What are some rare types of clouds? how are they formed? explain your answer

The Kelvin Helmholtz Waves is one of the rarest clouds. along with the Lenticular Cloud, Fallstreak Hole, Roll Cloud, and Nacreous Cloud.

Explanation: The Kelvin Hemholtz occurs when two different layers of air in our atmosphere are moving at different speeds (a phenomenon known as shear). ... The most obvious example is wind blowing over water, in which fast-moving air can create the waves on the slower-moving water. The lengicular cloud is caused when the air is stable and winds blow across hills and mountains from the same or similar direction at different heights through the troposphere. The nacreous cloud is caused when  the temperature in the stratosphere is below a chilly -78°C, which turns any moisture in the air into super-cooled liquid or ice crystals. ... This light is refracted by the ice crystals in the clouds, a process known as cloud iridescence, producing the shimmering rainbow effect.

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).