The three hypotheses (tropics as cradle, museum, and destination) are not mutually exclusive. For example, it could be that the tropics have high speciation rates (cradle) AND low extinction rates (museum) — or that the tropics have low extinction rates (museum) AND high immigration rates (destination). In fact, all three of these factors could be behind the millions of the species that we find in the tropics. However, to figure out which of these processes are actually contributing to the LDG, we’ll have to look at the evidence for each separately.
Testing hypothesis #1: Where are species and genera born?
Do organisms living in the tropics have higher rates of diversification than organisms living other places on Earth? In other words, do the tropics have higher “species and genera birth rates” than other regions do? To get a handle on this, paleontologists study the fossil record to try to identify the oldest fossil of a particular species or genus and where that fossil was found. This is called a “first occurrence.” The first occurrence of the giant clam Tridacna, for example, is represented by a fossil found in Oman on the Arabian Peninsula. These rocks are about 22 million years old, and at that time — the time that the clam died and the fossil started to form — the rocks were located in a tropical region, as they are today. This means it’s very likely that the giant clam was generated by a speciation event that occurred in the tropics.
By collecting lots of data about first occurrences, paleontologists can get a picture of where on Earth speciation is most likely to happen.