RNA has Uracil instead of Thymine. All the other three bases also occurs in RNA, adenine, guanine and cytosine. Therefore a ATA DNA codon will be transcript to UAU by RNA. Both Thymine and Uracil have the same role in however have minor chemical differences.
If the beavers need large teeth to collect wood for their dams, the population of beavers with larger teeth will live longer because they can collect wood. They will them be able to reproduce therefore passing down the gene with larger incisor teeth
