Firstly, most fatalities were of a very narrow age group who just happened to be of fighting age at the time. Why should their surviving siblings, who shared their genes but happened to be too young or too old to fight, and who lived to have children, be any less 'brave and best' than the killed? The same applies to children of the dead already born or conceived.
Secondly, the industrial scale and nature of the killing, especially in WW1, meant that whether you survived or not was largely a matter of chance: whether or not you happened to be brave was irrelevant.
Thirdly, the majority of fighting men, on all sides, were conscripts: and conscription doesn't select for bravery or any other qualities except a basic level of physical competence.
Finally, in the case for instance of the UK, the numbers killed, terrible as they were, were simply not a large enough fraction of the male population significantly to affect the gene pool. (In the case of some combatants, eg Serbia, the fraction was approaching the level where there could have been such an effect.)
Thanks for the comment.
Not entirely sure if I agree, although you may have A point to an extent.
WW1 German population was 67-68 million. So 34million males, many were too old or young so almost 2 million dead or seriously wounded for life is at least 10% of the young generation males.
WW2 90 million population but also about 4 million dead/seriously wounded also another 10% at least. So basically 2 generations almost in a row had over 10% of their healthy and intelligent males taken out of the population.
Just read 18% of Young French men died in WW1 who served, and nearly all healthy, intelligent young men were drafted.
Interesting article that does not approach the ideas of war causing selective breeding and change in genomes.
https://www.cairn-int.info/article-E_POPSOC_510_0001—lost-generations-the-demographic.htm
That might be enough to create a genetic drift, to what extent I’m unsure.
The first wave of fighters, and casualties, were mostly volunteers. Conscription was introduced only after they ran out of volunteers.