Mendelian genetics are used by creationists to argue against the Darwinian claim of evolving life forms. The evidence from genetic research shows that characteristics of an organism remain constant. However, there exists sources of variation which are able to explain the differences between different organisms. Environmental variations may have importance on the living individual, but they are never passed along to offspring. For example, large muscles acquired from lifting weights cannot be inherited by one's offspring. Gene recombination is another source of variation. This shuffling of genes may hide certain genes for generations, but the encoded traits are never truly lost. Recombining genes allows for certain variations within a population, but the variations only occur because the genes had always remained. One more major source of variation is mutation. Occasionally, during DNA replication, mistakes occur and sometimes result in the mutation of a gene. These mutations are typically harmful or fatal. Even when a mutation involves a drastic helpful change, it is never relevant to the evolving of one creature into another. The final source of species variation claimed by creation science is, simply, creation. The commands of the Creator are the final explanation for the great diversity of life. Only the work of an intelligent designer outside of nature can explain such complexities as the human brain or the plethora of different organisms on Earth.