Read the pdf and answer the questions

Falling CO2 levels are clearly implicated in the evolution of C4 photosynthesis, but it’s thought that this was helped along by stress factors such as low precipitation and high temperatures (C4 is recognized broadly as an adaptation to both hot and dry conditions). What stress factor has long been thought to be the main evolutionary driver of C4 in eudicots? How is this different from the assumed main driver of C4 evolution in grasses? Why is the evolution of C4 in grasses thought to be different from its evolution in eudicots in this way?

Why does it matter that all of the studies indicating that C4 and C3 grasses differ in temperature regime sampled their C4 species from one group within grasses (the PACMAD clade) and C3 grasses from another group (the Pooideae clade)? In other words, why is this potentially a problem? 

Why is it important to identify the nodes (i.e., branching points) on the tree that represent evolutionary transitions between C3 and C4 photosynthesis?

Two C3 grass lineages (groups), Pooideae and Danthonioideae, seem to inhabit colder environments than other grasses, but the other C3 lineages are similar to C4 lineages (Figure 1). What does this imply for the hypothesis that evolution of C4 photosynthesis in grasses was driven by hotter temperatures?

What does figure 2 show? What is its main message?

What does figure 3 show? What is its main message? 

What is interesting and surprising about this paper? Summarize the authors’ main point and the “received wisdom” about C4 evolution in grasses that they are challenging.

Why do the authors conclude that temperature may still be an important driving force for the evolution of C4 at the leaf level, and why doesn’t this show up in their analysis?