@sfmatheson’s post makes for an excellent thread that should be easy to find! Here’s the list contained within the post:
If you are looking for examples of speciation (or the processes that lead to it) happening rapidly and observably in living species, here is a reading list. These are examples of great scientific investigations, some involving decades of work and some involving experimentation in the wild. All of them examine rapid morphological change. The links go to basic overviews, but there are tons of articles on each of these topics that can keep you transfixed for days.
Guppies in Trinidad: rapid evolution and experiments involving “transplantation” of populations between habitats
Anole lizards in the Caribbean: one famous lab at Harvard is led by author of a very recent book on evolution and convergence
Stickleback fish in lakes and rivers all over the continent
Corn and teosinte (my own blog, from 10 years ago, great story)
Darwin’s finches (Peter and Rosemary Grant, two titans of evolutionary biology; the book about them won the Pulitzer almost 25 years ago)
Cichlid fishes in African rift lakes
Nearly instantaneous speciation in plants
Various populations on the “sky islands” of Southeastern Arizona
My opinion is that only for very specific religious reasons would anyone even wonder about whether speciation happens or whether living species descend from ancestral species that were different from them. I don’t mean to claim that you are expressing real skepticism, but I do mean to say that there is no reasonable cause for doubt. Mountains form, sometimes slowly, sometimes quickly, by the accumulation of change. So do species. It’s not mysterious.
Post that inspired this independent thread:ERV evidence for pastor with a lawyer's mind: