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.

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.