Wanna Speciate? Come to Canada

A recent Science paper by Wei and Schluter (2007) asks whether speciation rates really are faster in the tropics as widely believed. They looked at 309 sister species of mammals and birds in North and South America. Sister species are closely related species that have apparently diverged within the past few million years. The time since divergence was estimated by comparing the sequences of the mitochondrial cytochrome b gene. This gives an estimate of the rate of speciation by cladogenesis for each pair of sister species.

The range of each species was estimated from literature data and the time of speciation results were plotted in relation to the midpoint latitude of the range. The result was quite striking.

Near the equator, the ages of sister-species pairs spanned the past 10 million years, with a mean age of 3.4 million years ago. As the distance from the equator increased, the upper limit and mean ages of sister species declined significantly. At the highest latitudes, all of the sister species diverged less than 1.0 Ma.

It's widely known that there are far more species in the tropics than in temperate or arctic climates. How do we explain this apparent discrepancy?

Weir and Schluter (2007) estimated extinction rates at various latitudes and discovered that the rate of species extinction also increased with distance from the equator but the rate of increase was greater than the rate of increase in speciation. Thus, although there were more speciation events in temperate zones, there were also more extinctions, and the extinctions cancelled out the effect of frequent cladogenesis.

The net effect is more species in the tropics even though speciation rates are higher in temperate zones.

John Wilkins is an expert on species. He points out that there's no universal definition of species. I wonder if this result isn't biased by different ways of recognizing species. Perhaps populations and sub-species are more easily named in temperate zones because there's more room for them to spread out into non-overlapping ranges. Does anyone know whether "species" in temperate zones are more likely to be similar in appearance than in the tropics?

In any case, the result is intriguing. It suggests that things move pretty slowly in hot climates. If you want some fast speciation action you need to move north to a cooler place.

Weir, J.T. and Schluter, S. (2007) The Latitudinal Gradient in Recent Speciation and Extinction Rates of Birds and Mammals. Science 315: 1574-1576.

[Hat Tip: RichardDawkins.net; Cold is hot in evolution -- Researchers debunk belief species evolve faster in tropics]