Glossary of "Ecology (6th ed.)" by Krebs (2009)
- natural selection that favors traits either above or below the average of the population, so that over time the average moves in one direction.
Glossary of "Plant Biology (1st ed.)" by Graham et al. (2003)
- A process of selection that favors individuals with a trait at the extreme of its character range over individuals with the average or opposite extreme for that trait.
Glossary of "Evolution (4th ed.)" by Futuyma & Kirkpatrick (2017)
- Selection for a value of a character that is higher or lower than its current mean value.
Glossary of "Life (9th ed.)" by Sadava et al. (2011)
- Selection in which phenotypes at one extreme of the population distribution are favored. (Contrast with disruptive selection, stabilizing selection.)
- Futuyma, D. J. & Kirkpatrick, M. 2017. Evolution, 4th ed. -xviii + 602 pp. Sinauer Associates, Inc., Sunderland.
- Graham, L. E., Graham, J. M. & Wilcox, L. W. 2003. Plant Biology. -xxxv + 497 pp. Pearson Education, Inc., Upper Saddle River.
- 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.