Diseases that primarily exist in animals, but may be transmitted to humans are called

desert perennials often survive by remaining dormant during dry periods of the year, then springing to life when water becomes available. most annual desert plants germinate only after heavy seasonal rain, then complete their reproductive cycle very quickly.

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).