Despite the paltry human diversity outlined in Chapter 2, are there some genes that still explain the differences in important traits that we care about? Traits that underlie how we look, think, feel, act, heal, perform, and so on? Among individuals, definitely. There’s a genetic component to most aspects of both brawn and brains. Often a substantial one. Between populations, though, genetic effects are typically weak if they exist at all. Most of the human genome looks pretty much the same on average all over the world, even if it varies within each population. But there are a few genes that have diverged dramatically among continents.
These genes are rare exceptions that stand out against the background of more typical genes. Many of them have to do with infectious disease, diet, or climate. Factors that clearly aren’t the same in different environments. There are no known effects of these genes on personality or cognitive ability. And even for the most extreme genes, there are no absolute differences among populations, just average differences. One version of a gene is more common in some populations, while an alternate version is more common elsewhere.
A genome is a product of billions of years of evolution. A creationist might believe that is was designed by an Architect. But reality is not nearly so coherent. Genomes are messy. In genetics, there is usually no one-to-one association between a gene and a trait. Many genes and other factors contribute to each part of the final product. Most functional genetic variants are more like a person’s comment at a town meeting when a building is being designed. It might have an eventual effect on the number of toilets, but only in an indirect and contingent way.
So can we take genetic data and directly predict the effects on bodily traits? Yes, but only after controlling for other factors at play. This is relatively easy if you are working with, say, mice in a laboratory setting. It’s much harder if you are looking at humans across different societies. It’s still doable for simple traits like blood type. But it’s especially hard for complex traits like behavior that are affected by many genes as well as culture.
Even a highly heritable trait can be influenced by the environment, often in subtle ways. Skin color is determined genetically, but it can also vary because of tanning. Analogously, a trait with high heritability could differ between two populations, without the between-population differences having anything to do with genetics.
Thus, heritability isn’t some fixed feature of a trait, but depends on the environment. Imagine if all bicycles were exactly the same style and of a uniform, non-adjustable size. In that case, ability to ride a bike would be highly heritable because you would need to have just the right leg and arm length, etc. In the real world, almost anyone can find a bike that fits their body, and whether you learn to ride or not depends mostly on personal choice. So, simply by revising a piece of technology, it’s possible to change heritability.
Suppose two genetically-distinguishable groups of humans differ in average SAT score. That does not imply that they have different brain wiring because of their DNA. Consider the following analogy. There are two populations of flowers. The western population has more pink pigment than the eastern one, and also produces more seeds, on average.
Different populations are one thing. But what about different sexes? Chapter 4 will address the most striking genetic variant of them all.
👖👖👖👖👖👖👖👖👖👖👖🩳👖👖👖
Most jeans are the same, but not all
These genes are rare exceptions that stand out against the background of more typical genes. Many of them have to do with infectious disease, diet, or climate. Factors that clearly aren’t the same in different environments. There are no known effects of these genes on personality or cognitive ability. And even for the most extreme genes, there are no absolute differences among populations, just average differences. One version of a gene is more common in some populations, while an alternate version is more common elsewhere.
Genes with adaptive differences among human populations:
🩸🦟malaria resistance: ACKR1
🥛🧀digest dairy: LCT
👂💦ear wax & sweat odor: ABCC11
🆎🅾️blood type: ABO
🏔️🗻breathe at elevation: EPAS1
🤚🏿🤚🏻skin tone: SLC24A5
☃️❄️cold tolerance: TRPM8
Those are the outliers. They are few and far between in a genome of over 20,000 genes (the exact number is still debated). And while you sometimes hear that these variants have major overall effects on how bodies function, this is wrong.How to actually eat for your🩸 type:
🅰️:🥕🥗🌯🥜🍣🍠🥖🍅🧀
🅱️:🥕🥗🌯🥜🍣🍠🥖🍅🧀
🆎:🥕🥗🌯🥜🍣🍠🥖🍅🧀
🅾️:🥕🥗🌯🥜🍣🍠🥖🍅🧀
There are other genetic variants with moderate levels of divergence among populations. Such moderate differences are more numerous, and it’s hard to say what all of them do. Some are associated with personality traits, for example in the gene MAOA, sometimes called the “warrior gene” because it’s linked to aggressive behavior(McDermott et al. 2009). It’s an overly exaggerated moniker: its effect is minor, and as Chapter 4 will explain, MAOA isn’t even the main gene we should associate with aggression. And regardless, one cannot conclude from any specific gene that aggressive behavior differs innately among populations. Complex traits are influenced by many genes, as well as by the environment. MAOA only explains a small proportion of the variance in violent behavior, and even then it depends a great deal on a person’s upbringing. Unless we can tally up the effects of all of the many genes and other factors that influence aggression — and we have nowhere near the ability to do that — we shouldn’t expect populations to differ in any particular way.Relative abundances of genetic variants in people with ancestry in western Africa and northern Europe:
Variant near ACKR1🩸🦟:
🔷🔷🔷🔷🔷🔷🔷🔷🔷🔷🔷🔷W Africa
🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶N Europe
Variant near LCT🥛🧀:
🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶W Africa
🔷🔷🔷🔶🔶🔶🔶🔶🔶🔶🔶🔶N Europe
Variant near MAOA 🤬😡:
🔷🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶W Africa
🔷🔷🔷🔶🔶🔶🔶🔶🔶🔶🔶🔶N Europe
A more typical variant 😐😑:
🔷🔷🔷🔶🔶🔶🔶🔶🔶🔶🔶🔶W Africa
🔷🔷🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶N Europe
Another typical variant 😐😑:
🔷🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶W Africa
🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶🔶N Europe
Part of the problem comes from thinking of DNA like a blueprint. This is a common metaphor, even among scientists. It’s a nice shorthand for the idea that DNA records information that is later manifest in the physical construction of the body. But it’s not a perfect metaphor, and it’s easy to misinterpret. In an actual blueprint, as used by architects, every symbol has a precise meaning. Toilets are symbolized by little shapes that look like a toilet as viewed from above. The number of these symbols on the blueprint will exactly equal the number of toilets in the finished building. This is true whether the building will be constructed in Paris or Bangkok.⬜⚪→🚽
⬜⚪,⬜⚪→🚽🚽
⬜⚪,⬜⚪,⬜⚪,⬜⚪→🚽🚽🚽🚽
Blueprint: the number of toilet symbols equals the number of actual toilets
A genome is a product of billions of years of evolution. A creationist might believe that is was designed by an Architect. But reality is not nearly so coherent. Genomes are messy. In genetics, there is usually no one-to-one association between a gene and a trait. Many genes and other factors contribute to each part of the final product. Most functional genetic variants are more like a person’s comment at a town meeting when a building is being designed. It might have an eventual effect on the number of toilets, but only in an indirect and contingent way.
🗨️🙋🏽♀️🙍🏿♀️🙍🏽♂️🙍🏼♀️ → 🚽🚽🚽🚽🚽🚽🚽
Not a blueprint
So can we take genetic data and directly predict the effects on bodily traits? Yes, but only after controlling for other factors at play. This is relatively easy if you are working with, say, mice in a laboratory setting. It’s much harder if you are looking at humans across different societies. It’s still doable for simple traits like blood type. But it’s especially hard for complex traits like behavior that are affected by many genes as well as culture.
Predicting traits from🧬:
🐁 🆚 🐀 = easy
🆎 🆚 🅾️ = easy
👩🎨 🆚 👩🏫 = hard
Even if you don’t know the specific genes involved, you can tell if a trait is heritable if it runs in families. Can we use heritability to estimate innate differences among populations? Not really. Heritability is easy to misinterpret. It has to do with how much variation in one particular population at one particular time is due to genetics. A trait like “number of legs”, while obviously influenced by DNA, actually has really low heritability in humans. Almost all of the variation is due to wars and accidents, not genetics.🦵🦵 🆚 🦵🦿
Not a genetic trait
Even a highly heritable trait can be influenced by the environment, often in subtle ways. Skin color is determined genetically, but it can also vary because of tanning. Analogously, a trait with high heritability could differ between two populations, without the between-population differences having anything to do with genetics.
December:
👶🏼🧕🏼👱🏻♂️🧔🏿👵🏼👲🏻👧🏼🕵🏻♀️👴🏽🎅🏻
→
July:
👶🏽🧕🏽👱🏽♂️🧔🏿👵🏽👲🏽👧🏼🕵🏾♀️👴🏾🎅🏽
Not an example of evolution in action
Thus, heritability isn’t some fixed feature of a trait, but depends on the environment. Imagine if all bicycles were exactly the same style and of a uniform, non-adjustable size. In that case, ability to ride a bike would be highly heritable because you would need to have just the right leg and arm length, etc. In the real world, almost anyone can find a bike that fits their body, and whether you learn to ride or not depends mostly on personal choice. So, simply by revising a piece of technology, it’s possible to change heritability.
🚴🏽🚴🏽🚴🏽🚴🏽🚴🏽🚴🏽🚴🏽🚴🏽
🆚
🚴🏽🚴🏽🚴🏽🚴🏽🚴🏽🚴🏽🚴🏽🚴🏽
Heritability depends on the tools available
Suppose two genetically-distinguishable groups of humans differ in average SAT score. That does not imply that they have different brain wiring because of their DNA. Consider the following analogy. There are two populations of flowers. The western population has more pink pigment than the eastern one, and also produces more seeds, on average.
Natural situation:
🌺🌺🌺 🌼🌼🌼
🌰🌰🌰 🌰🌰🌰
🌰🌰🌰
A botanist wants to know why this is so. It could be that some aspect of the environment, like the soil, is simply different between the habitats. If so, growing the flowers together in a common garden should eliminate the differencePoor soil:
🌺🌺🌺 🌼🌼🌼
🌰🌰🌰 🌰🌰🌰
Rich soil:
🌺🌺🌺 🌼🌼🌼
🌰🌰🌰 🌰🌰🌰
🌰🌰🌰 🌰🌰🌰
Or, it could be that the difference is innate, and doesn’t depend on the soil:Any habitat:
🌺🌺🌺 🌼🌼🌼
🌰🌰🌰 🌰🌰🌰
🌰🌰🌰
Or, it could be that the soil matters, but it has an effect early on. For example, plants grown in poor soil produce nutrient-deficient seeds which themselves will grow up to produce only a few seeds. To test this hypothesis, you’d need to raise a few generations of plants in the same garden first.Mom grown in poor soil:
🌺🌺🌺 🌼🌼🌼
🌰🌰🌰 🌰🌰🌰
Mom grown in rich soil:
🌺🌺🌺 🌼🌼🌼
🌰🌰🌰 🌰🌰🌰
🌰🌰🌰 🌰🌰🌰
Or, there might be something else going on. What if bees favor pink flowers and are more likely to pollinate them? Then the question is really about the genetic basis of flower color, which only indirectly influences seed count. Flower color could be genetic, or due to soil, or anything else.Indirect effect of pollinators:
🌺🌺🌺 🌼🌼🌼
🐝🐝🐝 🐝
🌰🌰🌰 🌰🌰🌰
🌰🌰🌰
It’s not that pink flowers are innately better at seed production. Their seed production machinery has nothing to do with it. It’s an arbitrary preference of the bees. Release some butterflies that enjoy all flowers equally, and the difference disappears. Change the environment, change the trait. Even if the trait depends on genes.Equal-opportunity pollinators:
🌺🌺🌺 🌼🌼🌼
🦋🦋🦋 🦋🦋🦋
🌰🌰🌰 🌰🌰🌰
🌰🌰🌰 🌰🌰🌰
An analogous thing happens with humans. Is success in life due to nature or nurture? Any serious scientist would agree that both genes and environment play important roles. But they can interact in complex ways. Genes influence physical appearance. In a society that judges people by appearances, these genes influence professional achievement. Even a seemingly objective test like the SAT is heavily affected by how a person has been treated in the past. But just like the flowers, this is an indirect and arbitrary effect. It could be changed.🧠🧠🧠🧠🧠🧠🧠🧠🧠🧠🧠🧠🧠🧠🧠
These aren’t that different after all
Different populations are one thing. But what about different sexes? Chapter 4 will address the most striking genetic variant of them all.