This is a follow-up to questions about a paper discussed in a seminar which I gave two weeks ago at the School of Environmental Sciences and Natural Resources Management.
I have given a causal explanation of the much greater species richness in tropical than in cold-temperate environments in my post “Faster Evolution in the Tropics ? Effective Evolutionary Time”. A recent paper by Wright, S., Keeling, J. and Gillman, L. in PNAS (Proceedings of the National Academy of Science) 103, 7718, 2006) specifically tests this hypothesis using molecular evidence from tropical and cold-temperate tree species and finds convincing evidence. The authors used the ITS (internal transcribed spacer) –region of ribosomal RNA-encoding DNA of 45 pairs of phylogenetically independent congeneric or conspecific rainforest trees. One representative of each pair was from the lowland or low mountainous region in the tropics, the other from the highest possible latitude and altitude. The pairs did not overlap in their geographical distribution, and trees belonged to genera that were as species-rich or richer at high latitudes than in the tropics (which excludes the possibility that higher molecular substitution rates in the tropics were a consequence of faster speciation). In order to reduce the effect of genetic drift, population sizes of all species examined were large. – The speed of molecular evolution was found to be twice as great in the tropics than at high latitudes.
A brief note on this paper appeared in Science NOW Daily News, 1 May 2006, based on interviews with Jim Brown of the University of New Mexico, and me. Here is the interview with me, which clarifies some points and implications of the paper.
1) Have other scientists put forward empirical evidence that species evolve faster in the tropics before this study, or is this the first? And if there have been other kinds of studies, how is this one different or more enlightening?
There are several studies which provide strong evidence in support of various partial aspects of the hypothesis of faster evolutionary rates in the tropics, such as accelerated mutation rates, shorter generation times, and availability of many vacant niches which can absorb newly evolved species. There also are some studies which suggest faster rates of molecular evolution. However, the latter have never solved the problem of whether faster molecular evolution is the result of underlying metabolic rates (mutation rates especially) as suggested by the hypothesis, or whether faster molecular evolution is the result of higher speciation rates (in other words, which way round should the story be read). Allen et al. 2002 provide very strong evidence that metabolic rates are correlated with generation times and mutation rates, and they conclude that these finding support Rohde’s hypothesis, but only by inference. The present study, to my knowledge, is the first that provides direct and convincing empirical evidence that the hypothesis in toto is indeed valid.
2) What factor do you believe this study suggests might be responsible for the difference in molecular evolution rate between temperate and tropical regions?
As just said, the factor responsible was shown, in this study, to be increased mutation rates at higher temperatures, (as well as shorter generation times).
3) Are there weaknesses in the method that this group used?
I cannot see any. The authors have been very cautious: they state that further studies are necessary to definitively rule out the alternative explanation that genetic drift in small populations may be responsible. But this alternative really is only a remote possibility, considering that temporary reductions in population size in the evolutionary past are not more likely for tropical than cold-latitude species (as pointed out by the authors themselves).
4) What are the implications of species evolving faster in the tropics, either on how biodiversity is distributed around the world
The hypothesis predicts that evolution is faster not only in the tropics but in habitats which have higher temperatures and energy inputs generally, for example in deepsea sites which are fed by volcanic hot water upwellings. Scientists should therefore put special emphasis on studying such spots, if they want to get a true understanding of marine diversity. – Also, global warming will affect different areas of the globe differently. A re-distribution of diversity patterns must therefore be expected, although changes due to different evolutionary rates will take some time.
– or on how scientists conduct studies of evolution?
Looking at the vast number of evolutionary and ecological studies, some using very sophisticated statistical analyses, one is struck by the fact that potential DIRECT temperature effects on diversity until recently have been largely neglected. Such effects may be far greater than any effects due, for example, to area, heterogeneity of the habitat etc. etc. Many of the older studies are therefore quite useless. I don’t want to mention specific authors or books, but even some of the most widely cited monographs on general ecological and evolutionary patterns lead the reader astray because of the neglect of such direct temperature effects, concentrating instead on factors that are at best of secondary significance, such as area.- (It may be of interest that one of the truly great contemporary ecologists, when I presented my ideas at a symposium in the U.S., at first thought me “mad” (he told me and others), but he is now fully “converted”).
5) Does this speedier pace have any impact on species conservation in tropical locales?
It is obvious, of course, that devastation of tropical habitats has far greater implications for reducing global plant and animal diversity than devastation of cold-latitude habitats, because of the vastly greater species diversity in the tropics. Important here is to realize that species numbers in such tropical (and other) habitats are vastly underestimated, because parasites, the huge number of small invertebrates and microorganisms, most of which have not even been described yet, are usually ignored. Such species are not only of aesthetic and theoretical interest, but may play very significant roles in ecosystems. The role they play is even less well known than the species themselves. – To understand the cause of increased diversity in the tropics is of great significance: it may lead to more realistic estimates of diversity of these smaller organisms, because evolutionary rates will vary depending on the metabolism of these organisms. However, metabolic rates of these organisms are largely unknown. The present study may induce others to follow this up and look at mutation rates and their temperature dependence in various groups of organisms lower down the “hierarchy”.
Altogether, I believe that this paper is of extreme importance and will stimulate a lot of further research.
Here is the reference by Allen et al.:
Allen, A.P., Brown, J.H. and Gillooly, J.F. (2002). Global biodiversity, biochemical kinetics, and the energetic-equivalence rule. Science, 297, 1545-1548.