In nature, natural selection is most commonly stabilizing. The average members of the population, with intermediate body sizes, have higher fitness than the extremes. Natural selection now acts against change in form, and keeps the population constant through time.
Natural selection can also be directional and disruptive.
Figure: stabilizing selection on human birth weight. Infants weighing 8 lb at birth have a higher survival rate than heavier or lighter infants. The graph is based on infants born in London from 1935 to 1946. From Karn and Penrose (1951).
Does stabilizing selection operate on birth weight in humans?