müller cells help act like optical fibers to guide light through this awkward arrangement. The design also forces a trade-off between a clear optical path and the blood supply the retina needs. The system works, but it is hardly what a tidy engineer would draw from scratch. It also comes with structural vulnerabilities: the retina can detach, and when it does, vision can be permanently damaged. The octopus, whose camera eye evolved independently, ended up with a more direct setup. Its retina is everted rather than inverted, so the nerve fibers are routed behind the photoreceptors and there is no comparable blind spot.
Our jaws and teeth tell a similar story. Over time, human faces and jaws have become smaller without tooth size shrinking in perfect proportion. The result is a mismatch between tooth size and available arch space. Softer and more processed diets appear to worsen the problem by reducing the chewing demands that help stimulate jaw growth. So many modern humans grow jaws that are simply too small for the full dental package they still carry, leading to orthodontic problems, crooked teeth, and impacted wisdom teeth.
The broader point for me is simple: evolved traits are not always morally admirable or functionally ideal. Some have to be restrained, some have to be accommodated, and some have to be counterbalanced by culture. If we want to become more humane, we need rules, norms, education, medicine, and institutions that limit certain inherited tendencies, compensate for the problems they create, and cultivate better ones.
Cultural Evolution
A parallel kind of evolutionary logic shows up in culture. Dawkins coined the term “meme” for cultural units that replicate, but the broader point matters more than the label. Ideas, phrases, rituals, fashions, and institutions can spread, vary, compete, split, and disappear.Language evolution is a very good example. Languages branch and drift. Over time, groups can become mutually unintelligible--they become different language "species." Linguists can reconstruct family trees and infer common ancestors in ways that are strikingly analogous to biology. Proto-Indo-European—the distant ancestor of languages as varied as English, German, Greek, Russian, Persian, and Hindi—existed about six to eight thousand years ago. English, German, and Dutch are much closer cousins inside the Germanic family, with a common ancestor a little over two thousand years ago. French, Spanish, Italian, Portuguese, and Romanian are cousin languages that began diverging from Latin roughly fifteen hundred to two thousand years ago. And the same logic extends beyond Europe. Mandarin and Cantonese are also cousins. Their shared recognizable ancestor lies in Middle Chinese, roughly a thousand years in the past. What begins as a dialect can, given enough time and separation, harden into a clearly distinct language.
And language divergence does not require millennia only. Some changes become obvious in just the last few centuries. Afrikaans, for example, diverged from colonial Dutch over roughly the last three to four hundred years after Dutch settlement at the Cape in 1652. Even before a speech form is officially labeled a separate language, strong regional varieties can become difficult for outsiders to follow. There are strong accents within our own country that can be difficult to understand. Mutual understanding can erode gradually long before people agree on where to draw the boundary.
Religions also behave this way. Doctrines split. Schisms occur. New denominations form. We see this clearly in Christianity: the East–West Schism of 1054 formalized the great split between Roman Catholic and Eastern Orthodox Christianity; the Reformation is conventionally dated to 1517; and those Protestant branches splintered further into Lutherans, Calvinists, Anabaptists, Anglicans, Methodists, Baptists, Pentecostals, and countless smaller groups. This fragmentation is not just ancient history. In the United Methodist Church, more than 7,600 U.S. congregations left between 2019 and the end of 2023, roughly a quarter of the denomination’s earlier U.S. total. Buddhism diversified too. The older matrix was early Buddhism. Theravada traces itself to one early line of that tradition, while Mahayana arose later as another major descendant branch around the beginning of the Common Era. Islam experienced its defining Sunni–Shia split in the struggles over succession after Muhammad’s death. The family-tree metaphor is not perfect, but it is illuminating: religions do not descend from heaven as finished products. They branch, drift, quarrel, and split inside history.
Psychologically, that has an important implication: people often treat their own local, historically contingent version of a faith as if it were timeless and universal, when in reality it carries the fingerprints of geography, conflict, institutions, inheritance, and politics.
Sexual Selection
Another important evolutionary idea is sexual selection: traits can spread not because they help survival directly, but because they affect mating success. The peacock’s tail is the classic case—beautiful, costly, cumbersome, and yet selected because it becomes desirable within the mating preferences of the species. Darwin understood that biology is not only about staying alive long enough to reproduce; it is also about courtship, display, preference, and attraction. Richard Prum has argued, persuasively in my view, that sexual selection can include a genuinely aesthetic component: preferences themselves can become evolutionary forces, and traits can spread because they are found attractive, not merely because they advertise some practical advantage.This is relevant to humans not because we are peacocks, but because it reminds us that biology is not only grim survival calculus. Human mate choice also attends to looks, voice, movement, confidence, style, humor, conversation, and perhaps aspects of intelligence itself. Some theorists—notably Geoffrey Miller—have argued that traits such as humor, creativity, artistry, music, and parts of intelligence may have been shaped at least partly by sexual selection because they function as displays: they can signal mental agility, creativity, or the ability to hold another person’s attention. The evidence here is mixed and the details are debated, but the underlying idea is serious and worth considering.
Temperament, Inheritance, and Religion
Religiosity itself is not only cultural. Which particular religious group a person belongs to is largely a cultural and family-transmitted matter. But the broader tendency to be religious at all—to find religion important, compelling, consoling, or identity-defining—shows a meaningful inherited component. Twin research repeatedly suggests that adult religiosity has a moderate hereditary component, but the size depends a great deal on what exactly is being measured. Broadly speaking, estimates often land from the high 20s into the low 60s. More outward or socially enforced measures tend to sit lower, while more inward, identity-heavy, or conversion-like dimensions can sit higher. And when researchers say that some dimension of religiosity has a heritability of 60%, they do not mean that 60% of one person’s religion is “caused by genes.” They mean that, in the population being studied, about 60% of the variability between people on that trait is statistically associated with genetic variability.It is also worth noting that intelligence and religiosity show a modest negative association on average in meta-analytic work. This is not a claim about any individual believer—many brilliant people are religious. The more useful point is simply that cognitive style varies. Some people are more comfortable with analytical doubt and ambiguity; others are more drawn to certainty, authority, and communal reinforcement.
A trait dimension that is relevant here is schizotypy: a spectrum involving unusual perceptions, magical ideas, and strong pattern-finding. Most religious people are not schizotypal. But a person who is more prone to unusual experiences and to seeing hidden connections may be more likely to interpret inner events as messages, revelations, or signs coming from outside the self. Schizotypy itself also appears to be substantially heritable, often in roughly the 30% to 50% range. And there is repeated evidence suggesting that higher levels of schizotypic traits—especially the unusual-experiences side—are associated with stronger paranormal beliefs.
Moral psychology belongs here too. Some people are more temperamentally drawn to moral themes like loyalty, authority, and purity; others prioritize harm reduction and fairness more strongly. These inclinations are also significantly heritable. Religions, especially organized and more traditional ones, tend to be associated with stronger emphasis on loyalty, authority, and purity, with less emphasis on harm reduction and fairness. That is one reason some people feel deeply at home in religious cultures while others experience them as alienating.
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