Fig. 2: Results of tests for assortative mating.

Patterns of assortative mating over three experimental generations (see Supplementary Table 4 for exact p values). The y-axis shows the proportion of social pairs that were assortative with regard to traits that can only have been culturally transmitted such as song (blue) and traits that have been genetically inherited such as body size (red). The black dotted line marks the random expectation of 50% assortative pairs given an equal number of birds in each category. The two replicates, 1 and 2, are indicated by solid and dashed lines, respectively. The total number of pairs in each of the two replicates are indicated above or below the dots. ***p < 0.0001, binomial test, two-tailed; **p < 0.001; *p < 0.01. In Generation 1, where populations differed culturally and genetically, most individuals paired assortatively by population. In Generation 2, after cross-fostering, individuals mated assortatively by cultural background (population of their rearing parents) and disassortatively by genetic background (population of origin). In Generation 3, where tests were carried out within each genetic background but included groups that differed in cultural background, pairs formed assortatively by cultural as opposed to genetic background.