A reader writes:
According to Galenson, the mortality rate on slave ships (a little more than 10% for both Slaves and Europeans) was constant per time unit regardless of the length of the journey. That is about 2 per 1000 per day died. More would die in longer journeys, but only proportionally. The incremental difference between other stops and Jamaica should therefore be all but negligible.
The fact that the European crew died at such a high rate (ships with only whites tended to have 4-5% mortality rate) indicates it was the mixing of disease that was the main cause, not “harshness” of the journey per say. The slaves for example had enough food, so testosterone would not help the strong ones get more than the weak.
Many more slaves tended to die when the ships contained slaves from different regions. Again disease is the likely cause. I guess testosterone increases immune defence somewhat. But the direct selection for disease immunity most be vastly more important than testosterone in the selection.
But in the larger picture, what Aiken is pointing to is being a slave was a new evolutionary environment. He's focusing only on the flashy middle passage environment, to be sure. But it seems to me that even once they got off the boat, the slaves were facing a distinctly harsher environment (in many ways) than they had in West Africa. If it is true that being a slave in the West Indies was different in terms of what was "fit" from being a random peon in West Africa, then we should expect natural selection to differentiate the populations. A selection bottleneck which selected along the same lines would, in this case, speed the process along.
Rather than the middle passage, what I think may be more important was the astounding death rates the slaves had even once here. Consider this:
"By the middle of the 17th century, British Jamaica and French Saint-Domingue had become the largest and most brutal slave societies of the region, rivaling Brazil as a destination for enslaved Africans. The death rates for black slaves in these islands were higher than birth rates. The decrease averaged about 3 percent per year in Jamaica and 4 percent a year in the smaller islands. The main causes for this were overwork and malnutrition. Slaves worked from sun up until sun down in harsh conditions and supervised under demanding masters, with little medical care. Slaves also had poor living conditions and consequently they contracted many diseases."
A 4% per year decrease in population is an impressively powerful selection environment!
Right. The sugar plantation regions of Brazil and the West Indies were much more deadly than the tobacco plantation region of Virginia. The sugar growers worked slaves to death and replaced them with cheap imports from across the Atlantic.
It was more expensive to import slaves all the way to Virginia. Plus, the climate was quite healthful for Africans -- cooler than the tropics, so tropical diseases were less virulent, but not frigid, so Africans didn't suffer as much from respiratory diseases in Virginia as they did in New England. So, Virginia slaveowners had economic incentives to make sure that the birth rate was higher than the death rate among their slaves. (By the time the deep Southern cotton belt opened up fully, the slave trade had been outlawed, so American owners still had an incentive to care for the health of their slaves.)
Off the top of my head, though, I can't see any particular traits that the sugar plantations selected for. Take sprinting ability: West Indians (sugar) and African-Americans (mostly not sugar) are both outstanding, while black Brazilians are not. Not bad, just not great.
On the other hand, a black Brazilian named Ronaldo da Costa set the marathon world record in 1998, which is almost unimaginable for an African-American or West Indian.
Why? A. There is less connection between looks and racial background in Brazil than in America due to the lack of a color line and preference for fair women. Consider two sisters in Brazil, one fair and one dark. The fairer one is more likely to marry a richer, whiter man, while the darker one is more likely to marry a poorer, black man. Repeat for 12 generations and you've substantially disconnected the genes for looks from all the other genes. (In the U.S., the color line largely prevented this process.) So, perhaps, even somebody as black-looking as da Costa could have the athletic genes of a Portuguese distance runner.
B. The alternative theory is that da Costa is largely descended from South or East Africans. Brazil got a lot of its slaves out of those regions that today produce so many fine distance runners. In contrast, the U.S. got most of its slaves from West Africa, which has the same imbalance today as African-American between outstanding sprinters and virtually non-existent distance r