What scientist is credited with proposing the principle of uniformitarianism?

James Hutton

Uniformitarianism is the principle or assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe. This concept became known as uniformitarianism and can be summarized by the phrase "the present is the key to the past." 

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).

your spelling is a little off : ]