Fig. 3. Selection can purge standing variation from a population, and evolutionary capacitors can maintain it. Suppose alleles encoding `red' or `yellow' proteins, which fold differently to yield corresponding colors, segregate in an ancestral population. (A) In a yellow environment, conspicuousness to predators will soon eliminate the red allele from the population. (B) In a red environment, conspicuousness to predators will soon eliminate the yellow allele from a population. (C) If selection eliminates the red allele in the population in Environment A and this population then encounters the red environment, pre-existing variation may be insufficient for natural selection. The population may become extinct unless new variation arises. (D) An exemplary putative evolutionary capacitor, the molecular chaperone Hsp90 (CHAP), results in the folding of the red protein so that its conformation is the same as the yellow protein, yielding a yellow phenotype. Because the red phenotype is not expressed, genetic variation persists even in the yellow environment. (E) If the environment then becomes red and heat stress ensues, damaged proteins will out-compete the red proteins for chaperone, allowing the red proteins to fold to yield the red phenotype, which is beneficial in this environment. Compare the outcome here with that in C.