... by¹

Assuming that there are no genotype $\times$ environment interactions.

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... linear²

And we were willing to do this when we were studying the evolution of only one trait, so why not do it now?

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... scale.³

To measure on this scale the data is simply transformed by setting $y_i = (x_i - \bar x)/s$, where $x_i$ is the raw score for the $i$th individual, $\bar x$ is the sample mean for the trait, and $s$ is its standard deviation.

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... differential⁴

The cumulative effect of selection on the change in mean phenotype.

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... number.⁵

1000 progeny from 100 dams.

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... fitness.⁶

P is the phenotypic variance-covariance matrix and s is the vector of selection differentials.

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