Question

I need help my understanding in evolutoin is weak.

What can establish that selection acted directly on the specific trait of interest?

Answer 1

Selection acts at the phenotypic level, therefore its strength can be estimated if it has acted directly or indirectly. Selectional differentials that are used, measure total selection on a particular trait. However, phenotypes impacts fitness directly hence can be correlated. There can be indirect selection where phenotypes can be correlated with other traits that affect fitness. So, the mode and intensity of selection can be estimated by 'regressing relative fitness' of a particular trait of interest on phenotypic values. It is characterized by linear relationship between phenotype and fitness, slope of which can be calculated using linear regression. This will establish the strength of selection. The change in phenotypic means it is occuring due to selection.

On standardizing he values of phenotype by subtracting the population mean and dividing by population standard deviation , the slope of the fitness regression curve is equal to selection differentials (S), i. e, covariance between fitness and trait.