How does natural selection enhance or reduce tha variability of a species?

Natural selection does both. As it favours certain allele, it reduces variability by causing that genes with those allele will out reproduce genes without.

On the other hand, since it happens independently at many locus natural selection will also increase heterozygosity (or allele diversity), thereby increaing species-level variation of genes which in turn allows population to better adapt to the shifting envireonment.