Of mountain passes and migration in the olden days

The paper that I read today is Why Mountain Passes are Higher in the Tropics by Daniel H. Janzen. I found this paper by browsing the paper “100 articles every ecologist should read”, which was published in Nature in 2017.

What were the obstructions to the migration of species of animals in the ancient world, and what made species in temperate regions more suited to migration than those in the tropical regions?

One might imagine that tall mountains would be an obstruction to migration. High mountains would prevent people living in the valley from travelling through the impossible mountain passes to other parts of the world. However, they were less of an obstruction to species’ migration in temperate regions of the world, as compared to tropical regions. Why was that?

One of the factors that could affect migration is this: if the climate in the valley and in the high mountain passes were comparable, then species, that were already thriving in the valley, could easily travel through the mountain passes and spread. Maybe the weather conditions of mountain passes and valleys were much more similar in temperate regions as compared to the tropics? That can explain the fact that species migrated much more in temperate regions than in tropical regions? The authors, after a statistical study, conclude the following:

The climatic differences between the mountain passes and the valley in tropical and temperate regions were comparable. Hence, this wouldn’t be a deciding factor in migration.

What was the deciding factor in migration, however, was that temperatures varied a lot more annually in the temperate regions than in the tropics. Anyone living on the East Coast can attest to that. Hence, by natural selection, the species in temperate regions were a lot more resistant to wildly varying temperatures than their tropical counterparts, who were used to similar temperatures throughout the year.

These toughened and highly resistant species would then cross mountain passes, and spread out over a much larger area than species in the tropics.

This also explains another interesting observation: there is a much larger variety of species found in the tropics than in temperate regions. This is because of the following reason: when a few unusually resistant individuals of a tropical species were able to cross mountain passes and travel to other regions, they would not be followed by their less resistant counterparts. Hence, they could go to new regions and populate them with their evolutionarily advantaged genes, without any mixing and interference from their unfit counterparts still stuck in their old habitat.

Hence, mountain passes are not always higher in the tropics. The ability to cross these mountain passes were found to be lacking, however.

<Insert moral about climbing mountains or something similar>


  1. https://www.wsl.ch/lud/biodiversity_events/papers/niklaus.zimmermann@wsl.ch_2017_12_12AmNat_Janzen_1967.pdf
  2. https://www.nature.com/articles/s41559-017-0370-9

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Graduate student

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