  Trait differences between species may be attributable to natural selection.
However, quantifying the strength of evidence for selection acting on a
particular trait is a difficult task. Here we develop a population-genetic test
for selection acting on a quantitative trait which is based on multiple-line
crosses. We show that using multiple lines increases both the power and the
scope of selection inference. First, a test based on three or more lines
detects selection with strongly increased statistical significance, and we show
explicitly how the sensitivity of the test depends on the number of lines.
Second, a multiple-line test allows to distinguish different lineage-specific
selection scenarios. Our analytical results are complemented by extensive
numerical simulations. We then apply the multiple-line test to QTL data on
floral character traits in plant species of the Mimulus genus and on
photoperiodic traits in different maize strains, where we find a signatures of
lineage-specific selection not seen in a two-line test.
