What type of variation is nessary for natural selection to occur

Genetic variation

Explanation:

Genetic variability (diversity) stems from two basic processes: mutation and recombination.

If there were no mutations, all genes would exist in only one form, they would not have alternative forms.

Since phenotypic variability also depends on genetic variability, it is clear that the processes of mutation and recombination contribute to the creation of a large number of different phenotypes.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.