A phyloGENETIC tree is not the same thing as a taxonomic tree, which is what Cal described. rRNA and molecular studies are used in phylogenetics to determine relations between species.
1 As the light intensity increases, the rate of photosynthesis increases. 2  However, the rate will not increase beyond a certain level of light intensity. 3  At high light intensities the rate becomes constant, even with further increases in light intensity, there are no increases in the rate.
Photosynthesis increases linearly with light intensity at low intensity, but at higher intensity this is no longer the case (see Photosynthesis-irradiance curve). Above about 10,000 lux or ~100 watts/square meter the rate no longer increases. Thus, most plants can only utilize ~10% of full mid-day sunlight intensity.