Why might one variation have a selective advantage over another variation?

Variation becomes an advantage if neither of the individuals is having detrimental traits. If neither of the individuals is having the detrimental trait then that means that a bad gene is being introduced in the population. Other times, variation is an advantage since it provides the population with better traits and devices to use on their environment. This is crucial since the environment changes over time.

the supermassive black hole is in the middle of the galaxy.