Top: evolution of gene number through the three WGD events. Estimates of gene numbers at the time of the duplications were made under the simplifying assumption that further gene loss affected only the new duplicates. Bottom: focus on an inter-WGD period. Just after WGD, genes subject to dosage constraints (that is, genes in complexes or metabolic pathways) and highly expressed genes are preferentially retained as duplicates. Differential loss of paralogues in subpopulations will create barriers to reproduction, favouring speciation. Gradually, the duplicates will erode as the dosage effects are compensated for by new mutations. This massive retention of duplicates across a long evolutionary time enables selection to retain a few new genes with new derived functions.