STANDARDIZED COEFFICIENTS ([beta]) FROM LINEAR REGRESSION REPRESENT THE DIRECTIONAL SELECTION
To distinguish direct and indirect effects of directional selection
, we analyzed mating success through a multivariate approach.
The linear regression models for directional selection
accounted for a higher fraction of variance in the annual standardized fitness in comparison to E.
By substituting the definition of the phenotype from equation (14) into equation (3) and focusing only on the directional selection
gradient, we get the fitness equation:
On the hillside, CL flowering date is the only trait for which there was consistent, strong directional selection
suggesting that CL flowering date is the trait most important for superior relative fitness of HL plants on the hillside.
Strength and tempo of directional selection
in the wild.
The consequence of directional selection
on a trait on the mean and variance of other, correlated traits is the topic of Chapter 5.
A central theme of the book is evaluation of the extent to which novel forms on islands are generated predominantly via founder effects alone (including population bottlenecks, loss of genetic variation, and subsequent random drift and genome reorganization) versus adaptation via directional selection
for new phenotypes in new insular environments (see especially Chapter 8 for an overview of alternatives).
The way in which genetic variation is affected by such variability will determine the extent to which traits under directional selection
will evolve in nature.
implies proportionately higher nest failure at one end of a habitat gradient [ILLUSTRATION FOR FIGURE 1B OMITTED].
As a result, the mimetic form is under strong directional selection
to resemble the model.
Flower size would experience positive directional selection
if larger flowers attract more pollinators (Young and Stanton 1990), but it could be under negative directional selection
if pollen transfer is more efficient in smaller flowers (Galen and Newport 1987, Armbruster 1990).
An estimate of the directional selection
gradient is obtained from the partial regression coefficient of a linear regression of relative fitness on the character (a measure of the slope).
I estimated the standardized directional selection
gradient, B, as the partial regression coefficients of relative fitness vs.
The common pattern of seasonally declining survival prospects of offspring and/or declining clutch sizes, has usually led workers to suggest that timing of breeding is subject to directional selection
toward earlier dates (Perrins 1970).