I believe all of the above!
Genetic drift is defined as a shift in the allelic frequency of a population. This can be caused by natural selection which is the differential survival and reproduction of individuals across a population due to differences in phenotype. Natural selection favors particular traits in a population hence these individuals are able to reproduce while the individuals with disadvantageous traits will pass their genes to succeeding generations. This way the allelic frequency of succeeding generation will be diferent from that of the parent population.
Guessing you mean "impulse".
It means to have a sudden strong desire to do something or a small amount of energy that moves from one area to another.
Hope this helps.
