How did natural selection most likely result in this evolutionary changeHow did natural selection most likely result in this evolutionary change?

Bright mice were not able to blend into their environment as easily as dark mice, so they did not continue to survive and reproduce.
Bright mice produced fewer offspring than dark mice, so the bright mice became outnumbered within days.
Random changes in the genetics of bright mice produced more dark mice, so the bright mice quickly died off.
Random changes in the genetics of dark mice produced fewer bright mice, so the dark mice took over the population.

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant. 

Butterfly wingspan is 7.7