Sometimes the news on the BBC World Service brings up strange associations (and I don't mean the Conservative Party). Yesterday morning I heard a report about the latest findings on the evolution of elephants. The first thing that struck me was how good the report was: concise, informative, and went for accuracy rather than sensationalism. The other thing that struck me was that elephants don't have balls.
The BBC report was about a paper in PLOS biology, which reports firstly the first complete mitochondrial sequence of the extinct American mastodon. This sort of thing is now de rigour: extracting DNA from a tooth that is at least 50 000 years old, sequencing all the little bits, and then putting them back together to give an almost complete sequence. They then compare this sequence to sequences of other proboscideans, i.e. African and Asian elephants and a mammoth. From this they could work out the phylogenetic relationship between the species, and the dates when the different species diverged:
The shape of the tree is roughly what was expected. What was interesting what the timings of the divergences. A couple of technical points need to be raised to see why this is interesting. First, any phylogenetic tree gives the distance between different species, but it does not show where the tree started. This is because the processes of sequence evolution are indistinguishable whether we run them forwards or backwards in time. Hence, we can't find the start of the tree - the root. The usual solution to this is to include a species which is known to have been the first to diverge from the rest. This is called an outgroup: often is is selected on the basis of the fossil record.
Previous studies of elephants and their ilk have used the dugong or hyrax as an outgroup. This is a problem because these diverged from the proto-elephants at least 60 million years ago (Mya), but the elephants started diverging from each other less than 30 Mya. Hence, there is a lot of time in which the sequences could have diverged. This affects the dating of the divergences because (a) the molecular clock has had plenty of time to start running faster or slower, and (b) the differences between the sequences can start to saturate. The second point is important because the methods for estimating the times of divergence assume that the number of differences between sequences is proportional to time. When saturation occurs, this is no longer true. Hence, it would be better to use an outgroup that diverged more recently. The mastadon sequence provides this, as the fossil record can be used to date their divergence to about 25 Mya.
With this new outgroup, the divergences are pushed back in time, suggesting the mammoths and elephants diverges about 6 to 9 Mya. As the authors of the paper note, this is about the same time that humans, chimpanzees and gorillas diverged from each other. Now, this might be happenstance, but it could also be related to the changes in the environment at the time, as grassland spread throughout the world.
But, you are wondering, what about the balls? Well, the authors also found that the mutation rate is about half as fast in the elephants as in primates. The authors do not give a good explanation for this (they admit it themselves!). I don't have a good explanation either, but it did remind me of another paper, which was thrust into my hands whilst I was stood on the stairwell of the University of Helsinki museum. This paper pointed out that, according to the fossil record, elephants evolve really quickly, and hence could adapt to varying environments. They then pointed out that elephants don't have scrota: the testicles are kept near the kidneys, rather than migrating to somewhere cooler (this is called testicondy). It is well known that the mutation rate increases with temperature, and this has been used to explain why such a delicate piece of the anatomy hangs around outside the body. The suggestion, therefore, was that having the testicles inside the body increased the mutation rate, and hence induced more variation, so that there was more opportunity for selection to work: in other words, evolution could go faster.
How does this line up with the finding of a low mutation rate in the mitochondria? Well, one reason why mitochondria are studied is that they are only inherited from the mother. So, their evolution is obviously not affected by testicondy. Is it possible that the lower mutation rate in mitochondria offsets the higher rate in male nuclear DNA? It seems curious to me that there would be such a difference. But perhaps the elevated mutation rate in males is enough, so an increased mutation rate in females would increase the mutation rate above the optimum.
Werdelin L., Nilsonne Å., Fortelius M. (1999) Testicondy and ecological opportunism predict the rapid evolution of elephants. Evolutionary Theory 12: 39-45.
Rohland N., Malaspinas A.S., Pollack J.L., Slatkin M., Matheus P., Hofreiter M. (2007) Proboscidean Mitogenomics: Chronology and Mode of Elephant Evolution Using Mastodon as Outgroup. PLoS Biol 5(8): e207 doi:10.1371/journal.pbio.0050207