The two hypotheses are: 1) Regions of the earth that are now tropical have had a climate that could support more diverse life for longer in evolutionary history. 2) The resources available in tropical regions are greater than anywhere else on earth.
Work Step by Step
1) This hypothesis is somewhat self-explanatory. For much of the earth's living history, the overall climate has been significantly colder. The areas near the equator, where tropical regions exist, are warmer and would therefore have created opportunities for adaptation and speciation much earlier than anywhere else. 2) The area around the equator has more direct, strong sunlight and more moisture than anywhere else on earth. These two resources are foundational in supporting plant life and diversity, which could then scale up to animals that feed on plants and animals that feed on those animals.