We adopted an empirical outlier approach to identify potential targets of directional selection
by obtaining top 0.1% of highly divergent regions as well as top 0.1% of strong reduction of genetic diversity in Thoroughbreds (Excoffier et al., 2009).
The magnitude of directional selection
([beta]) on life history traits detected in this study (range 0.109 to 0.256) is higher than the average magnitude estimated for plants and animals (0.08; Kingsolver et al.
The analysis of standardized selection differentials revealed only directional selection
on tibia length favoring females (Fig.
Unlike least squares regression, this procedure may only reveal directional selection
, but says nothing regarding nonlinear selection (Price and Boag 1987).
A simulation study conducted by Lacy (1987) also showed that even with a population size of 20, genetic variation remained in the population for nearly 100 generations in the absence of directional selection
. He showed that strong directional selection
in a finite population is more likely to quickly deplete the genetic variance than genetic drift.
This suggests an empirical test for this particular hypothesis for divergence: the degree to which isolated parts of a population diverge under accelerating selection should be reduced if the initial population from which they are drawn is first moved away from a canalized state, perhaps by being subjected to strong directional selection
or crossed with a different population.
gradients ([Beta]) describe the direct linear relationship between traits and fitness.
However, the two genes, Tas2r49 and Tas2r3, were associated with bitter taste and were under directional selection
between the QIN and non-QIN populations, showing no signal of directional selection
between MIN and QXL populations.As a form of olfactory communication, odor perception is crucial for reproduction and survival of giant pandas in the dense forest.
Predicting microevolutionary responses to directional selection
on heritable variation.
The expected change in the mean values for a set of traits depends on phenotypic selection and genetic variation by the equation [delta]z = g[beta], in which [delta]z is a vector of predicted changes in mean values, G is the matrix of additive genetic (co-)variances among the traits, and [beta] is the selection gradient vector for directional selection
on the traits (Lande 1982, Iwasa et al.
The same procedure was followed to test the significance of the linear trend in the allelic frequency changes that might result from directional selection
Lande and Arnold 1983), we show how simple stabilizing or directional selection
on maternally affected offspring traits results in functional integration of these traits, favoring particular combinations of maternal and offspring genetic effects.