Chapter 1 explained why we shouldn’t expect much evolutionary diversification since the last human common ancestor. And yet, if you look at humans from around the world, you’ll see body parts in a variety of colors and shapes:
One often hears comments about the richness of human diversity. But interestingly enough, our species actually has relatively little genetic diversity.
To demonstrate this, imagine choosing any gene from your genome. You have two copies of it, one from your dad and one from your mom, so pick one at random. Do the same for another human with a different racial background. Compare their DNA with your own. You’ll see a few differences. But you’d see even more differences between the two copies that exist side-by-side within one individual rabbit, gibbon, or sparrow. People are genetic paupers. We have less total biodiversity than most other animals(Leffler et al. 2012).
However, let’s not oversimplify things. Overall, two copies of a chromosome chosen at random from two humans will be 99.9% the same and 0.1% different. But what does that mean? A single mutation could have no effect, or it could mean the difference between life and death. So, there is no straightforward connection between variation in DNA sequences and variation in traits we can actually see.
What, then, does it mean for a species to have a lot of biodiversity? Consider, for example, the common side-blotched lizard from western North America. This is a single species, but among these lizards are several “morphs” the look and act differently(Corl et al. 2010). Orange-throated males guard large harems of several females. Blue-throated males are monogamous and guard a single mate. Yellow-throated males sneak into the territories of orange-throated males and mate with the females there, but forego such trysts with the ever-guarded mates of the blue throats. Meanwhile, orange-throated females produce many small eggs, yellow-throated females produce a few large eggs, and there are no blue-throated females. Most populations include all three colors, but some have just one or two. These ratios vary with geography, and along with other genetic differences they divide the lizards into several “subspecies.”
It should be obvious that human biodiversity pales in comparison. People practice different strategies for finding mates, but these are influenced culturally, not genetically. Unlike the lizards, a person of any genotype could grow up to be a celibate, a monogamous spouse, or a polygamist.
Furthermore, even if prehistoric humans like Homo erectus were still around, you would have no trouble sorting out Homo sapiens from any other species, as you might with the longwing butterflies. Human populations may vary, but we don’t differ in what kind of fluid we are able to breathe, like freshwater versus saltwater fish. Nor in the presence or absence of an entire sexual organ, as in strawberries. For these reasons, humans are not classified into morphs, subspecies, or breeds(Norton et al. 2019). There is one standard type of human, with minor customizable flourishes.
To be clear, the variation we do possess is not distributed equally around the world. Humanity is not homogeneous. If you send a geneticist your DNA, they could get a pretty good estimate of where your recent ancestors lived. If you showed those ancestry estimates to someone else, they could make a few general predictions about your overall physical appearance.
But DNA is not cleanly partitioned by homeland. Remember the archipelagos. Populations geneticists often make plots of individuals that actually look a lot like islands in an ocean. However, these plots are easy to misinterpret. There is no literal ocean or any other geographical space. Positions on the plot are defined by multivariate statistics. Genetically similar individuals cluster together. Consider the following classic principal component analysis (PCA), for example(López Herráez et al. 2009).
In reality, human populations have been mixing and remixing since the dawn of our species. Every “pure” population is really descended from two or more other populations that used to be separate, which themselves have a mixed ancestry, and so on(Reich et al. 2018). We are all admixed.
More importantly, images like the PCA above are meant to highlight subtle dissimilarities by turning the contrast up to eleven. They doesn’t say anything about the size of the differences among those ancestry groups. As an analogy, consider the following ten bags of groceries, each with twenty items:
Just by looking at their contents, could you easily classify the bags into distinct categories? Probably not. Or if you did it would be arbitrary. You wouldn’t expect other people to make the same decisions. All the bags seem to contain a fairly random assortment of items. However, a computer algorithm could readily detect nuanced patterns and place the bags into clear groups. Here’s a PCA. The first axis explains 16% of the variation among bags. A, E, F, H, and (to a lesser extent) C all seemingly share something.
For most genes, diversity within our species and divergence among populations is neither large nor meaningful. But is that true for every gene? Chapter 3 will explore whether and how some genes may impact important traits that vary among populations.
💪🏿💇🏻♀️👂🏾🦶🏻💋💪🏻🤚🏾👃🏼💇🏼♂️👂🏽💅🏻🦵🏾👣🦵🏼👃🏿👄
Disembodied diversity
One often hears comments about the richness of human diversity. But interestingly enough, our species actually has relatively little genetic diversity.
👨🏽👶🏿👱🏿♀️👧🏾👨🏼👩🏾👦🏽👵🏾👶🏻👩🏻👱🏻♀️👦🏿👧🏽👴🏿<🐰
A typical non-human species harbors more genetic diversity than humans do
To demonstrate this, imagine choosing any gene from your genome. You have two copies of it, one from your dad and one from your mom, so pick one at random. Do the same for another human with a different racial background. Compare their DNA with your own. You’ll see a few differences. But you’d see even more differences between the two copies that exist side-by-side within one individual rabbit, gibbon, or sparrow. People are genetic paupers. We have less total biodiversity than most other animals(Leffler et al. 2012).
Genetic diversity per species:
0.0000
0.0005🦦
0.0010🧑
0.0015🐼🦍🐺🐘🌵
0.0020🐸🐗🐃
0.0025🐒
0.0030🦧
0.0035🕷️🦐
0.0040🐁🐋🦆🐡🌲
.
.
.
0.0060🦎
0.0065🐦
.
.
.
0.0085🐰🐝
0.0090🦗
.
0.0100🌳
.
.
.
.
.
0.0130🦂
.
0.0140🦟
.
.
.
.
.
.
.
.
.
.
.
0.0200🦋
.
.
.
0.0220🌻
The reason for our dearth of diversity lies in our history. Our species appeared quite recently, initially as only a few thousand individuals. We have since expanded into the billions, but there hasn’t been enough time for new mutations to spread and reflect our new global ubiquity. Genetically, we still look like a rare species(1000 Genomes Project Consortium 2015). And if there is little genetic variation in our species as a whole, there it is little that can be partitioned out among humans in different places.👥👥👥→🏙️🏙️🏙️🏙️🏙️🏙️🏙️🏙️🏙️🏙️🏙️🏙️
We quickly went from a few scattered people to a swarm of enormous cities
However, let’s not oversimplify things. Overall, two copies of a chromosome chosen at random from two humans will be 99.9% the same and 0.1% different. But what does that mean? A single mutation could have no effect, or it could mean the difference between life and death. So, there is no straightforward connection between variation in DNA sequences and variation in traits we can actually see.
🦟🦟🦟🦟🦟🦟🦟🦟🦟🦟🦟🦟🦟🦟🦟🦟
Even with high genetic diversity, every member of a species could look the same
What, then, does it mean for a species to have a lot of biodiversity? Consider, for example, the common side-blotched lizard from western North America. This is a single species, but among these lizards are several “morphs” the look and act differently(Corl et al. 2010). Orange-throated males guard large harems of several females. Blue-throated males are monogamous and guard a single mate. Yellow-throated males sneak into the territories of orange-throated males and mate with the females there, but forego such trysts with the ever-guarded mates of the blue throats. Meanwhile, orange-throated females produce many small eggs, yellow-throated females produce a few large eggs, and there are no blue-throated females. Most populations include all three colors, but some have just one or two. These ratios vary with geography, and along with other genetic differences they divide the lizards into several “subspecies.”
🟠🦎 🆚 🟡🦎 🆚 🔵🦎
There are many other species with similarly mind-blowing diversity. The Numata longwing is a South American butterfly that comes in seven forms, each with a completely different pattern of orange, yellow, and black pigment on its wings. If you didn’t know better you’d assume they were separate species, and in fact they have each evolved to mimic a different poisonous butterfly(Joron et al. 2011). Steelhead and rainbow trout are actually the same fish species. The former is more than twice as large and migrates to the sea to hunt, while the latter maintains a more laid-back lifestyle in the watershed of its birth. Woodland strawberries have distinct subspecies with totally different sexual systems: one includes females, while the other consists entirely of hermaphrodites(Staudt 1989). In these animals and plants, the different morphs or subspecies each have their own ecological roles and adaptive strategies.🦎🦋🐟🍓🚷
Species with outstanding biodiversity. Humans need not apply.
It should be obvious that human biodiversity pales in comparison. People practice different strategies for finding mates, but these are influenced culturally, not genetically. Unlike the lizards, a person of any genotype could grow up to be a celibate, a monogamous spouse, or a polygamist.
👨🏽 🆚 🤵🏽👰🏻 🆚 👯♀️🕺🏽👯♀️
Your ancestry doesn’t define your lifestyle
Furthermore, even if prehistoric humans like Homo erectus were still around, you would have no trouble sorting out Homo sapiens from any other species, as you might with the longwing butterflies. Human populations may vary, but we don’t differ in what kind of fluid we are able to breathe, like freshwater versus saltwater fish. Nor in the presence or absence of an entire sexual organ, as in strawberries. For these reasons, humans are not classified into morphs, subspecies, or breeds(Norton et al. 2019). There is one standard type of human, with minor customizable flourishes.
🚙🚙🚗🚕🚕🚙🚙🚗🚗🚗🚕
🆚
🚙🚕🏎️🚓🚜🛺🚗🚐🚑🚚🚛
Some variety (top), but not as much as there could be (bottom)
To be clear, the variation we do possess is not distributed equally around the world. Humanity is not homogeneous. If you send a geneticist your DNA, they could get a pretty good estimate of where your recent ancestors lived. If you showed those ancestry estimates to someone else, they could make a few general predictions about your overall physical appearance.
🇯🇵🇲🇽🇿🇼🇵🇼🇪🇸🇮🇳🇲🇬🇧🇷🇮🇩🇮🇸🇦🇹🇬🇦🇸🇦🇻🇳
There’s no perfect match between your DNA and your ancestors’ nationalities, but there are correlations
But DNA is not cleanly partitioned by homeland. Remember the archipelagos. Populations geneticists often make plots of individuals that actually look a lot like islands in an ocean. However, these plots are easy to misinterpret. There is no literal ocean or any other geographical space. Positions on the plot are defined by multivariate statistics. Genetically similar individuals cluster together. Consider the following classic principal component analysis (PCA), for example(López Herráez et al. 2009).
⬛⬛⬛⬛⬛ → 🔲 ← ⬜⬜⬜⬜⬜
A hybrid of two pure populations: not a useful way to think about human genetics
In reality, human populations have been mixing and remixing since the dawn of our species. Every “pure” population is really descended from two or more other populations that used to be separate, which themselves have a mixed ancestry, and so on(Reich et al. 2018). We are all admixed.
🥘🥘🥘🥘🥘 → 🍲 ← 🍜🍜🍜🍜🍜
A stew of mixed ingredients plus another stew of mixed ingredients yields an especially mixed stew
More importantly, images like the PCA above are meant to highlight subtle dissimilarities by turning the contrast up to eleven. They doesn’t say anything about the size of the differences among those ancestry groups. As an analogy, consider the following ten bags of groceries, each with twenty items:
Bag A:
🍪🎂🥐🌽🥒🍇🍍🍑🍩🍓
🍉🍊🍌🥓🥕🥖🥜🥦🥔🥚
Bag B:
🧀🥨🥧🥥🥝🥚🍠🍗🍐🍏
🍊🍇🥕🥔🍓🥒🥐🌽🍎🍩
Bag C:
🌶️🌽🍇🍒🍗🍞🍬🍓🥓🥕
🥖🍉🍋🍌🍏🥜🥝🥦🍠🍊
Bag D:
🥥🥦🍐🥖🌽🍫🍪🍤🍍🍌
🍋🍓🌶️🧀🥔🥐🍠🍗🍎🍇
Bag E:
🍌🍐🍋🍑🥖🥥🍓🥑🍪🍉
🥕🥔🍠🥦🥓🧀🍏🍞🥚🎂
Bag F:
🍠🍞🥚🥝🥔🍆🥦🍐🥓🍌
🍏🍪🍓🥨🍬🍊🍎🌽🥥🥜
Bag G:
🍆🍋🍍🍎🍐🥔🥕🥜🥦🥧
🥨🍑🍠🍤🍩🍓🍪🥐🍏🧀
Bag H:
🥕🍏🍪🍉🍩🍍🍊🍅🌽🥔
🍠🥜🎂🍎🍓🍒🥖🥚🍤🍞
Bag I:
🌽🥨🍇🍉🍌🍒🍗🍞🍓🍤
🍩🥕🥦🍫🥑🍋🍎🍑🍠🧀
Bag J:
🥐🥔🍓🥧🌽🍉🍒🍩🥦🥨
🍗🍇🍅🍊🥓🥝🍐🍤🍋🥕
Ten grocery bags that were randomly packed. Or were they?
Just by looking at their contents, could you easily classify the bags into distinct categories? Probably not. Or if you did it would be arbitrary. You wouldn’t expect other people to make the same decisions. All the bags seem to contain a fairly random assortment of items. However, a computer algorithm could readily detect nuanced patterns and place the bags into clear groups. Here’s a PCA. The first axis explains 16% of the variation among bags. A, E, F, H, and (to a lesser extent) C all seemingly share something.
➕🍌🥪🍎🍐🍇🍿🍩🍋🍒🍑🍠🥭🥜🍞🥓🥖🥚🥐🥨🥧🍤🧀🍗🍬🍫🥫🎂🍪🍉🍊🍓🍕🌮🍅🥑🍆🌶️🥒🥦🍒🥔🥚🍫🥦🍪🍉🥝🥓🍭🍈🥟🌶️🥖🥨🥐🍞🍊🍅🥧🍿🎂🥑🍆🥒🍌🥪🍑🌽🍍🥥🍇🥭🍬🥫🥑🍩🍋🍤🧀🍏🥧🍭🍈🍏🥜🍐🍓🍕🌮🥕🥧🍗🍠🍍🥥🥝🥟🌽🥕🥔… (x 200)
That’s a really big grocery bag
For most genes, diversity within our species and divergence among populations is neither large nor meaningful. But is that true for every gene? Chapter 3 will explore whether and how some genes may impact important traits that vary among populations.