Every individual in a population of a species is more or less unique; even a pair of identical twins or clonal individuals may differ from each other in environmentally induced details. Natural selection acts on this variation among the members of a species, but only the genetically determined variation is relevant from one generation to the next.
Intuitively, one expects limits on the variability of a character. Fanciful legends and children’s tales notwithstanding, giant men or thumb-sized boys cannot exist, because numerous morphological and physiological constraints would prevent such creatures from being viable. In natural populations what controls the limits of variability is stabilizing selection. For example, in a more or less stable environment, the medium-sized members of a population are usually more likely to survive than those that are either very small or very large. Therefore, it should not be too surprising that the relative standard deviation values for the dimensions of various anatomical characters, not only of mammals1, but probably of any animal species lie between 4 and 10%.
This means that under the watchful eyes of natural selection, mothers2 are not any better than tiny snails3 when it comes to going beyond the limits.
1. Simpson, G.G., Roe, A., Lewontin, R.C. 1960. Quantitative Zoology. Harcourt, Brace & World.
2. The data for the heights of 1052 British mothers are from Pearson, K. & Lee, A. 1903. On the Laws of Inheritance in Man: I. Inheritance of Physical Characters. Biometrika, 2:357-462.
3. The data for the diameters of 131 adult specimens of the tiny snail (mean diameter= 2.2 mm) Vallonia excentrica, collected in my backyard, are from an unpublished study of mine.
4. I acknowledge that a comparison of the variabilities of the heights of humans and snail shells was first presented, using different sets of data, by A. E. Boycott. 1928. Conchometry. Proceedings of the Malacological Society, London. 18:8-31.