Well more than just one...
Basic evolution: a small number of more or less "random" DNA mutations are observed in every offspring of every species.
Most are harmless and serve really only to help us track our ancestry through passed-down harmless mutations.
Of those which have some affect, most are harmful and often get acted on by natural selection.
But a very small percentage can be helpful and those can accumulate over many generations leading to speciation.
That's evolution in a nutshell.
So speciation can be observed at every step of the way, from simple short-term adaptations which create, for example, new breeds & varieties, to much more significant & long-term changes creating sub-species, species & even genera.
An example of one mankind forced & witnessed from the beginning, is the creation of dogs as a species separate from wolves.
Another example illustrating degrees of biological separation are polar bears & brown bears, once classified as separate genera in the Ursidae family, now discovered to be simply two species in the genus Ursus.
Point is: evidence clearly shows increasing degrees of biological separation -- evolution -- on a continuum from mere breeds through species, genera, family, class etc.
And fossil records reveal these degrees of separation take longer times (millions of years) the greater the separations.
So mankind has been around just long enough to create a separate species from wolves, dogs, but not yet long enough to have witnessed, say, the separation of Indian elephants as a distinct genus from African elephants.
However, evidence of it is all around us, from the fossil record to comparisons of their DNAs.
Is that a good enough start for you?
No, adaptation within species is an argument by association. Which is the weakest form of argument.
The ability of a computer program to adapt to differing conditions and random inputs also has to be programmed in.
When moderns look at the fossil record they assume that similarities between species implies that evolution was the cause.
But see, now we are getting glimpses into the complexity of the genome, which is complex beyond all imagining. No serious mind can look at the human genome or bovine genome and walk away from that knowledge thinking that any of it arose from inorganic chemistry and random unguided processes.
Charles Darwin was an ignorant fool, and those who continue his stupid legacy do so in the face of mountains of information theory that have come to light in the last 100 years.
I know my first parents by name. That little rib thingy that God took from Adam to make Eve, what do you think it was? I’ll tell you, it was an X chromosome. Moses was shown it, had to write about it in language that mankind would comprehend, knowing that we would not see it for thousands of years.
Here is a prediction of mine. As scientists continue to crawl over the human genome they will find a counter that increments by one every time a generation has occurred. When that happens, each person will be able to have their DNA examined, telling them how many generations have passed up until now. That number will not correspond to millions and millions of generations.