Two hypothetical classes of coexisting proteins, X and Y. Proteins of class X are essential for ecological adaptations, and proteins of class Y are essential for basic biological processes. This schematic picture shows the evolutionary importance of the proteins of each class for ecological adaptation and survival. Organisms having proteins Y can live only in the water environment (upper fig.). Organisms possessing proteins Y and X can exist in the water, in the terrestrial environment and in the mud (central fig.). An organism that loses its Y proteins in the terrestrial environment is evolutionarily eliminated (bottom fig.).
Pawlowski et al. BMC Genomics 2013 14:172 doi:10.1186/1471-2164-14-172