An obvious question, which follows from the observation that this relationship is widespread amongst modern organisms, is to ask about its geological history. Clearly, the type of explanations we use to explain this pattern could be different if it has been common for tens or hundreds of millions of years compared to being a product of particular conditions on the modern Earth. It turns out that there is good evidence that, in at least some groups, this pattern has a long geological history.38 For example, foram fossils extracted from marine sediments suggest that the tropics have had more species than temperate regions for at least 10 million years; interestingly, they also suggest that the difference has become more pronounced over this period, with tropical diversity increasing much more than temperate diversity over time (although this conclusion is based on rather limited data).33
There is also evidence of a long-established (geological) latitudinal gradient in flowering plants (angiosperms). Ever since the origin of this group in the tropics approximately 145 million years ago,39 the angiosperms have remained a predominantly tropical group. Again it is interesting that this gradient appears to have become more pronounced during the past few million years.39,40 Clearly, any explanation for latitudinal gradients in species richness needs to be applicable to the geological past as well as the present, although it is possible that one or more of the relevant processes may have become more pronounced over the past few million years than it was over longer spans of geological time.
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