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The Power of Natural Selection Sufficient Statistical Power to Detect Typical Andrew P 17.2 n&v 691 MH 11/2/05 5:42 pm Page 694 news and views Winterthurerstrasse 190, CH-8057 Zurich, 4. Karbowski, M. & Youle, R. J. Cell Death Differ. 10, 870–880 (2003). Burdened by the practical needs of gradua- Switzerland. 5. Frank, S. et al. Dev. Cell 1, 515–525 (2001). ting, I soon bowed out of the project and did e-mail: [email protected] 6. Kinchen, J. M. & Hengartner, M. O. Curr. Top. Dev. Biol. 65, not see the results until 2001. Surprisingly, it 1–45 (2004). seemed that Endler’s conclusions had swung 1. Jagasia, R., Grote, P.,Westermann, B. & Conradt, B. Nature 433, 7. Parrish, J. et al. Nature 412, 90–94 (2001). 754–760 (2005). 8. Horvitz, H. R. Cancer Res. 59, 1701s–1706s (1999). the pendulum too far back when Kingsolver 2. Danial, N. N. & Korsmeyer, S. J. Cell 116, 205–219 (2004). 9. Labrousse, A. M., Zappaterra, M. D., Rube, D. A. et al. emphasized that “directional selection 3. Hengartner, M. O. Nature 407, 770–776 (2000). & van der Bliek, A. M. Mol. Cell 4, 815–826 (1999). on most traits and in most systems is quite weak”7.This conclusion was largely based on the observation that most estimates of Evolutionary biology selection were non-significant and centred around zero. A particularly worrisome finding was that most studies did not have The power of natural selection sufficient statistical power to detect typical Andrew P. Hendry strengths of selection7,8.Perhaps the pendu- lum should swing all the way back to Darwin: Adaptation by natural selection is the centrepiece of biology. Yet natural selection really is weak in nature, evolutionary biologists may be deluding themselves if they think they except in exceptional situations. have a good handle on the typical strength of selection in nature. Enter Hereford et al.1,who argue that pre- vious reviews did not have objective criteria he one constant in our world is change of selection (Box 1).Endler’s review heralded by which to judge whether selection was — change often wrought by our own a shift in our perceptions when he empha- weak or strong. They suggest that this prob- Tdevices. In consequence, some of the sized that “strong selection is not rare and lem can be resolved if selection estimates for populations and species with which we may even be common”4, basing this conclu- individual traits are standardized to allow cohabit have difficulty persisting.Yet organ- sion largely on the observation that some comparison with the expected strength of isms should be able to adapt to changing studies documented quite strong selection. selection on fitness itself (Box 1). Selection environments, as they have done for billions Another way to infer the power of selec- on fitness,they argue,provides a clear bench- of years,diversifying into a bewildering array tion is to actually measure evolutionary mark for strong selection.In reviewing many of environments. But extinctions are also a changes in natural populations5,6.Studies of the same studies as Kingsolver et al.,Here- prominent feature of the past.Were these lost taking this approach often document sub- ford et al.conclude that selection estimates organisms unable to adapt to change? If so, stantial changes over short time intervals, are,on average,54% as strong as selection on are the rapid changes now being driven by suggesting that natural selection does indeed fitness (31% after correction for a statistical humans too much for adaptation to combat? have the power to drive rapid adaptation. bias). In their view, these values represent At the heart of these questions is the power of Darwin was too modest,it seemed,about the “extremely strong selection overall” and natural selection to bring about evolution- power of his idea. “such large estimates clearly cannot be repre- ary adaptation in natural populations. Fast-forward to 1998, when I joined a sentative of selection on all traits”1.They Writing in Evolution,Joe Hereford and discussion group led by Joel Kingsolver at then consider reasons for why current esti- colleagues1 bring this matter into stark relief. the University of Washington. This group mates of selection might be biased. The primary mechanism of adaptive set about analysing all studies of natural These results1 raise some perplexing evolution is natural selection, whereby org- selection published since Endler’s book. questions. Principal among them is the anisms possessing traits that improve their evolutionary ‘fitness’ — their survival and reproduction — contribute more genes to Box 1Measuring selection in natural populations subsequent generations. Yet perceptions of the power of selection have recently swung at On the small island of standardized strength of the end of a pendulum. Charles Darwin felt Daphne Major in the selection on beak depth that “natural selection will always act very Galapagos Islands, Peter and during the drought was A. P. HENDRY slowly, often only at long intervals of time, Rosemary Grant and Sǃ0.63 and ȋǃ0.53. That is, and generally on only a very few of the inhab- colleagues9 measured the selection favoured large itants of the same region at the same time”2. beak size of all medium beaks because such beaks If Darwin was right,natural selection should ground finches (Geospiza could crack the harder seeds be almost imperceptible, and adaptation fortis, pictured) before a that remained. must require “the long lapse of ages”2.This drought. The abundance of Endler4 and Kingsolver perception held sway for more than a cen- seeds (particularly soft et al.7 compiled standardized tury before it was challenged by a series of seeds) decreased during the can be obtained by dividing S or ȋ values for many empirical studies — most famously those drought and finch mortality S by the variance for the studies and traits. Hereford showing dramatic changes in the coloration was high. When the drought trait (ȋ can also be obtained and colleagues1 took a of peppered moths during industriali- ended but before from a regression of the trait similar approach, except that zation3.These studies inspired a wave of reproduction started, the on a measure of fitness, ȋ values were standardized interest in actually measuring selection and Grants determined the beak in this case survival). by the mean for the trait, adaptation in natural populations. size of all surviving finches. Selection estimates can be rather than its standard By the mid-1980s, enough studies had The difference in mean standardized by dividing S or deviation. Hereford et al. accumulated for John Endler to profitably beak size from before to after multiplying ȋ by the standard argue that the benefit of review them in his classic book Natural Selec- the drought is one measure deviation of the trait. ȋ is standardizing selection by tion in the Wild 4.Reviews of this sort typi- of the strength of selection additionally useful because it the mean is that the cally collate and combine selection estimates (S). A related measure is the can account for correlations corresponding value for for a variety of traits and studies so as to selection gradient, ȋ, which among traits. The fitness should be 1. A.P.H. address general questions about the strength 694 NATURE | VOL 433 | 17 FEBRUARY 2005 | www.nature.com/nature © 2005 Nature Publishing Group 17.2 n&v 691 MH 11/2/05 5:42 pm Page 695 news and views apparent paradox that typical studies of investigate how humans are changing selec- selection do not have the statistical power tion pressures,and whether populations and necessary7,8 to detect selection that appears species will be able to adapt accordingly. ■ unrealistically strong1.Unfortunately, this Andrew P. Hendry is in the Redpath Museum and paradox will not be resolved simply by accu- Department of Biology, McGill University, mulating more data of the same ilk, as all Montreal, Quebec H3A 2K6, Canada. reviews identify problems with our current e-mail: [email protected] 1,4,7,8 methods .How, then, are we to obtain a 1. Hereford, J., Hansen, T. F. & Houle, D. Evolution 58, 2133–2143 100 YEARS AGO good handle on the true power of selection (2004). What mutation is in biology, conversion 2. Darwin, C. On the Origin of Species (John Murray, London, 1859). in nature? 3. Kettlewell, H.B.D.The Evolution of Melanism: The Study of a is in psychology, and revolution in sociology. Evolutionary biologists will have to Recurring Necessity (Oxford Univ. Press, 1973). It may be said that to assume such parallels resolve this uncertainty by determining how 4. Endler, J. A. Natural Selection in the Wild (Princeton Univ. Press, is merely to beg the question, but I think that best to measure and judge the strength of 1986). the apparent parallelism cannot be without 5. Hendry, A. P. & Kinnison, M. T. Evolution 53, 1637–1653 selection, and by conducting more robust (1999). significance… If the supposed analogy studies of selection. Meanwhile, we are 6. Stockwell, C. A., Hendry, A. P. & Kinnison, M. T. Trends Ecol. is a valid one, it appears to follow that only deluding ourselves that we have a Evol. 18, 94–101 (2003). mutability is due to the same general 7. Kingsolver, J. G. et al. Am. Nat. 157, 245–261 (2001). good handle on the typical power of selec- 8. Hersch, E. I. & Phillips, P. C. Evolution 58, 479–485 (2004). causes as ordinary variability (just as tion in nature. Once we do, we can begin to 9. Grant, P. R. & Grant, B. R. Evolution 49, 241–251 (1995). change of opinion and reform are due to the same general causes as conversion and revolution), but that there is this difference Planetary science — mutability represents an explosion of energy, as it were, in a given direction, and therefore differs from ordinary variation Saturn’s mixed magnetosphere somewhat as the firing of a gun differs Fran Bagenal from the explosion of a loose heap of powder… [T]he chance of mutations When interplanetary shock waves hit the Cassini spacecraft and then succeeding from the first is comparatively Saturn in January 2004, it presented a unique opportunity to study the remote, though such a thing is quite planet’s magnetosphere and to compare it with that of Earth.
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