The basis for the latitudinal-species diversity gradient is still being debated. One hypothesis , the diversity pump, simply states that the tropics have a higher rate of speciation than other latitudes, and that this is the reason for its increased diversity. Another hypothesis has the tropics acting as a diversity accumulator; the rate of speciation would be equivalent to that at other latitudes, but the rate of extinction would be much lower. Many scientists have proposed some combination of the two to be responsible.
Recent studies have shown that the 'diversity pump' hypothesis is most likely the correct one. Analysis of post-Paleozoic marine orders shows significantly more first appearances of species in tropical waters. The result of this study also found that body plans which have greater potential to accumulate further derived characteristics originate more frequently in the tropics.
(Nature, vol 365)