In general, characteristics that an organism survive and reproduce become more common in a population over time. what mechanism of evolution cause this change?

Natural selection.

Explanation:

Natural selection means that those with good phenotypes are more likely to survive and reproduce. On the other side of the same coin, organisms with less beneficial characteristics are less likely to survive. Because of this, they can't pass their genetic information down, making it less common, and vice versa.

it also supports my hypothesis. if you rest first you can squeeze the clothespin more times. from the data i collected the average of the eight people concluded exercise makes your muscles more fatigued and less able to squeeze the clothespin compared to resting before squeezing.