What kind of variability plays a leading role in evolution?

The main role is played by hereditary variability. Thanks to her, evolution was possible. Its essence lies in the fact that when genes are mixed, beneficial mutations and other changes in the organism of the offspring take root and are transmitted. If genetic information was not changed or transmitted, there would be no evolution in principle. Those organisms that accidentally received useful mutations and adaptations would simply die after some time and would not pass them on to their offspring. It would turn out that unique features were only in some organisms, and not in the whole species.