Darwin studied actual birds on the galapagos islands instead of using a simulation, as you did in this lab. studying natural selection in the field can be challenging. list at least three possible challenges darwin might have faced.

In Darwin's time, natural selection was a strange theory among researchers and scientists. Before him various other scientists had postulated something regarding evolution, thus Darwin would have few people on his side, so it would have taken him some time to prove his theory.  

Another major issue Darwin would have faced would be the lack of technology, as in order to travel, Darwin had to use boats to go too far places, and thus, it would have taken ample of time.  

The final issue Darwin would have faced is that evolution is a natural phenomenon that takes millions of years to take place, as it does not happen overnight. Therefore, it would have taken him quite a long time to observe, conduct his experiments again and again, and then to finally come to a conclusion.  

i am going to guess that it is co2.

as in mitosis, dna replication occurs during interphase.   in meiosis, it occurs specifically in s phase of interphase 1.