“…Although the notion of duplication producing redundant genes is central to current theories of duplicated gene evolution, the short-term benefits of gene duplications are well known. This is illustrated by the numerous observations of adaptive gene amplifications in response to antibiotics [ 31 , 32 , 33 ], anticancer drug treatments and exposure to various toxins [ 34 , 35 , 36 , 37 , 38 , 39 ] or heavy metals [ 40 , 41 , 42 , 43 , 44 ], nutrient limitations [ 32 , 33 , 45 , 46 , 47 , 48 , 49 , 50 ], pesticide treatments [ 51 , 52 , 53 ], extreme temperatures [ 54 , 55 ] and symbiotic and parasitic interactions [ 56 , 57 ]. Combining this information with the observations that recently duplicated genes evolve under purifying selection ([ 21 ] and our present work), it seems reasonable to hypothesize that a majority of duplicated genes that achieve fixation in a population increase fitness when present in two or more copies in a genome and thus are subject to purifying selection from the moment of duplication.…”