r-selected species exploit less-crowded ecological niches, and produce many offspring, each of which has a relatively low probability of surviving to adulthood. In contrast, K-selected species are strong competitors in crowded niches, and invest more heavily in fewer offspring, each of which has a relatively high probability of surviving to adulthood.In this conceptual framework, humans are clearly a K-selected species. Frogs and spiders, in contrast, are examples of r-selected species.
Intraspecies differences exist as well. In Race, Evolution, and Behavior, Philippe Rushton describes human populations as existing along an r/K continuum, with blacks nearest the r side, Asians nearest the K side, and whites in between.
My purpose is not to detract from or defend that conceptualization, but instead to present the relationship between fecundity and infant mortality at the national level. The following graph depicts this, with the total fertility rate along the x-axis and infant mortality per 1,000 live births on the y-axis:
The correlation between the two is .83 (p=0), very strong for the social sciences. This does not imply that the causes of national differences are primarily genetic in nature. Over the last 60 years, the infant mortality rate in the US has nearly been cut in half while the birth rate has declined by more than one-third over the same period of time. For American blacks, the infant mortality rate is 13.6, compared to 44.4 for the entire country of South Africa, the most developed nation in sub-Saharan Africa.
Still, the strength of the relationship is interesting.