Ernst  Mayer, considered  as one of the world’s leading evolutionary biologists of the 20th Century once wrote

Anyone who writes about “Darwin’s theory of evolution in the singular, without segregating the theories of gradual evolution, common descent, speciation, and the mechanism of natural selection, will be quite unable to discuss the subject competently.

So if you are struggling with the concepts around the mechanisms  of  `speciation’  this entertaining short video clip might just be for you.

Speciation: Of Ligers & Men – Crash Course Biology #15
https://www.youtube.com/watch?v=2oKlKmrbLoU