To the (fortunately few) people who don't think there's evidence of evolution.
Evolution can be observed in a lab setting.
Step 1) Get a few hundred of those fruit flys that reproduce in three days time. Divide them into two containers. Mark one Control and one Experiment.
Step 2) Provide Control with food, water, and whatever else fruit flys need to survive.
Step 3) Do the same for Experiment, but at the same time introduce an element that makes it harder for the fruit flys to survive. Not something that will kill all of them, but a few. Maybe a glowing blue bug-zapper, or enough of a toxic substance in their water so that some (but not all) die off, or perhaps too little food. Anything that will kill off some, but not all of the fruit flies will generally work.
Step 4) Wait for a few months (keep feeding, watering, etc them and keep providing the same problem to the Experiment group).
Step 5) Compare the two groups. The fruit flys in the Control group will probably be pretty much the same, but the fruit flys in the Experiment group will have changed. If you introduced a glowing blue bug zapper, you will likely find that the Experiment group now avoids the color blue. They didn't learn this - but some of the individuals naturally disliked the color blue, and that trait was passed down to their offspring because they didn't get zapped. Introduced a slight poison into their water? Now Experiment is immune, or at least resistant to small amounts - they probably are no longer affected by the amount you've been putting in their water. Too little food? The new flys are probably smaller, or they've adapted to eat other material in their environment.
Scientists do this all the time. It's how a lot of medical research companies work. They take a creature and breed it, and then introduce different stimuli. Rats are another popular option, although it takes years to pull off even minor adaptations. And yet, people still use them, running experiments that last years and years, through hundreds of generations.
Ever heard of "penicillin resistant bacteria"? Those are bacteria that have evolved to ignore the effects of penicillin. How did they do this? Well, someone took penicillin, and 99.99% of the bacteria were killed. That last .01% had a natural immunity. They reproduced mitosis and created whole penicillin resistant diseases. Which is why we keep inventing new antibiotics - because the diseases adapt to the old ones. That is Evolution in action.
So saying that we can't observe evolution is absolute bull. We do observe it. Every day.
We fight against it (in regards to bacteria) and we use it to solve problems. Give a group of rats a disease, and breed the survivors. Give it to their offspring, and breed the survivors. Eventually the rats are immune - and you have yourself the basis for a vaccine or cure.