Why should they have to? Evolutionists can’t test their theory - it happens over millions of years. Can’t put that in a lab and get repeatable observable experiments.
ANd don’t give me the crap experiments of fruit flies. All that proves is that doses of radiation produces damaged, genetically inferior fruit flies. You get me an experiment where you get a non-fly from a fly, and can repeat it, then I will believe in one species coming from another. But radiation is not going to get you there.
Natural selection, mutations, and genetic drift work just fine.
One good example is what is called a ring species. You can see the two different species side by side with all the transitional populations (which we're told can't exist) still living in between them!
Ring species provide unusual and valuable situations in which we can observe two species and the intermediate forms connecting them. In a ring species:
- A ring of populations encircles an area of unsuitable habitat.
- At one location in the ring of populations, two distinct forms coexist without interbreeding, and hence are different species.
- Around the rest of the ring, the traits of one of these species change gradually, through intermediate populations, into the traits of the second species.
A ring species, therefore, is a ring of populations in which there is only one place where two distinct species meet. Ernst Mayr called ring species "the perfect demonstration of speciation" because they show a range of intermediate forms between two species. They allow us to use variation in space to infer how changes occurred over time. This approach is especially powerful when we can reconstruct the biogeographical history of a ring species, as has been done in two cases. Source
As far as I can tell there is no pursuit of knowledge with intelligent design. ID means "stop looking". Why should science concede an intelligent designer even if there is one?