This theory has a certain common sense appeal, because there clearly is an evolutionary trade off between quantity and quality, and that fascinates me because quality is a value judgement and scientists aren’t supposed to think in terms of some life forms being genetically superior to others. An r strategist is like fast-food cook (she makes tons of food fast and cheap) while a K strategist is like a fine dining chef (makes a few expensive masterpieces with exquisite care and craft)
I think where the theory went wrong is claiming that selection pressures that depended on population density led to K selection because when this hypothesis was actually tested, it wasn’t always true causing many to reject the whole concept prematurely.
In the 1980s J. Phillipe Rushton became perhaps the first person to ever apply the r vs K to humans, arguing more recently evolved and colder adapted races like Northeast Asians were more K than older tropical peoples. He viewed large genitalia and other developed sexual traits (including less obvious ones like breasts, buttocks, voice deepness, muscle salience) as a sign of r selection since they presumably lead to more and better sex, and thus reproduction. By contrast large brains were viewed as a sign of K selection because they are slow to develop and thus require more parental care.
Critics claimed Rushton was wrong and that arctic environments are actually more r selecting because they are less stable, and that tropical diseases are actually K selecting because they are infectious and less density dependent.
The critics were not completely wrong. In his book Lone Survivor, Christopher Stringer argued that one reason modern humans evolved in Africa instead of Northern Eurasia is that in the latter, sudden burst of extreme cold would kill entire populations before any progress could gain traction.
But to me, critics were making the whole debate more complicated than it needs to be. To me it’s very simple: if individual survival is luck-dependent, it’s better to have high quantity offspring (r selection) than high quality offspring (K selection) because luck favors large numbers. The more lottery tickets you buy, the more likely one will win.
How do you know if individual survival is luck dependent? It’s not as simple as asking whether you live in the tropics or the arctic, because an environment that is luck dependent at a primitive stage of technology, might be skill dependent at a more advanced stage.
Instead I suggest you look at how heritable life span is in a particular environment. The lower the heritability, the more luck-dependent your environment is, because luck is really just unexplained variance, and if there’s nothing about the genetic variance (which is the variance that matters in evolution) that can explain who lives or dies, then survival depends on luck.
Of course even if life span had a high heritability at first, natural selection would likely drive it down by removing unfit genetic variance, so it’s not as simple as just comparing heritabilities in the tropics to the arctic. Instead what we need to do is take a group of people who have not had time to genetically adapt to either the tropics or the arctic (middle easterners perhaps) and get one group of them to live as hunter-gatherers in the arctic, and another to live as hunter-gatherers in the tropics. Whichever group showed a higher life span heritability would be in the more K selecting environment.