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Evolution: Geology and climate drive diversification - eScholarship

Published on Jan 1, 2014
· DOI :10.1038/509297a
Rg Gillespie1
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Abstract
NEWS & VIEWS RESEARCH to replicate that effect perhaps underscores previously suggested 2,3 flaws in the blinding of earlier studies. So what might be going on in these birds? Several other external stimuli that stop ani- mals from responding to geomagnetic cues have been identified. Early studies of animal navigation noted that cues from the Sun or stars would take precedence over magnetic cues, leading to the idea that magnetism is the compass of last resort. It was then noticed that robins would ignore the magnetic field when the background intensity was shifted 20–30% outside the normal value 1 , and that pigeons raced poorly during geomagnetic storms. From an evolutionary perspective, ignoring geomagnetic cues at such times makes sense, because anomalies in the background field are often associated with iron deposits or lightning strikes. Some animals also stop using their magnetic compass in the presence of red-only light, but such light is present only at sunrise and sunset, when the Sun compass is most reliable 3 . Hence, radio-frequency noise might be just another cue that tells migrating animals to ignore their magnetic sense, but the puzzle is why this might have evolved. Surprisingly, there is a natural source of the radio-frequency electromagnetic noise identified as disruptive by Engels and colleagues — that produced by solar storms. Coronal mass ejection (CME) events from the Sun slam plasma into Earth’s magnetosphere every now and then, caus- ing it to ‘sing’ at frequencies from as low as around 20 kHz up to the MHz range 7 , some of which even leaks through Earth’s normally radio-opaque ionosphere; the lower end of this range is remarkably close to that identified by the authors. These CME events generate the beautiful polar auroras, disrupt our use of the medium-wave radio band, and sometimes perturb the background geomagnetic field at Earth’s surface enough to disturb animal navigation. All known sensory systems in animals are based on cells specialized to convert the stim- ulus of interest into a coded stream of action potentials that are sent to the brain 8 . If the effects of radio-frequency radiation are real, such cells must exist, but the mystery is in the biophysics. The lack of an enhanced effect at the Larmor frequency, and the low levels of radiation concerned, make it unlikely that a previously proposed mechanism 6 for radio- sensing, based on light activation of a cellular protein called cryptochrome, is involved. But some magnetic effects on animals (such as that of a short, sharp magnetic pulse 1 ) function through biological magnetite (Fe 3 O 4 ) in tissue — might this also be the radio-wave detector? If it is, how could such a detection mecha- nism have arisen? Early animals that had a simple compass patterned along the lines of magneto­tactic bacteria would have needed to survive geomagnetic excursions or reversals — periods in which Earth’s magnetic field weakened — and natural selection would have favoured individuals with higher cellular vol- umes of magnetite 3,9 . When the field recovered, animals would have been left with cells that have surprisingly large magnetic moments 9 (Fig. 1). Such cells might then have evolved to serve other functions, such as intensity-based magnetic navigation systems, increasing the amount of magnetite further. With large enough volumes of metallically conductive magnetite in these cells, direct detection of the small electric and magnetic vectors of radio- frequency radiation might have emerged, as Engels and colleagues suggest. Do the authors’ findings have implications for humans? It seems that geomagnetic sen- sitivity dates back to an early ancestor of ani- mals, and it is clearly present in many extant mammalian species. Human tissues also contain biological magnetite 10 . Many people claim to be bothered by radio transmissions, and some have even moved to live in radio- frequency ‘quiet zones’ around radio tele- scopes. Modern-day charlatans will undoubt- edly seize on this study as an argument for banning the use of mobile phones, despite the different frequency bands involved. However, if the effect reported by the authors stands the acid test of reproducibility, we might con- sider gradually abandoning our use of this portion of the electromagnetic spectrum and implementing engineering approaches to minimize incidental low-frequency noise, to help migratory birds find their way. ■ Joseph L. Kirschvink is in the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA, and at the Earth- Life Science Institute, Tokyo Institute of Technology, Japan. e-mail: kirschvink@gmail.com 1. Wiltschko, W. & Wiltschko, R. J. Comp. Physiol. A 2. Kirschvink, J. L. Bioelectromagnetics 13, 401–411 3. Kirschvink, J. L., Winklhofer, M. & Walker, M. M. J. R. Soc. Interface 7, S179–S191 (2010). 4. Kobayashi, A. K., Kirschvink, J. L. & Nesson, M. H. Nature 374, 123 (1995). 5. Engels, S. et al. Nature 509, 353–356 (2014). 6. Ritz, T. et al. Biophys. J. 96, 3451–3457 (2009). 7. LaBelle, J. & Treumann, R. A. Space Sci. Rev. 101, 8. Block, S. M. in Sensory Transduction (eds Corey, D. P. & Roper, S. D.) Ch. 1, 1–17 (Rockefeller Univ. Press, 9. Eder, S. H. K. et al. Proc. Natl Acad. Sci. USA 109, 10. Kirschvink, J. L., Kobayashi-Kirschvink, A. & Woodford, B. J. Proc. Natl Acad. Sci. USA 89, 11. Schumann, D. et al. Proc. Natl Acad. Sci. USA 105, This article was published online on 7 May 2014. EVO LU TI O N Geology and climate drive diversification Data from the Galapagos Islands exemplify how geology and climate can interact to cause episodes of isolation and fusion of the biota across a landscape. Different scales of such cycles dictate varying mechanisms of species generation. ROSEMARY G. GILLESPIE & GEORGE K. RODERICK W riting in the Journal of Biogeography, Ali and Aitchison 1 examine geolo­ gical and climatic events over the past 700,000 years, namely island ontogeny and shifting sea levels, and their effects on biodiver- sity in the Galapagos Islands. The authors pro- pose a process that can be considered a general evolutionary mechanism: that the dynamics of isolation caused by geological and clima- tological processes plays a fundamental part in shaping diversity. Whether these processes promote or constrain species diversifica- tion, however, depends on the spatial (global, regional or local) and temporal (multimillion, multi­millennial or multi­decadal) scales and periodi­city of isolation and coalescence. Geological events have long been known to mould and shape biodiversity. A breakthrough in understanding the underlying mechanisms came with the recognition that ancient split- ting of landmasses resulted in shared diversity. The concept of vicariance biogeography — the separation of a group of organisms by a geo- graphical barrier — provided the means for rigorous hypothesis testing in a hitherto largely descriptive field. This established that vicari- ance resulting directly from geological events can cause diversification, such that geological history will be clearly reflected in the result- ing biotic assemblages. The isolation created by ancient geological events is fundamental. Yet, what is given is frequently taken away — separate land masses can become connected and biotic assemblages reunited to various degrees. For example, the Great American 1 5 M AY 2 0 1 4 | VO L 5 0 9 | NAT U R E | 2 9 7 © 2014 Macmillan Publishers Limited. All rights reserved
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