Galapagos finch beak size decreased from 1975 to 1977, this corresponded to a severe drought when seeds became smaller and smaller, birds with large beaks were less able to manipulate the seeds during feeding 3. Salmon body size and age at first reproduction decreased from 1984 to 1990, this likely is due to increased fishing pressure that removes the largest individuals 4. Cliff swallows mean wing length increased in only six days, there was a severe storm that prevented feeding so the smaller birds with less energy reserves died 5. Using this definition, evolution is indisputable.
Evolution does happen.
HOW it happens is considered evolutionary theory. If you remember the definition of theory, you should realize that there is a huge amount of evidence for the mechanisms behind evolution. That does not mean that we cannot argue over the mechanisms and change our ideas. We have argued and we have changed some of our thinking. The answer that is usually given is Darwin.