The molecule (DNA) that passes genetic material on and the code used by the molecule. It seems like basic biology now but until then we only knew it was happening but not how. Once we knew that it was possible to demonstrate common ancestors etc.

One of the more important examples involves bacteria, in particular, those that have developed resistant strains. The over use of antibiotics have provided a challenging environment to bacteria, and they evolved to handle their new environment.

the thing that makes Darwin so indespensable to the field of evolution is that virtually every discovery in the discipline since the propagation of his work is the outgrowth of efforts to verify the accuracy of his conclusions. To my mind, so great is his importance in the field that were it decided that the term evolution should revert to the hideously awkward “Darwinism” that our fundamentalist friends still employ, I’d go along with it.
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The ability to sequence and study DNA and RNA. We can now follow Mitochondrial DNA through family trees.
The discovery of the molecular clock that tracks the mutation rate of biomolecules so we can determine when life forms diverged.
The discovery of extreme life forms that live in the coldest and hottest climates in the lowest and highest pressure.
The development of a lower cost DNA sequencing method which economically enables study of many more life forms and helps fills in the evolutionary charts.
mutation

https://en.wikipedia.org/wiki/E._coli_long-term_evolution_experiment

Just about everything in modern medicine.

To follow up on what @Lightlyseared mentioned, it’s worth noting that Darwin didn’t just lack knowledge of DNA—he didn’t even have the concept of a gene. Even though Mendel was studying genetics and heredity around the same time that Darwin was studying natural selection, Darwin knew nothing of Mendel’s work.

It wasn’t until the early 20th century that the two were put together (forming what is called the “modern evolutionary synthesis”). Putting the two discoveries together was one of the most significant advancements in evolutionary theory, and the subsequent discovery of DNA allowed us to put evolutionary theory into practice. And this led to the next big discovery: epigenetics.

On a macro level of human evolution, the discovery of the australopithicines has provided a lot of insight into our own origins.

Darwin is special because he saw a relationship between animals and humans that no one else dared acknowledge.

In one sense, Darwin did not discover anything. He did not do any experiments or find any new species. He took the same observations that were available to everyone else and came up with a way of explaining them. Similarly, Copernicus and Einstein did not discover anything. They all came up with theories that changed how we look at the world.

And they ALL got in TROUBLE!