See figure 18.14 , which summarizes hypotheses for why species richness is higher in the tropics. Panel B has an error that has confused students in previous years. What is the error? The blue temperate line should be above the red tropical line. There is no problem with this panel If only time available for diversification differs, both temperate and tropical species diversity should be plotted on the same curve. Tropical should be plotted on the curve at younger times. If only time available for diversification differs, both temperate and tropical curves should have the same asymptote (reach the same level of species richness). FIGURE 18.14 Hypotheses Proposed to Explain the Latitudinal Gradient in Species Richness (A) The tropics have a higher diversification rate (speciation rate - extinction rate) than temperate areas do, so they have accumulated species faster. (B) The tropics have had more time for diversification than temperate areas have, so they have accumulated more species. (C) Because their productivity is higher, the tropics have a higher carrying capacity than temperate areas, so more species can coexist there. (After G. G. Mittelbach et al. 2007. Ecol Lett 10: 315-331.) Most lineages (species, genera, and broader clades) in North and South America have very different evolutionary histories since North America (Nearctic region) was once part of the supercontinent Laurasia. Laurasia was separated from Gondowana (the supercontinent that contained South America (Neotropic region) for more than 100 million years. Choose is the best explanation for this pattern. The long separation limited dispersal until 60 million years ago. Different lineages evolved on each continent. North and South America were separated until 6 million years ago, limiting dispersal. Different lineages evolved on each continent. North and South America have different abiotic conditions (different biomes), so different lineages have evolved on each continent. Most species in North and South America have very similar evolutionary histories, but North America has experienced a higher extinction rate due to habitat fragmentation and thus has fewer species than South America. Terrestrial (on land) biodiversity is higher in tropical (near the equator) than in temperate (midlatitude) regions. Which of the following best explains the reason for this pattern? (Check all statements that you think apply.) Tropical regions have had a longer evolutionary history than the northern and the southern hemispheres since the tropics worked as a refuge during several ice ages. A longer evolutionary history means that species have been able to diversify over a longer time. The annual average temperature in tropical regions is more variable than in the northern and the southern hemispheres. Higher temperature variability is likely to generate a wider variety of species. Tropical regions are more pristine, with less human development and thus less species extinction. Tropical regions.