|MadSci Network: Evolution|
Because humans are long-lived and produce few offspring, they would be a poor choice of organism to study stabilizing selection. One can, however, cite reasonable speculations concerning stabilizing selection in humans. For example, consider the human brain. For many millions of years the brain size to body size ratio of the hominid line leading to man got bigger and bigger. Presumably, this increase occurred because bigger-brained individuals were on average more intelligent, and more likely to survive/procreate (directional selection). Since the time of the Cro-Magnons, however, the increase in the human brain/body ratio appears to have reached a plateau (i.e., selection has stabilized). Why has the positive selection for large-brained individuals ceased? Two possibilities seem likely. First, large-brain individuals might have been selected against because they could not pass through the narrow human birth canal without killing themselves and/or their mothers. Second, having a large brain is metabolically quite expensive. Johanson & Shreeve (Lucy’s Child) point out that “modern humans spend about 20% of their metabolic energy keeping their brains running, as opposed to 10-13% in a relatively cerebral nonhuman like a monkey. In human infants and children under four, the brain devours closer to 50 percent of total body metabolism.” The modern brain size/body size ratio of humans is probably stable because smaller-brained individuals are selected against because they are not smart enough,while larger-brained individuals are weeded out either because their brains are metabolically too costly or because they are unable to pass through the birth canal.
Try the links in the MadSci Library for more information on Evolution.