That’s a romantic myth which is periodically trotted out but which has little or no substance. The losses, terrible as they were, were nowhere near large enough, and were concentrated in too narrow an age range, to seriously affect the gene pool. The genes of those who died survived in older or younger siblings too old or too young to fight, or in children conceived before their deaths. The industrial scale of the killing meant that whether you died or survived was random, and your bravery or otherwise were irrelevant. And most of the casualties in all combatant armies were conscripts. Conscription does not select for bravery or anything other than minimal physical attributes.