Whole-genome duplication events have driven to a large degree the evolution of angiosperm genomes. Although the majority of redundant gene copies after a genome duplication are lost, subfunctionalization or gene balance account for the retention of gene copies. The Arabidopsis 80S ribosome represents an excellent model to test the gene balance hypothesis as it consists of 80 ribosomal proteins, all of them encoded by genes belonging to small gene families. Here, we present the isolation of mutant alleles of the APICULATA2 (API2) and RPL36aA paralogous genes, which encode identical ribosomal proteins but share a similarity of 89% in their coding sequences. RPL36aA was found expressed at a higher level than API2 in the wild type. The loss-of-function api2 and rpl36aa mutations are recessive and affect leaf development in a similar way. Their double mutant combinations with asymmetric leaves2-1 (as2-1) caused leaf polarity defects that were stronger in rpl36aa as2-1 than in api2 as2-1. Our results highlight the role of combined haploinsufficiency and purifying selection in the retention of these paralogous genes in the Arabidopsis genome.