Answer: Leeuwenhoek biggest contribution is to the first tenet of the cell theory.
Explanation: the first tenet of cell theory states that every living being is made of cells and even an organism can be made of just one cell able to reproduce by itself.
Leeuwenhoek observations stablish the existence of microscopic structures alive some with movement. This helped other scientists to determinate this structures were cells that can make vital processes such as feeding, breathing or reproducing which are determinant in biological definition of life.
Natural selection or survival of the fittest can cause a major evolution as the species at risk need to stay alive and therefore need to become more adapted to the situation at hand. The species can evolve through generations to become more crafted to the predatorial habits of their predators. If the females are less at risk than the males then the males might evolve to become more protected or if some of the species live in a different situation maybe not even that far away, that can have a big impact on the evolutionary habits of the species at hand.
I hope I'm right
