>>19295711R selected vs K Selected
R selection is a high stress environment biological response that favor a higher number of offspring in the assumption that in the dangerous environment many of them will die but if you make enough some will live. R selection favors the big butt because like this other anon says big butt tend to indicate wider hips which in turn means easier childbirth less likely to cause lasting damage to the mother and thus enable her to pop out many babies for many years. R selected creatures happen in dangerous environment where the chances of children and/or parents dying young is fairly high, so priority is placed on producing as many offspring as possible as quickly as possible wiithout too much regard for individual quality in hope that predators wont be able to get them all and some genetic lineage will survive long enough to reproduce.
K selection is an opposite trait where the number of offspring is lowered is favor of high individual investment in offspring by parents in order to produce high quality individually competitive offspring. To this end big tits are preferable, you're only going to have a few babies anyway so you can chance the birth of each but once born a mother with big tits can give the baby a lot of a milk enabling rapid muscle, organ and most importantly brain development in the infant, the amount of natural breastmilk a kid gets in the first year of life is directly correlated to brain tissue growth and intelligence in the longer term. If you want smart kids INSIST your waifu breastfeeds them, if she does not want to, ditch her and find another she will give you retards. This is the reason for the big push to "liberate" women from breastfeeding. Bottle babies are less intelligent and thus less independent.
TLDR
Big ass for producing lots of babies quickly without regard to their intelliegence.
Big tits for producing fewer, more intelligent, more competitive babies.
