Glossary of Ecology (6th ed.) by Krebs (2009)
- natural selection that favors the norm, the most common or average trait in a population, so the population mean stays constant.
Glossary of "Life (9th ed.)" by Sadava et al. (2011)
- Selection against the extreme phenotypes in a population, so that the intermediate types are favored. (Contrast with disruptive selection.)
- Krebs, C. J. 2009. Ecology, 6th ed. -xvi + 655 pp. Pearson Benjamin Cummings, San Francisco.
- Sadava, D., Hillis, D. M., Heller, H. C. & Berenbaum, M. R. 2011. Life: The Science of Biology, 9th ed. -xliv + 1266 pp. Sinauer Associates, Inc., Sunderland.