When I joined Twitter three years ago, I thought I would get into a lot of arguments with creationists. I mean, I’m an evolutionary biologist. We hear so much about the widespread rejection of evolution and the war on science. But I almost never interact online with anyone who questions evolution. Maybe the movement is losing energy, as least so far as directly confronting scientists is concerned. Instead, who do I find myself arguing with the most?

Racists.

Specifically racists of the “human biodiversity” variety. Folks who love evolution as much as I do. But they think it means that each ethnicity has evolved a distinct brain type. They think the cultural and technological differences among human societies are primarily due to genetics. To be clear, these beliefs are absolutely false. But when framed with scientific language, they can sound plausible. That makes them dangerous.

These pseudoscientific racists get a few things right. They often claim to be correcting common misconceptions. However, I think it would be hard to find many people who disagree with these facts:

You could take an unidentified human DNA sample and infer the major geographic homeland(s) of the person’s ancestors.

Some of these ancestry-associated genetic differences cause measurable average differences in anatomy and physiology.

Most human traits, including psychological traits, have a heritable component.

The human biodiversity crowd is also sophisticated enough to avoid some simplistic racist fallacies. They correctly accept that:

Genetic differences among populations are not absolute, but statistical averages. For example, a variant of a gene occurs in 20% of Peruvians and 40% of Nigerians.

Traditional racial categories, while correlated with ancestry, are a poor proxy for it. For example, “black” includes African Americans with primarily European heritage, as well as several African peoples as distantly related to each other as Greeks are to Koreans.

There is no such thing as a “gene for” a complex behavioral habit like altruism or creativity. Such aptitudes emerge from a medley of genetic and environmental factors.

But after that, the Darwinian racists head straight into fantasyland. They claim the genomics revolution has validated their form of bigotry. After all, we now know that natural selection has favored genetic changes in some racial and ethnic groups. But a closer look actually lays waste to their beliefs. What parts of the genome show this recent adaptation? Very few. Just the genes underlying the same superficial aspects that we can observe in other ways. Skin color. Ease of digesting particular foods. Ease of breathing at varying altitudes. Resistance to local diseases. Natural selection logically should act differently on these traits in places with differing climates, parasites, and available meals. In contrast, we do not see such differences in genes underlying mental traits. That’s also to be expected, because the main thing driving natural selection on our brains is other people. Our need to impress, outwit, and cooperate with each other provides the adaptive basis for our cerebral abilities. The human drama of gossip, romance, revenge, and fraternity doesn’t change whether you are a tropical hunter-gatherer or a farmer on a frigid plain.

The racists aren’t convinced. Their thinking goes like this. Natural selection is a powerful force, right? And people have lived such different lives for such a long time. Shouldn’t even slight intercontinental differences select for different brain wiring? It sounds reasonable at first. But it’s not, because the default assumption in evolution should be that traits do not change. The main effect of natural selection on most biological features is to keep them the same. This is even true of our minds.

Consider the armadillos. Sharp noses, weak eyes, crunchy shells, wimpy teeth, serious claws. These are nocturnal, insect-loving beasts with a similar lifestyle to the earliest placental mammals. Those ancient ancestors probably had an armadillo-like intellect. The armadillo lineage hasn’t had a brainy breakthrough since the days when it scuttled under sauropod feet. Now it so happens that most other mammal species diverged from armadillos about 100 million years ago. Woodchucks. Cows. Manatees. Shrews. Aardvarks. Walruses. For most of these animals, I know of no evidence that they are any smarter than armadillos. A handful of mammalian families did become smarter. We’ve all heard about the intelligence of dogs, dolphins, pigs, and of course all of us primates. So how fast does mammalian intelligence evolve on average? There isn’t a very measurable answer to that question, since intelligence isn’t a single concrete quantity like body weight. We need to go with our gut sense of what we mean by “intelligence,” and do some very rough estimating. As a back-of-the envelope guess, I might say that the average modern placental mammal is about twice as smart as an armadillo. Over 100 million years, that corresponds to an average rate of becoming about 0.1% smarter every 100,000 years. But human populations only began to split up about 100,000 years ago. Suppose human IQ evolved at something close to the mean mammalian rate during the past 100 millennia. If so, any variation among races is miniscule. The idea that the French must have different brains than the Ethiopians, simply because they live at different latitudes, just doesn’t hold up. Evolution, as a rule, is much more conservative than that.

But wait, say the racists. Why assume a typical mammalian rate for humans? Aren’t we an exception with our rapidly expanding craniums? True, our grey matter did evolve very quickly prior to the emergence of anatomically modern humans. But here are three reasons why this still doesn’t imply substantial racial differences.

First, how much smarter than armadillos are we, anyway? I’m going to a take a wild, human-flattering stab and say 20,000 times. 20,000 is a typical adult vocabulary size. And armadillos can learn at least one command via target-training. To get 20,000 times smarter over 100 million years, how fast did we need to evolve? Not as fast as you think. Remarkably, only an order of magnitude higher than the mean mammalian rate: 1% smarter every 100,000 years. Even this elevated rate couldn’t lead to big disparities among races.

Second, you shouldn’t conclude that our current rate of neurological evolution is still unusually fast. My ballpark math here, which assumes a consistent rate, is an oversimplification. Evolutionary trajectories don’t continue indefinitely. We didn’t sprout extra opposable thumbs. Imagine a gambler on a winning streak, betting for another big score. The streak is just a statistical fluke, and it’s foolish to expect it to continue. After flipping twenty heads in a row, you should still expect 50% heads on the next few coin tosses.

And third, suppose our intellects are really still on an evolutionary fast-track. Then wouldn’t we all keep evolving in parallel? Maybe we all got 1% smarter in the past 100,000 years. I’m just talking about the genetic component of intelligence, of course. We actually have gotten smarter recently, because of cultural changes. This gets to the root of racial prejudice: differences due to external causes are assumed to be innate.

So who should we target with evolutionary biology outreach? People who reject evolution for religious reasons? Or people who misinterpret evolution for racist reasons? The former gets a lot of attention, which is important, but let’s not ignore the latter. It’s even a more achievable goal, because these people are already excited about science. That’s actually a good thing. I don’t believe they’re a lost cause. If I’m right, then today’s Darwin-loving racists could be tomorrow’s objective evolution enthusiasts. If not, well, maybe 20,000 was an overestimate.

Religious fundamentalism is on the decline, thanks in part to countless science literacy activists. Racism is on a decline too, but it’s not going quietly. The presumptive GOP presidential nominee is no Bible thumper, but his whole campaign is founded on racism. So yes, let’s keep working to promote acceptance of what evolution has accomplished. But don’t stop there. People also need to know what evolution hasn’t done.