No new species has ever been seen to arise
For a long time this was a valid argument; even though we had frequently observed what the fundies refer to as micro-evolution (small adaptations within a species), we had not fully witnessed macro-evolution (development of a new species by evolution). Even though all people of rational mind can understand and (usually) believe that this “macro-evolution” does occur we understand that due to the timeframe these changes require the likelihood of actually witnessing macro-evolution are very small.
However in 2008 a study by Blount, Borland and Lenski showed proof of this “macro-evolution," and not only has it been witnessed, and all data samples maintained to prove that this did happen, the process is repeatable. This is an amazing discovery and definitive proof of evolution.
The link to paper is below, I would advise having a read through, it is quite an impressive article!