While climatic events can drive dramatic selection events in natural populations, it is (sort of) difficult to think of tornadoes as triggering specific adaptations. Maybe that’s because we are particularly impressed by the power of these (gosh!) _really big_ storms. Maybe there’s another bias in that we mostly think of life as mainly animal based, and it’s even harder to think of a potential adaptations to hurricanes when dealing with squirrels or rodents of some sort (actually, if you are biased toward inverts, hexapodic adaptation to tornadoes might be easier to get through… but we’re dealing with plants today, okay? ;).
A recent study nevertheless showed that such things happen*: in palms, tolerance and survival to tornadoes is significantly higher for specimens from cyclone-prone provenances than for palms coming from places with more gentle winds. Storm effects are actually not difficult to imagine in trees: weaker branches break and thus the number of flowers and then fruits, that is, the fitness of trees, will be directly correlated to the effects of the storm. No special effort is needed to understand that more resistant trees will directly benefit from their resistance advantage. And resistance is only a matter of lignin chemistry in branches, with strong genetic effect.
So coming back to palms, storms, and even hurricanes since the study is dealing with such events, are undoubtly an important factor of evolution in regions where they occur regularly. This is even more obvious that damage may be more drastic, since palms are plants with a monopodial growth (unbranched monopodial arborescence); that is, there’s only a single place where development is actively organized, at the top of the trunk and crown of leaves. Any unfortunate accident is thus letal to the plant, this is a quite fragile condition, but this is also a reason to expect adaptations to the whirlwind bad tricks. Amazingly, crown structure, leaf shape or overall morphological organization seem to play little role in wind resistance, because fairly different species can achieve comparable levels resistance and tolerance, providing a good case for more subtle chemically basis for these adaptations.
But there’s really another very interesting thing with this study. It’s the fact that resistance really correlate with biogeography (or more accurately, bioclimacy), and very little with phylogeny (the line of descent within palm species). This means that resistance and survival (to tornadoes) is not a matter of ancient ancestry, this is really something that has to do with relatively modern selective events. You don’t have much resistance when you happen to be living in a place where hurricanes are rare, even if your long term ancestors would have been. This kind of illustrating a rather frequent situation in biology, involving tradeoffs, and this is really suggesting that resistance is also maybe associated with a cost and counterselected where it does not provide any further benefit. It indeed tells us these very adaptations are actually maintained by selection and not driven like ghosts of some ancient evolutionary past. That is, not only is natural selection against tornados effective, but it’s also part of the natural history of palms. You know, natural selection, a motor of evolution under the very shameful adaptationistic view of evolution… (yes, I’m a crypto-adaptationist, and proud of being one!).
* M. Patrick Griffith, Larry R. Noblick, John L. Dowe, Chad E. Husby, and Michael A. Calonje (2008). Cyclone Tolerance in New World Arecaceae: Biogeographic Variation and Abiotic Natural Selection. Annals of Botany 102: 591-598.