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]
On what I think are standard stochastical models of speciation/extinction, it actually doesn't matter much if you are talking species or subspecies or whatnot, since subspecies are thought to be incipient species.
ReplyDeleteWhat would cause a problem is a geographic bias, e.g. if North American/European taxonomy is more developed and detailed than tropical taxonomy. This seems plausible although presumably it too could be tested, e.g. by comparing well studied groups (e.g. vertebrates) with less well-studied groups.
Actually, it does matter how you identify species. If there was a bias in favor of recognizing temperate subspecies as species while applying stricter criteria to tropical subspecies, then you'd get the result they report.
ReplyDeleteIt would look like speciation happened faster in temperate zones because the so-called "sister" species are more closely related.
I don't think there is a bias in this direction but I'm troubled by the fact that naming species is such a subjective thing. It's a problem in a paper like this and it makes the result much less reliable.
When it comes to invertebrates, especially insects, I think the bias is in the opposite direction. There seems to be a tendency to identify every minor variant of beetle (for example) as a species. That's why you get these inflated statistics about the number of species in the tropical rain forest. If you did that with humans then we would be subdivided into several dozen species. You and I would probably be different species. :-)
The mechanism for this phenomenon is climate change; temperature variations in both global warming and global cooling are greater at the poles than at the equator.
ReplyDeleteChepe Noyon says,
ReplyDeleteThe mechanism for this phenomenon is climate change; temperature variations in both global warming and global cooling are greater at the poles than at the equator.
It is widely believed that most speciation is connected in some way to climate change. The fact that all available evidence refutes this idea doesn't seem to make any difference to the true believers.
Why is that?
And what available evidence is that?
ReplyDeleteYeah, I've never heard that speciation is particularly tied to climate change.
ReplyDeleteA higher extinction rate, however, is quite plausibly tied to e.g. the continental glaciation events that regularly happen in northern latitudes.
Also, if you believe in Unified Neutral Theory (see The Unified Neutral Theory of Biogeography and Biodiversity, by Stephen Hubbell), then the number of species in region is a function of the number of individual organisms in the region. More individual organisms = lower chance of extinction and larger number of successful speciation events (where speciation and extinction are modeled as stochastic processes that are random at a coarse scale of analysis).