The gravity of the Sun and Moon pulls earth's surface in opposite directions:​

The Moon's gravity pulls more on the planet than the water on the opposite side. These two water bulges on opposite sides of the Earth aligned with the Moon are the high tides. Spring tides occur when the Earth, the Sun, and the Moon are aligned, increasing the gravitational pull on the oceans.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).