…Despite Being 99.9% Genetically Identical
In The Human Story textbook, this is a lecture on chapter 3 “Human Variation” (sections 3.1-3.2)for Intro to Anthropology. The next lecture concludes the chapter with Race, Biology, and Power.
Key Concepts
- Human beings are one interbreeding species and “99.9% the same genetically speaking” (66) — among the most genetically uniform species on Earth.
- The visible differences we see come from “relatively recent evolutionary adaptations to new and localized environmental challenges” that “altered our visible phenotypes” (51) — not from deep genetic division.
- Genotype is the genetic code; phenotype is the set of physical traits, shaped both by genotype and by the environment one grows up in (52). Humans have unusually high phenotypic plasticity.
- Inheritance is discrete, not blended: Mendel’s work showed traits pass as units that can hide for generations and recombine, preserving the variation natural selection requires.
- The Modern Synthesis (61) combined Darwin’s natural selection with Mendel’s inheritance — assembled in the 1940s and 1950s, long after both men had died.
- Most human traits are polygenic and multifactorial (57, 58); even dimples are an “irregular dominant trait” rather than a simple dominant one.
- Whether a mutation is advantageous depends entirely on environment (59-60); evolutionary fitness means reproductive success, not health or athletic ability.
- About 85% of human genetic variation sits within any local population (65) — there is more genetic difference within the groups we call races than between them.
- Variation is clinal (65): traits change gradually across geography, with no sharp line dividing one group from another. Most human genetic variation is found in Africa (64).
The Paradox: Genetically Identical, Visibly Different
Human beings are one continuous interbreeding species. We’ve been that way for at least 300,000 years. From the tallest to the shortest, from any population anywhere in the world, when people of reproductive age get together, they can reproduce—and their offspring can too. We are “99.9% the same genetically speaking” (66). We’re not exactly cousins, but we’re pretty darn close.
We also have what the textbook calls a “penchant for migration and intermingling” (66). Even before planes, boats, and automobiles, humans walked, ran, and moved to different places. We migrate, we trade, we interact. Yes, we fight each other too—but we also intermingle. Human beings love testing out the biological species concept, not always nicely, but constantly.
So here’s the puzzle: if all this is true, why do people from different regions “appear to be so physically different” (66)? Look around any room or around the world—we see remarkable visible diversity. How can that be if we’re basically all the same genetically?
The Answer: Phenotype, Not Genotype
The textbook somewhat hides the answer in a caption, but it’s crucial: “Relatively recent evolutionary adaptations to new and localized environmental challenges have altered our visible phenotypes” (51). Our visible appearances have been recently altered—partly through evolutionary adaptations, partly because humans have tremendous plasticity in response to environment.
Your genotype is your genetic makeup—the DNA code, all that CGAT stuff (52). Your phenotype includes the physical traits we can see and measure. The phenotype is partly an expression of genotype, but here’s what makes it interesting: it’s also significantly influenced by the environment in which we grow up.
Take two people with basically the same genotype. Raise them in very different environments—different food, different protein levels, different exposure to sunlight or cold, different altitudes—and different genes will express themselves throughout development. You’ll see very different outcomes in things like lung capacity, body size, and visible features, even though the underlying genetic code is nearly identical.
This genotype-phenotype relationship is especially complicated in humans. We are particularly prone to phenotypic variability without needing huge genetic shifts. Part of this comes from culture—we can express ourselves differently because we’ve invented glasses, for instance, reducing selective pressure for perfect eyesight. We don’t need dramatic genetic change to have lots of visible diversity.
For an advanced treatment of how environment shapes developing bodies—including the argument that DNA functions as a regulator, not a blueprint—see Tim Ingold on why walking and cycling are both skills that literally shape our skeletons.
Why Inheritance Matters: Variation, Not Blending
Your textbook uses dimples and Punnett squares to explain inheritance (57). But why learn this stuff? Here’s the anthropological “why.”
When Charles Darwin developed his ideas, people thought inheritance worked like mixing paint—red and white make pink, traits blend together. But if inheritance actually worked that way, variation would wash out in each generation. Everything would just blend together, leaving nothing for evolution to work on.
Gregor Mendel’s pea plant experiments—conducted independently, never communicated to Darwin—showed something different. Traits come as discrete units, not blended paint. Recessive genes can hide for generations and then reappear. Traits recombine independently rather than coming as fixed packages. This preserves the variation that evolution depends on.
The “Modern Synthesis” (61) combined Darwin’s natural selection with Mendel’s discrete inheritance—but this didn’t happen until the 1940s and 1950s, long after both were dead. It’s what we study today in biology classes.
Here’s the complication for humans: “many traits are determined by a much more complex process than a single gene with only two alleles” (57). Most human phenotypes are polygenic (influenced by multiple genes) and multifactorial (genes plus environment interacting) (58). Even dimples—supposedly a simple dominant trait—turn out to be an “irregular dominant trait” that can appear, disappear, or develop later in life. Almost nothing in human genetics is as simple as a Punnett square suggests.
Random Forces and Gene Flow
Natural selection gets all the attention, but here’s what most people don’t know: you need variation for selection to work on anything. Another word for variation? Diversity. If every generation were identical, evolution would have nothing to do. Those who want to eliminate diversity are being anti-evolutionary—evolution depends on difference.
Mutations provide some variation. Most are neutral (they don’t do anything). Some are deleterious (harmful). A few might be advantageous—but advantageous where? Whether a mutation helps or hurts depends entirely on environment (59-60). A mutation for lighter skin might be advantageous where sunlight is limited; the same mutation would be disadvantageous under intense UV radiation. The environment determines whether traits help or hurt.
Lots of random stuff also shapes human populations: genetic drift (61), the founder effect (62) when a small group colonizes a new area, genetic bottlenecking when populations get trapped or reduced. These aren’t adaptive—they’re just random.
But the big one for humans is gene flow—different populations exchanging genes. And we do a lot of it. “The current abundance of gene flow in humans… means humans are now less likely to evolve into different species than at any time in our hominin history” (64). There’s zero chance we’re splitting into separate species. We have more gene flow now than ever, but we’ve always had it. Even when populations say they hate each other and won’t intermarry, somebody’s always sneaking around testing the biological species concept. Gene flow has always kept us “remarkably similar genetically” (66).
The Key Facts About Human Variation
Here’s what actually matters about human genetic variation:
We’re a young species. Two to three hundred thousand years is nothing in evolutionary terms—scientists usually talk in millions of years (64). We haven’t been here long enough to diverge significantly.
85% of genetic variation is within any local population (65). Go anywhere in the world—that population contains about 85% of total human genetic variation within it. There is always more difference within any human group than between human groups.
Variation is clinal—it changes gradually across geographic ranges (65). If you lined everyone up geographically, there’s no line where you could say “here’s where one type ends and another begins.” Skin color, blood type, most measurable features—they all vary gradually, not categorically.
Most genetic variation exists in Africa (64). Because human evolution centered there, Africa contains the largest range of genetic variation. If everyone outside Africa disappeared, almost all human genetic variation would still exist within that continent. Those of us with heritage elsewhere are, genetically speaking, a subset of African variation.
What This Means
We look different from each other because of recent environmental adaptations and human developmental plasticity—not because of deep genetic divisions. The groups we call “races” have more genetic difference within each group than between them. We’re all part of one interbreeding species where visible diversity doesn’t reflect actual genetic distance.
Understanding this scientifically helps us see the difference between meaningful biological diversity and socially constructed categories that have been used—and misused—to justify hierarchy and exclusion. In the next class on Race, Biology, and Power, we examine how these categories developed and why they persist despite the biological evidence.
Frequently Asked Questions
If humans are 99.9% genetically identical, why do we look so different? Because visible traits are shaped by recent, localized environmental adaptation and by human phenotypic plasticity, not by deep genetic divergence. The textbook states it plainly: “relatively recent evolutionary adaptations to new and localized environmental challenges have altered our visible phenotypes” (51).
What is the difference between genotype and phenotype? Genotype is the genetic code itself (52); phenotype is the set of observable physical traits. The phenotype is partly an expression of genotype but is also significantly influenced by the environment in which a person develops, which is why people with similar genotypes can look quite different.
Why do we learn Punnett squares and dominant-recessive inheritance? Because discrete inheritance, unlike the old “blended paint” model, preserves variation across generations — and that variation is what natural selection acts on. Without it, traits would average out and evolution would have nothing to work on.
Do human races have a biological basis? No. About 85% of human genetic variation exists within any local population (65), so there is more genetic difference within a so-called race than between them. Variation is clinal — gradual across geography — with no trait marking a sharp boundary between groups.
Could humans evolve into separate species? Extremely unlikely. “The current abundance of gene flow in humans… means humans are now less likely to evolve into different species than at any time in our hominin history” (64). We are also a young species, having emerged within the last few hundred thousand years.
Previous: Doing Anthropology
Next: Race, Biology, and Power