Final answer:
The statement describes stabilizing selection, which favors average phenotypes and selects against extremes, leading to reduced genetic variance in the population.
Step-by-step explanation:
The type of selection described in the statement "Babies born in the mid-range for weight have a greater chance of surviving birth than do babies who are too small or too large" is referred to as stabilizing selection. This is because stabilizing selection favors an average phenotype by selecting against the extremes at both ends of the phenotypic distribution, which results in a reduction of genetic variance in the population over time. In contrast, directional selection would favor one extreme phenotype, leading to a shift in the population's characteristics toward that trait.