3. La Terra, S., English, C.N., Hergert, P.,McEwen, B.F., Sluder, G., andKhodjakov, A. (2005). The de novocentriole assembly pathway in HeLa cells:cell cycle progression and centrioleassembly/maturation. J. Cell Biol. 168,713–722.

4. Delattre, M., Canard, C., and Gonczy, P.(2006). Sequential protein recruitment inC. elegans centriole formation. Curr. Biol.16, 1844–1849.

5. Pelletier, L., O’Toole, E., Schwager, A.,Hyman, A.A., and Muller-Reichert, T.(2006). Centriole assembly inCaenorhabditis elegans. Nature 444,619–623.

6. Swallow, C.J., Ko, M.A., Siddiqui, N.U.,Hudson, J.W., and Dennis, J.W. (2005).Sak/Plk4 and mitotic fidelity. Oncogene24, 306–312.

7. Habedanck, R., Stierhof, Y.D.,Wilkinson, C.J., and Nigg, E.A. (2005). ThePolo kinase Plk4 functions in centrioleduplication. Nat. Cell Biol. 7, 1140–1146.

8. Pelletier, L., Ozlu, N., Hannak, E.,Cowan, C., Habermann, B., Ruer, M.,Muller-Reichert, T., and Hyman, A.A.(2004). The Caenorhabditis eleganscentrosomal protein SPD-2 is required forboth pericentriolar material recruitmentand centriole duplication. Curr. Biol. 14,863–873.

9. Leidel, S., and Gonczy, P. (2003). SAS-4 isessential for centrosome duplication inC. elegans and is recruited to daughtercentrioles once per cell cycle. Dev. Cell 4,431–439.

10. Basto, R., Lau, J., Vinogradova, T.,Gardiol, A., Woods, C.G., Khodjakov, A.,and Raff, J.W. (2006). Flies withoutcentrioles. Cell 125, 1375–1386.

11. Dammermann, A., Muller-Reichert, T.,Pelletier, L., Habermann, B., Desai, A., andOegema, K. (2004). Centriole assemblyrequires both centriolar andpericentriolar material proteins.Dev. Cell 7, 815–829.

12. Leidel, S., Delattre, M., Cerutti, L.,Baumer, K., and Gonczy, P. (2005). SAS-6defines a protein family required forcentrosome duplication in C. elegans andin human cells. Nat. Cell Biol. 7,115–125.

13. Peel, N., Stevens, N.R., Basto, R., andRaff, J.W. (2007). Overexpressingcentriole-replication proteins in vivoinduces centriole overduplicationand de novo formation. Curr. Biol.17, 834–843.

14. Rodrigues-Martins, A.,Bettencourt-Dias, M., Riparbelli, M.,Ferreira, C., Ferreira, I., Callaini, G., andGlover, D.M. (2007). Drosophila SAS-6organizes a tube-like centriole precursorstructure and its absence suggestsmodularity in centriole assembly. Curr.Biol. 17, 1465–1472.

15. Bettencourt-Dias, M., Rodrigues-Martins, A., Carpenter, L., Riparbelli, M.,Lehmann, L., Gatt, M.K., Carmo, N.,Balloux, F., Callaini, G., and Glover, D.M.(2005). SAK/PLK4 is required for centrioleduplication and flagella development.Curr. Biol. 15, 2199–2207.

16. Rodrigues-Martins, A., Riparbelli, M.,Callaini, G., Glover, D.M., andBettencourt-Dias, M. (2007). Revisitingthe role of the mother centriole incentriole biogenesis. Science 316,1046–1050.

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Samuel Lunenfeld Research Institute,600 University Avenue, Room 1075A,Toronto, Ontario M5G 1X5, Canada.E-mail: pelletier@mshri.on.ca

DOI: 10.1016/j.cub.2007.07.025

DispatchR773

Adaptive Evolution: The Legacy ofPast Giants

The wire syndrome shared by plants in New Zealand and Madagascarappears to have evolved convergently as a defence against herbivoryfrom now extinct avian giants.

Hannes Dempewolfand Loren H. Rieseberg

Evolutionary biologists are oftenaccused of adaptive story telling,in which adaptive explanationsare developed to account forparticular organismal features orbehaviour. While some of thesehave been validated by rigorousevolutionary study, in manyinstances they remain untested,‘just-so’ stories [1]. Evolutionarypsychologists have beenespecially imaginative, explainingrape, for example, as anadaptation of lower status malesto guarantee their reproductivesuccess [2], the killing ofnewborns as a means formanaging limited parentalresources [3], and depression asa mechanism of conflict

avoidance for individuals inlower social classes [4,5]. Mostspectacularly, 20 adaptive andone non-adaptive explanationshave been offered for femaleorgasm [6].

Plant evolutionists have putforward some ingenioushypotheses as well. For example,a shift from a high-browsing tolow-browsing dinosaur fauna inthe Early Cretaceous is correlatedwith the emergence ofangiosperms. This observationspawned a line of adaptivereasoning in which thelow-browsing dinosaurs withsophisticated jaw structures arespeculated to have decimated slowgrowing gymnosperm saplings,thereby creating a favourableenvironment for the origin anddiversification of smaller, weedier

early angiosperms [7,8]. Thatthese adaptive stories survivedpeer review is a testament both tothe charisma of dinosaurs (howelse are we to interest ouroffspring in botany?) and to thedifficulty of disprovinghypotheses about events thattook place 130 million years ago(but see [9]). However, anadaptive story linkingmodifications in plant architecturewith herbivory by giant aviandinosaurs –– elephant birds andmoas –– has been validated bya recent study [10]. At least inthis case, the legacy of pastgiants is clear.

Elephant birds and giant moas,which were native to Madascarand New Zealand, respectively,are believed to have been theworld’s largest ‘modern dinosaurs’(birds). Giant moas reached 3.6 min height and elephant birds couldexceed 500 kg in weight — abouttwice the weight of a grizzly bear.Both giant birds were driven toextinction by humans within thepast six hundred years, so if theyhad a major impact on plant form, itshould still be apparent. Indeed,

Current Biology Vol 17 No 17R774

Figure 1. Artist’s impres-sion of an elephant birdthat is attempting to feedon a plant with the ‘wiresyndrome’ (drawing byAnya E. Gangaeva).

scientists have speculated that thedivaricate plant architecture —wide-angled branches, smallleaves, thin, wiry branches, and fewor no leaves in the outer canopy —which is common in the NewZealand flora, evolved as a defenceagainst herbivory by giant Moas[11]. This hypothesis is supportedby results from feedingexperiments with emus andostriches, in which divaricatearchitecture, particularly hightensile strength of stems, detersbrowsing by large flightless birds[12]. Ecophysiologicalexperiments, however, suggestthat this architecture mightinstead be an adaptation to coldclimates [13].

To distinguish between thesehypotheses, Bond and Silander[10] searched for plants witha divaricate architecture inMadagascar, the former home ofgiant elephant birds. Madagascarhas a very different climate thanNew Zealand, so convergence inplant architecture, if observed,could be attributed to parallelselection pressure from giantflightless birds. They reportspecies from 25 families and 36genera that share a suite oftraits –– the so-called wire-syndrome –– with divaricate plantsfrom New Zealand. The ‘wiresyndrome’ includes wide-angledbranches, small leaves, and thin,wiry branches (Figure 1). Unlikedivaricate plants from NewZealand, however, the leaves of

Malagasy wire plants are notclustered inside the plant. Thus,while the wire syndrome appears tohave evolved as a defense againstgiant flightless birds, the paucityof leaves in the outer canopy ofNew Zealand plants may wellrepresent an adaptation to cold.

Although this combination ofexperimental and comparativeanalyses makes for a convincingargument, the evidence iscorrelative only and based ona single comparison. What if someother ecological or phylogeneticfactor was responsible for theconvergent wire syndrome? Tominimize this possibility, Bondand Silander [10] also comparedthe architecture of South Africanplants with Malagasy species fromsimilar habitats and/or closephylogenetic relationships. Thewire plant syndrome offers littledefense against the kinds oflarge ungulate browsersresponsible for the bulk ofherbivory in South Africa. Aspredicted, South African plants failto exhibit the wire plant syndrome,even when ostriches are present.Thus, both the absence ofterrestrial ungulates in NewZealand and Madagascar, andthe presence of avian giants,may be required for the evolutionof the wire plant syndrome. Theproverbial ‘fly in the ointment’is the presence of divaricate plantsin Patagonia [14], where bothflightless birds (rheas) andterrestrial ungulates roam.

The wire plants of New Zealandand Madagascar should be addedto the pantheon of examples ofadaptive convergence in plants,such as the evolution of spiny,succulent stems in Cactaceae inthe Americas and Euphorbiaceae inAfrica and the multiple independentevolution of arborescence onislands. While rigorous tests ofadaptive hypotheses remain far tooinfrequent, they have the potentialto transform the image ofevolutionary biologists fromadaptive storytellers to expertwitnesses.

References1. Gould, S.J., and Lewontin, R.C. (1979).

The spandrels of San Marco and thePanglossian paradigm. A critique of theadaptationist programme. Proc. R. Soc.Lond. B Biol. Sci. 205, 581–598.

2. Thornhill, R., and Palmer, C.T. (2000). ANatural History of Rape: Biological Basesof Sexual Coercion (Cambridge,Massachusetts: MIT Press).

3. Pinker, S. (1997). Why They Kill TheirNewborns. The New York TimesMagazine, November 2, 52–54.

4. Price, J. (1967). The dominance hierarchyand the evolution of mental illness. Lancet2, 243–246.

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6. Lloyd, E.A. (2005). The Case of the FemaleOrgasm: Bias in the Science of Evolution(Cambridge, Massachusetts: HarvardUniversity Press).

7. Bakker, R.T. (1978). Dinosaur feedingbehaviour and the origin of floweringplants. Nature 274, 661–663.

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10. Bond, W.J., and Silander, J.A. (2007).Springs and wire plants: anachronisticdefences against Madagascar’s extinctelephant birds. Proc. Biol. Sci. 274,1985–1992.

11. Greenwood, R.M., and Atkinson, I.A.E.(1977). Evolution of the divaricating plantsin New Zealand in relation to moabrowsing. Proc. New Zeal. Ecol. Soc. 24,21–33.

12. Bond, W.J., Lee, W.G., and Craine, J.M.(2004). Plant structural defences againstavian browsers: the legacy of NewZealand’s extinct moas. Oikos 104,500–508.

13. Howell, C.J., Kelly, D., andTurnbull, M.T.H. (2002). Moa ghostsexorcised? New Zealand’s divaricateshrubs avoid photoinhibition. Funct. Ecol.16, 232–240.

14. McQueen, D.R. (2000). Divaricating shrubsin Patagonia and New Zealand. New Zeal.J. Ecol. 24, 69–80.

Department of Botany, University ofBritish-Columbia, Vancouver, BritishColumbia, Canada V6T 1Z4.E-mail: lriesebe@interchange.ubc.ca

DOI: 10.1016/j.cub.2007.06.054