NBA athletes now aren't bigger and stronger than NBA athletes in the 1960s because of evolution and gene mutation of the world's overall population. They're bigger and stronger because of a much larger pool of people that started playing basketball compe ively, thereby raising the standards. The best athletes back in the 1960s were not all playing professional basketball. They were also playing baseball and football and ran track or went into boxing. But now, the NBA probably has a much, much larger percentage of the world's best athletes. You add the fact that the pool includes international players so the pool of possible NBA players is global now, yes, NBA players are bigger and stronger and more athletic than the players in the NBA back in the 1960s.
But there are also advancements, both nutritionally and with strength and conditioning techniques that have made a difference now. 99% of NBA players lift weights these days, and not just for recreation. They have to in order to compete at that level. Back in the 60s, I wouldn't be surprised if it were 10% or less of NBA players who lifted weights routinely. It wasn't a necessity like it is now. You add all the funky voodoo crap NBA training staffs put players through these days to maximize athleticism, yes, I think that has quite a bit to do with NBA players being more athletic and stronger than they were 40-50 years ago. It's not gene mutation. But in its own way, it's very much evolution. It's evolution of the game and the standards of NBA compe ion.