“Actually, most of my work in the medical research field is incredibly dependent on understanding the theory of evolution.”
More details, please. How exactly does the theory of evolution apply to your research, and why is it necessary?
Not saying that there is no such thing as survival of the fittest - that is reality - but which species has ever evolved into another species?
Where do you want me to start?
One example is the influenza virus. This is actually a group of viruses. Each virus in the group infects people for a few years, but each person who gets infected and survives develops an immunity to that virus. So the virus changes from year to year in response to the immunity. The RNA that codes for key proteins mutates, such that the proteins it makes are different. Sometimes, the mutations help the virus to evade the pre-existing immunity, and viruses with those mutations can multiply and infect people who are immune to previous "versions" of the virus. This is why new flu vaccines have to be made every year, and why the flu vaccines are sometimes ineffective (as what happened with the H3N2 component of the vaccine this year).
In addition to the influenza viruses being able to evade immunity year after year through mutations, they trade bits of genetic material with each other. In this way, new species of influenza viruses appear.
There are many other details about influenza virus biology which make it one of the most difficult viruses to study (and develop countermeasures for). The bottom line is that nothing about the complicated biology of the influenza virus is explicable or can be studied outside of the context of the evolutionary process.