Glossary of "Evolution (4th ed.)" by Futuyma & Kirkpatrick (2017)
- Limitation on the variation expressed in a phenotype (perhaps a protein) because many variants have impaired function and reduce fitness.
- Futuyma, D. J. & Kirkpatrick, M. 2017. Evolution, 4th ed. -xviii + 602 pp. Sinauer Associates, Inc., Sunderland.