outsidethebox wrote: I am referring to the building/maintaining of brood stock lines that indeed are more narrow in their genetic makeup. If one selects from the more average/center of the herd/flock the variability will be reduced significantly...and I am talking about culling the outliers from both ends of the bell-curve.
If one has the power to manipulate the bell curve (and I would argue that that power is limited if one avoids using math in their selection criteria), why would one not strive to move the entire population (i.e., shift the bell curve) in a desired direction? As Janet wrote, reducing variation lessens the ability to change the mean. It is a pretty bold statement to say a line is so perfect that no improvement is necessary. In general, most who take such a position fade into history as times change.
outsidethebox wrote: EBVs and EPDs simply informs one of the expected average. They do not inform one of anything about variability...please explain this otherwise.
For an individual:
Most BLUP evaluation also reports some form of accuracy, which provides a measure of the reliability of the estimate of the mean (EBV). Among many other things, accuracy takes into account the variation in the records for an animal and its relatives.
For a population:
1. Take a list of numeric EBVs for all individuals.
2. Compute variance for trait within population.
3. Hard to do without a list of numbers.