Selecting the hen who lays the most eggs doesn't necessarily...

Selecting the hen who lays the most eggs doesn’t necessarily get you the most efficient egg-laying metabolism. It may get you the most dominant hen, that pecked its way to the top of the pecking order at the expense of other hens. Individual selection doesn’t necessarily work to the benefit of the group, but a farm’s productivity is determined by group outputs.

Indeed, for some strange reason, the individual breeding programs which had been so successful at increasing egg production now required hens to have their beaks clipped, or be housed in individual cages, or they would peck each other to death.

While the conditions for group selection are only rarely right in Nature, one can readily impose genuine group selection in the laboratory. After only 6 generations of artificially imposed group selection - breeding from the hens in the best groups, rather than the best individual hens - average days of survival increased from 160 to 348, and egg mass per bird increased from 5.3 to 13.3 kg. At 58 weeks of age, the selected line had 20% mortality compared to the control group at 54%. A commercial line of hens, allowed to grow up with unclipped beaks, had 89% mortality at 58 weeks.

Optimizing a group is different than optimizing an individual