While others might have had similar ideas to Darwin (such as Alfred Russell Wallace) without the decades long research of Darwin the theory might not have been as accepted as it was. The theory of evolution was a hugely important landmark in scientific development.
Lincoln was only the (very eloquent) spokesperson for the already large and zealous abolitionist movement, and not even their first choice for that, being a compromise candidate for the 1860 election. The movement to abolish slavery would have gone on without Lincoln, but evolution would have been decades away from scientific prominence without Darwin.
Lincoln wasn't a spokesperson for the abolitionist movement. He didn't even run as an abolitionist. But that makes him more, rather than less relevant. If the Republicans had nominated Seward or some other fire-breather, there's a good chance that they wouldn't have won the EV and Douglass would probably have been chosen by the House.