First of all you aren't using the word species correctly. Different types of barnacles and birds that were observed in the nineteenth century were different species as were the different species observed on tropical islands that were related but clearly distinct from those on the mainland or from the fauna on other islands. In all of these cases only a hypothesis that they were related to a common ancestor could explain (1)how geographical variation seemed to imply corresponding changes in the fauna (2) how different species had 'adaptations' that provided advantages in their environment (3) why geographical variation seemed to be compounded the farther you got away from the mainland.

There is no other explanation besides natural selection that could explain these basic facts. Besides Darwin other scientists were independently converging on the exact same solution.

In the same way only natural selection could explain the existence of vestigial features among related families of species such as the existence of hind limbs in whales or the curious why that the eye of the vertebrates is constructed. This was recognized immediately in the 1860s once Darwin's work was published.

DNA evidence supports evolution 100%. Hypothesized descent from two families can be easily verified from DNA evidence. The whale really once had an ancestor that walked on the land with four limbs. There is simply no other explanation could produce DNA in the exact way that would be predicted from natural selection.

This is just the tip of the iceberg.