I think the answer would be needs. Hope this helps.
Answer:
It is true that studying natural selection in the fields can be challenging. Some of the challenges which Darwin might have faced could have been:
Darwin was only studying one trait for his results. But there might have been many other factors or traits affecting natural selection.
Natural selection does not happen rapidly hence Darwin would have had to wait for many generations to observe the results.
The environment is not such an easy factor to study. It is complex. It might have happened that the environment might have changed before any changes were seen in the population of the birds.
Just like individual players in a sports team, every photosynthetic pigments in a plant has their role in absorbing wavelength to be used in making food.
Chlorophyll - absorbs yellow and blue wavelength and reflects green
Carotenoids - they are accessory pigments and function as light-harvesting pigment
Phycobilins - absorb yellow, green, orange and red light.
