It makes sense to have spares of those things you might need to change at a moment's notice (such as tyres, light bulbs and fuses) but the evolutionary advantages of having two sets of chromosomes — diploidy — are less obvious. Now the experimental work in yeast of Zeyl and colleagues has shown that the prevalence of diploidy cannot be put down to a greater rate of adaptation compared to haploids.

It has been suggested that the more frequent production of adaptive mutations — double the alleles means double the opportunity for adaptive mutations to arise — might explain why seed plants and multicellular animals are predominantly diploid. Zeyl et al. tested this hypothesis by comparing five haploid and five diploid populations of Saccharomyces cerevisiae. As these populations were propagated through 2,000 generations in a liquid medium with limited nutrients — which forces cells to compete for these resources — mutants with higher rates of growth and survival arose and increased in frequency.

Measuring the fitness of the experimental populations relative to the unselected diploid ancestor every 200–300 generations showed that haploids adapted significantly faster than diploids to the new environment. This was good experimental evidence supporting the idea that in large diploid populations the additional opportunities for adaptive mutations to arise are outweighed by the increased time required for their fixation, because of their lower selective advantage in heterozygotes compared with haploids.

Repeating the experiment in small populations showed that the haploid advantage was lost (that is, the rates of adaptation were indistinguishable between haploids and diploids), confirming that in small populations the rate at which adaptive mutations arise is more of a limiting factor than it is in large populations.

The work of Zeyl et al. shows that diploidy does not necessarily lead to a greater rate of adaptation, so we cannot base a general explanation of its prevalence on this idea. The authors suggest that similar future theoretical and empirical studies should consider the consequences of different possible correlations between selection and dominance. In future, understanding how the dominance of adaptive mutations varies with respect to the fitness advantage they confer might well be crucial if we are to understand why diploidy is so popular.