Pattern in the geographical orientation of diversification

In 'Guns, Germs and Steel' Jared Diamond argued that European technological advance was enhanced by the East-West orientation of Eurasia which allowed domesticated animals and plants to easily migrate, as ecological conditions are generally more similar to the east and west than to the north and south. I wondered if this increased motility would lead to more or less diversification? Increased motility may result in larger ranges and hence a greater probability of vicariance (splitting and diversification). Alternatively, mobility may prevent divergence through increased admixture. Perhaps the relative importance of these factors changes through time?

To investigate this question, I calculated the bearing between the geographical centroid of the ranges of terrestrial mammal sister clades. I classified the lines connecting range centroids as north-south or east-west and plotted them on a map and as a temporal histogram.

North-South diversification is most common in South America (except the Atlantic coast), Western North America, Sub-Saharan Africa, S.E. Asia and Western Australia - New Guinea, but much rarer in Eurasia and the majority of North America. East-West diversification occurs everywhere, except polar regions. Glaciations will have severely eroded pattern in northern latitudes, but why is north-south diversification uncommon in western South America and Eastern Australia?

Interestingly, over the last 50 million years the frequency of east-west diversification has been consistently 2.5-3 times more frequent than north-south diversification. Temporal slicing of the map (not shown, but see) shows that pattern does vary though time with east-west connections between southern continents being more common further back it time - traces of the breakup of Gondwana.

So, these data seem to show that, yes, diversification is more 2-3 time more common east-west than north south, and that this pattern has been consistent in mammals for the last 50 million years. It also shows there is considerable geographical variation in north-south diversification, possibly related to recent environmental change destroying pattern.