It seems logical that natural selection would work toward genetic uniformity; the genotypes that are most fit produce the most offspring, increasing the frequency of adaptive alleles and eliminating less adaptive alleles. yet there remains a great deal of genetic variation within populations. describe factors that contribute to this variation.

I would guess that recessive genes in diploid animals would not manifest as traits, so if one parent is selectively fit  with a homozygous dominant (WW) gene and the other is selectively unfit with a homozygous recessive (ww) gene, the children will be heterozygous (Ww), but will still have the trait that is selectively fit. This ensures that genes that may not be selectively fit remain in the population. One must also consider that genetic mutation that occurs constantly, although slowly, introduces new genes into the population. You may also get breeding between one population and another, which results in the injection of news genes into a population. Finally, since environmental conditions can be fairly variable, you may find that the temporal variation in gene selection may be fairly high. So for example, you may have a drought year, which would provide selective advantage to plants that are better adapted to drought, but then the following year might be wetter, another set of genes is selectively advantaged.