www.sciencemag.org SCIENCE VOL 310 11 NOVEMBER 2005 979

The central claim of the theory of evo-lution as laid out in 1859 by CharlesDarwin in The Origin of Species is

that living species, despite their diversity inform and way of life, are the products ofdescent (with modification) from commonancestors. To communicate this idea,Darwin developed the metaphor of the“tree of life.” In this comparison, livingspecies trace backward in time to commonancestors in the same way that separatetwigs on a tree trace back to the same majorbranches. Coincident with improved meth-ods for uncovering evolutionaryrelationships, evolutionary trees,or phylogenies, have become anessential element of modern biol-ogy (1). Consider the case ofHIV/AIDS, where phylogenieshave been used to identify thesource of the virus, to date theonset of the epidemic, to detectviral recombination, to track viralevolution within a patient, and toidentify modes of potential trans-mission (2). Phylogenetic analysiswas even used to solve a murdercase involving HIV (3). Yet “treethinking” remains widely prac-ticed only by professional evolu-tionary biologists. This is a partic-ular cause for concern at a timewhen the teaching of evolution isbeing challenged, because evolutionarytrees serve not only as tools for biologicalresearchers across disciplines but also asthe main framework within which evidencefor evolution is evaluated (4, 5).

At the outset, it is important to clarifythat tree thinking does not necessarilyentail knowing how phylogenies areinferred by practicing systematists. Anyonewho has looked into phylogenetics fromoutside the field of evolutionary biologyknows that it is complex and rapidly chang-ing, replete with a dense statistical litera-ture, impassioned philosophical debates,and an abundance of highly technical com-puter programs. Fortunately, one can inter-

pret trees and use them for organizingknowledge of biodiversity without know-ing the details of phylogenetic inference.The reverse is, however, not true. One can-not really understand phylogenetics if oneis not clear what an evolutionary tree is.

The preferred interpretation of a phylo-genetic tree is as a depiction of lines ofdescent. That is, trees communicate theevolutionary relationships among ele-ments, such as genes or species, that con-nect a sample of branch tips. Under thisinterpretation, the nodes (branching points)

on a tree are taken to correspond to actualbiological entities that existed in the past:ancestral populations or ancestral genes.However, tree diagrams are also used inmany nonevolutionary contexts, which cancause confusion. For example, trees candepict the clustering of genes on the basisof their expression profiles from microar-rays, or the clustering of ecological com-munities by species composition. Theprevalence of such cluster diagrams mayexplain why phylogenetic trees are oftenmisinterpreted as depictions of the similar-ity among the branch tips. Phylogenetictrees show historical relationships, not sim-ilarities. Although closely related speciestend to be similar to one another, this is notnecessarily the case if the rate of evolutionis not uniform: Crocodiles are more closelyrelated to birds than they are to lizards, eventhough crocodiles are indisputably moresimilar in external appearance to lizards.

But what does it mean to be “moreclosely related”? Relatedness should beunderstood in terms of common ancestry—the more recently species share a commonancestor, the more closely related they are.This can be seen by reference to pedigrees:You are more closely related to your firstcousin than to your second cousin becauseyour last common ancestor with your firstcousin lived two generations ago (grand-parents), whereas your last common ances-tor with your second cousin lived threegenerations ago (great-grandparents).Nonetheless, many introductory studentsand even professionals do not find it easy toread a tree diagram as a depiction of evolu-tionary relationships. For example, whenpresented with a particular phylogenetictree (see the figure, left), people often erro-

neously conclude that a frog is more closelyrelated to a fish than to a human. A frog isactually more closely related to a humanthan to a f ish because the last commonancestor of a frog and a human (see the fig-ure, label x) is a descendant of the last com-mon ancestor of a frog and a fish (see thef igure, label y), and thus lived morerecently. [To evaluate your tree-thinkingskills, take the quizzes (6)].

Why are trees liable to misinterpreta-tion? Some evolutionary biologists haveproposed that nonspecialists are prone toread trees along the tips (1, 7), which in thiscase yields an ordered sequence from fishto frogs and ultimately to humans. Thisincorrect way to read a phylogeny mayexplain the widely held but erroneous viewthat evolution is a linear progression fromprimitive to advanced species (8), eventhough a moment’s reflection will revealthat a living frog cannot be the ancestor of

E V O L U T I O N

The Tree-Thinking ChallengeDavid A. Baum, Stacey DeWitt Smith, Samuel S. S. Donovan

D. A. Baum and S. D. Smith are in the Department ofBotany, University of Wisconsin, 430 Lincoln Drive,Madison, WI 53706, USA. E-mail: dbaum@wisc.edu;sdsmith4@wisc.edu S. S. Donovan is in the Departmentof Instruction and Learning, University of Pittsburgh,Pittsburgh, PA 15260, USA. E-mail: sdonovan@pitt.edu

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Which phylogenetic tree is accurate? On the basis of the tree on the left, is the frog more closely related tothe fish or the human? Does the tree on the right change your mind? See the text for how the common ances-tors (x and y) indicate relatedness.

Published by AAAS

980

a living human. The correct way to read a treeis as a set of hierarchically nested groups,known as clades. In this example, there arethree meaningful clades: human-mouse,human-mouse-lizard, and human-mouse-lizard-frog. The difference between readingbranch tips and reading clades becomesapparent if the branches are rotated so thatthe tip order is changed (see the f igure,right). Although the order across the branchtips is different, the branching pattern of evo-lutionary descent and clade composition isidentical. A focus on clade structure helps toemphasize that there is no single, linear nar-rative of evolutionary progress (1, 7).

There are other problems in reading rela-tionships from trees (9). For example, thereis a common assumption that trait evolutionhappens only at nodes. But nodes simplyrepresent places where populations becamegenetically isolated, permitting them toaccumulate differences in their subsequentevolution. Similarly, living species may bemistakenly projected backward to occupyinternal nodes of a tree. But it is incorrect toread a tree as saying that humans descendedfrom mice when all that is implied is that

humans and mice shared a common ances-tor. Thus, for all its importance, tree think-ing is fraught with challenges.

Tree thinking belongs alongside naturalselection as a major theme in evolutiontraining. Further, trees could be usedthroughout biological training as an effi-cient way to present information on the dis-tribution of traits among species. To thisend, what is needed are more resources:computer programs (10), educational strate-gies (11, 12), and accessible presentationsof current phylogenetic knowledge (13–15).

Phylogenetic trees are the most direct rep-resentation of the principle of commonancestry—the very core of evolutionarytheory—and thus they must f ind a moreprominent place in the general public’s under-standing of evolution. As philosopher of sci-ence Robert O’Hara (16) stated, “just asbeginning students in geography need to betaught how to read maps, so beginning stu-dents in biology should be taught how to readtrees and to understand what trees communi-cate.” Among other benefits, as the concept oftree thinking becomes better understood bythose in the sciences, we can hope that a wider

segment of society will come to appreciate theoverwhelming evidence for common ancestryand the scientific rigor of evolutionary biology.

References1. R. J. O’Hara, Syst. Zool. 37, 142 (1988).2. K. A. Crandall, The Evolution of HIV (Johns Hopkins

Univ. Press, Baltimore, 1999).3. M. L. Metzger et al. Proc. Natl. Acad. Sci. U.S.A. 99,

14292 (2002).4. D. Penny, L. R. Foulds, M. D. Hendy, Nature 297,197

(1982).5. E. Sober, M. Steel, J. Theor. Biol. 218, 395 (2002).6. See the two quizzes on Science Online.7. S. Nee, Nature 435, 429 (2005).8. J. L. Rudolph, J. Stewart, J. Res. Sci.Teach. 35, 1069 (1998).9. M. D. Crisp, L. G. Cook, Trends Ecol. Evol. 20, 122 (2005).

10. J. Herron et al., EvoBeaker 1.0 (SimBiotic Software,Ithaca, NY, 2005).

11. D.W. Goldsmith, Am. Biol. Teach. 65, 679 (2003).12. S. F. Gilbert, Nat. Rev. Genet. 4, 735 (2003).13. J. Cracraft, M. J. Donoghue, Assembling the Tree of Life

(Oxford Univ. Press, Oxford, 2004).14. R. Dawkins, The Ancestor’s Tale: A Pilgrimage to the

Dawn of Evolution (Houghton Mifflin, New York, 2004).15. Tree-Thinking Group (www.tree-thinking.org).16. R. J. O’Hara, Zool. Scripta 26, 323 (1997).

Supporting Online Material www.sciencemag.org/cgi/content/full/310/5750/979/DC1Tree-Thinking Quizzes I and II

10.1126/science.1117727

Some biological molecules, such as thosein visual or photosynthetic systems,have evolved to eff iciently convert

energy from one form to another. How dothese molecules channel energy rapidly andefficiently so that useful work can be per-formed without this energy being dissipatedineffectively into the surroundings?Dissipation of molecular vibrational excita-tion energy typically takes place on picosec-ond time scales, so biological molecules mustbe able to channel energy rapidly and effi-ciently if they are to be able to direct it in a use-ful manner. In biological systems excited bylight, the nonradiative electronic transitionscan occur on time scales (<< 10–12 ps) that areeven faster than vibrational energy dissipation(1–3), hinting at how nature solves the problemof directing energy flow. On page 1006 of thisissue, Kukura et al. (4) take an important stepforward in defining the process of directedenergy flow in the visual pigment rhodopsin.

Photoexcited biological molecules offera unique opportunity to monitor the evolu-tion of excitation energy as it transforms areactant molecule into its final products.With the advent of appropriate femtosec-ond laser techniques (5), it has become pos-sible to examine the underlying dynamicsof the elementary vibrational and electronicexcitations that guide the structural changesand, ultimately, the function of a variety ofbiomolecules (6–8). The work presented byKukura et al. enhances our ability to moni-tor rapid structural changes in such mole-cules by introducing the technique of fem-tosecond stimulated Raman spectroscopy(FSRS). In their report, Kukura et al. followthe evolution of the retinal chromophore asit is excited to photorhodopsin and decaysinto bathorhodopsin, all within the f irstpicosecond of the visual process. They dothis by taking advantage of the broad spec-tral bandwidth of their probe pulse to obtainvery high quality time-resolved stimulatedRaman spectra over the range of 600 to2000 cm!1.

How does this experiment generateultrafast time resolution, as well as the high

spectral resolution associated with Ramanspectra, without violating the uncertaintyprinciple? Although not emphasized in thereport by Kukura et al., these authors arefully aware (9) that the underlying timescale for the generation of the Raman pho-ton is dictated by the dephasing time of thecoherence between the initial and f inalvibrational levels of the material undergo-ing the Raman process. A typical time scalefor the vibrational dephasing time is on theorder of 10!12 s, which translates to a 10cm!1 Raman bandwidth. This means thatthe FSRS experiment reads out Ramanradiation from the sample that is averagedover its vibrational dephasing time window(that is, the stimulated Raman signals con-tinue to appear at the detector, even after theprobe pulse has passed through the sam-ple). Thus, there is no violation of theuncertainty principle. However, being ableto control the “gating” of the Raman coher-ence by changing the time delay betweenthe photochemical pump and the broadbandprobe allows the dephasing time window tobe moved so that rapid structural dynamicscan be monitored. Changes in the vibra-tional frequencies that take place within thedephasing time window affect the FSRSlineshape, and the authors have done a con-vincing job of simulating these lineshapechanges as shown in the supporting onlinematerial of their paper.

A key conclusion of the work onrhodopsin is that low-symmetry hydrogenout-of-plane (HOOP) wagging motions

C H E M I S T RY

Following the Flow of Energyin Biomolecules

Paul M. Champion

The author is in the Physics Department and theCenter for Interdisciplinary Research on ComplexSystems, Northeastern University, Boston, MA 02115,USA. E-mail: p.champion@neu.edu

11 NOVEMBER 2005 VOL 310 SCIENCE www.sciencemag.org

P E R S P E C T I V E S

Published by AAAS

www.sciencemag.org/cgi/content/full/310/5750/979/DC1

Supporting Online Material for

The Tree-Thinking Challenge

David A. Baum,* Stacey DeWitt Smith, Samuel S. Donovan

*To whom correspondence should be addressed. E-mail: dbaum@wisc.edu

Published 11 November 2005, Science 310, 979 (2005) DOI: 10.1126/science.1117727

This PDF file includes:

Tree-Thinking Quizzes I and II

Basic Tree Thinking AssessmentDavid A. Baum, Stacey DeWitt Smith, Samuel S. Donovan

This quiz includes a number of multiple-choice questions you can use to test yourself on yourability to accurately interpret evolutionary trees. Insofar as real biological examples havebeen used they are accurate based on current knowledge.

1) By reference to the tree above, which of the following is an accurate statement ofrelationships?

a) A green alga is more closely related to a red alga than to a mossb) A green alga is more closely related to a moss than to a red algac) A green alga is equally related to a red alga and a mossd) A green alga is related to a red alga, but is not related to a moss

2) By reference to the tree above, which of the following is an accurate statement ofrelationships?

a) A crocodile is more closely related to a lizard than to a birdb) A crocodile is more closely related to a bird than to a lizardc) A crocodile is equally related to a lizard and a birdd) A crocodile is related to a lizard, but is not related to a bird

3) By reference to the tree above, which of the following is an accurate statement ofrelationships?

a) A seal is more closely related to a horse than to a whaleb) A seal is more closely related to a whale than to a horsec) A seal is equally related to a horse and a whaled) A seal is related to a whale, but is not related to a horse

4) Which of the five marks in the tree above corresponds to the most recent common ancestor ofa mushroom and a sponge?

5) If you were to add a trout to the phylogeny shown above, where would its lineage attach to therest of the tree?

6) Which of trees below is false given the larger phylogeny above?

7) Which of the four trees above depicts a different pattern of relationships than the others?

8) Which of the four trees above depicts a different pattern of relationships than the others?

9) In the above tree, assume that the ancestor had a long tail, ear flaps, external testes, and fixedclaws. Based on the tree and assuming that all evolutionary changes in these traits are shown,what traits does a sea lion have?

a) long tail, ear flaps, external testes, and fixed clawsb) short tail, no ear flaps, external testes, and fixed clawsc) short tail, no ear flaps, abdominal testes, and fixed clawsd) short tail, ear flaps, abdominal testes, and fixed clawse) long tail, ear flaps, abdominal testes, and retractable claws

10) In the above tree, assume that the ancestor was a herb (not a tree) without leaves or seeds.Based on the tree and assuming that all evolutionary changes in these traits are shown, which ofthe tips has a tree habit and lacks true leaves?

a) Lepidodendronb) Clubmossc) Oakd) Psilotume) Fern

Basic Tree Thinking AssessmentDavid A. Baum, Stacey DeWitt Smith, Samuel S. Donovan

This quiz includes a number of multiple-choice questions you can use to test yourself onyour ability to accurately interpret evolutionary trees. Insofar as real biological exampleshave been used they are accurate based on current knowledge.

1) By reference to the tree above, which of the following is an accurate statement ofrelationships?

a) A green alga is more closely related to a red alga than to a mossb) A green alga is more closely related to a moss than to a red algac) A green alga is equally related to a red alga and a mossd) A green alga is related to a red alga, but is not related to a moss

2) By reference to the tree above, which of the following is an accurate statement ofrelationships?

a) A crocodile is more closely related to a lizard than to a birdb) A crocodile is more closely related to a bird than to a lizardc) A crocodile is equally related to a lizard and a birdd) A crocodile is related to a lizard, but is not related to a bird

‘b’ is correct. The most recentcommon ancestor of a greenalga and a moss is at node xwhereas the most recentcommon ancestor of a red algaand a moss is at the “deeper”node, y. If you picked ‘c’ youmight be reading along the tips.

x

y

‘b’ is correct. The most recentcommon ancestor of a crocodileand a bird is at node x whereasthe most recent commonancestor of a crocodile and alizard is at the “deeper” node, y.If you picked ‘a’ you might bereading along the tips.

x

y

3) By reference to the tree above, which of the following is an accurate statement ofrelationships?

a) A seal is more closely related to a horse than to a whaleb) A seal is more closely related to a whale than to a horsec) A seal is equally related to a horse and a whaled) A seal is related to a whale, but is not related to a horse

4) Which of the five dots in the tree above corresponds to the most recent common ancestor of amushroom and a sponge?

‘c’ is correct. The most recentcommon ancestor of a seal and awhale is at node y, as is the mostrecent common ancestor of aseal and a horse. Alldescendants of node x areequally related to the seal. If youpicked ‘a’ you might be readingalong the tips.

x

y

‘d’ is the correct answer. ‘a’ is aliving species and is not anancestor. ‘e’ is an ancestor of asponge but not of a mushroom.‘b’ and ‘c’ are commonancestors of a sponge and amushroom, but they are moreancient common ancestors than‘d’.

5) If you were to add a trout to the phylogeny shown above, where would its lineage attach tothe rest of the tree?

6) Which of the trees below is false given the larger phylogeny above?

‘c’ is the correct answer. Thisdepends on only the knowledge thata salmon and a trout are veryclosely related. Therefore theymust share a more recent commonancestor with each other than withany other included species.Position ‘c’ is the only place suchan ancestor could be.

‘d’ is the correct answer. ‘d’ showsyeast being more closely related toplants than it is to animals. Thetrue phylogeny (right) shows thatyeast is more closely related tohuman than to any of the plants(green algae, lily, and fern).

7) Which of the four trees above depicts a different pattern of relationships than the others?

8) Which of the four trees above depicts a different pattern of relationships than the others?

‘c’ is the correct answer. Inall the other trees C is moreclosely related to E and Dthan to B. In ‘c,’ C is moreclosely related to B than to Eor D.

‘a’ is the correct answer. In allthe other trees B is more closelyrelated to D and E than is C. In‘a,’ C is more closely related toD and E than is B.

9) In the above tree, assume that the ancestor had a long tail, ear flaps, external testes, and fixedclaws. Based on the tree and assuming that all evolutionary changes in these traits are shown,what traits does a sea lion have?

a) long tail, ear flaps, external testes, and fixed clawsb) short tail, no ear flaps, external testes, and fixed clawsc) short tail, no ear flaps, abdominal testes, and fixed clawsd) short tail, ear flaps, abdominal testes, and fixed clawse) long tail, ear flaps, abdominal testes, and retractable claws

10) In the above tree, assume that the ancestor was an herb (not a tree) without leaves or seeds.Based on the tree and assuming that all evolutionary changes in these traits are shown, which ofthe tips has a tree habit and lacks true leaves?

a) Lepidodendronb) Clubmossc) Oakd) Psilotume) Fern

‘a’ is the correct answer.Clubmosses are not trees. Oak(and yew) are trees but they haveleaves. Psilotum lacks leaves, butit is not a tree. A fern has leavesand is not a tree.

‘d’ is the correct answer. Tracingup from the ancestor to sea lions,one sees that the only changes arein tail length and testes position.For the other traits, they haveretained the ancestral condition.

Tree thinking quiz IID. A. Baum, S. D. Smith, and S. D. Donovan

Each question in this quiz is built around a Science paper that depicted a phylogenetic tree. It should not benecessary to consult the original article to answer the questions.

S. Mathews, M. J. Donoghue. The root of angiosperm phylogeny inferred from duplicate phytochrome genes.Science 286, 947 (1999).

1) The figure above shows the phylogeny estimated for a sample of flowering plants (angiosperms) fromPHYTOCHROME A and PHYTOCHROME C, a pair of genes that duplicated prior to the origin of the angiosperms.Which of the following sets of taxa constitute a clade (=monophyletic group) on one gene tree but not on the other?

a) Degeneria-Magnolia-Eupomatiab) All angiosperms except Amborellac) Austrobaileya-Nymphaea-Cabombaceaed) Nelumbo-Trochodendron-Aquilegia

J. X. Becerra. Insects on plants: macroevolutionary chemical trends in host use. Science 276, 253 (1997).

2) The dendrogram on the left clusters plant species by chemical similarity; each of the four main chemical groups isindicated with a different color. This tree does not depict descent relationships, just degree of chemical similarity.On the right, the evolution of these chemical types is reconstructed on a phylogeny of the plants (this does depictinferred evolutionary relationships). The colors correspond to the chemical groups on the left, and the gray branchesindicate uncertainty in character reconstruction. What does a comparison of these two figures tell us about theevolution of plant secondary chemistry?

a) The four groups of chemically similar species each constitutes a distinct evolutionary lineageb) The group colored “black” has the most advanced chemical defensesc) The red (3) and blue (1) chemical groups are most distantly relatedd) The chemical groups have each been gained and/or lost multiple times in evolution

F. Bossuyt, M. C. Milinkovitch. Amphibians as indicators of early tertiary "out-of-India" dispersal ofvertebrates. Science 292, 93 (2001).

3) This tree depicts inferred relationships among some major frog groups with branches drawn proportional toabsolute time. Error bars on internal nodes depict confidence intervals on the dates of estimated nodes. Assumingthis tree and the associated ages are correct which of the following statements is true?

a) No individual living before 70 million years ago is an ancestor of Raninaeb) Raninae and Dicroglossinae shared a common ancestor about 75 million years agoc) The divergence of Raninae and Nyctibatrachinae occurred more recently than the 85 million year old

separation of India from Madagascard) The last common ancestor of Micrixalinae and Dicroglossinae lived before India and Madagascar separated

(85 million years ago)

M. Berenbrink, P. Koldkjaer, O. Kepp, A. R. Cossins, Evolution of oxygen secretion in fishes and the emergence ofa complex physiological system. Science 307, 1752 (2005).

4) Retia mirabilia (sing. rete mirabile) are vascular bundles that allow fish to secrete O2. In the above figure, redbranches indicate lineages with choroid retia, blue branches indicate those with swimbladder retia, and whitebranches indicate absence of retia. Assuming the phylogeny and character evolution have been accurately inferred,we can see that:

a) Swimbladder retia predate choroid retiab) Gains of swimbladder retia primarily took place in lineages that already had choroid retia.c) Loss of choroid retia causes gain of swimbladder retiad) Choroid retia have been gained more often than swimbladder retia

V. Daubin, N. A. Moran, H. Ochman. Phylogenetics and the cohesion of bacterial genomes. Science 301, 829(2003).[The above is only a portion of the figure].

5) Each row in the table above lists a set of four bacterial taxa whose relationship follows the topology shown. Thuseach row can be read as a four-taxon tree. Which of the four trees below is compatible with the information in thethree rows of the table?

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L. Marivaux et al. A fossil lemur from the Oligocene of Pakistan. Science 294, 587 (2001).[The above is only a portion of the figure]

6) Lemuriformes are currently restricted to Madagascar, whereas Lorisiformes are found in Africa and Asia but notMadagascar, and Tarsius is Asian. The tree above was generated in order to assess the relationship of a fossil,Bugtilemur, found in 30 million year old deposits in Pakistan. Each branch of the tree has been annotated with twonumbers, the first of which is the bootstrap percentage, a measure of support. In order to hold that Bugtilemur ismore closely related to Lorisiformes than to Lemuriformes what is the minimum number of branches, with whatbootstrap support, that would need to be incorrect?

a) 1: 92%b) 2: 78%, 69%c) 4: 78%, 45%, 30%, 69%d) 4: 78%, 45%, 30%, 29%

M. J. Gibbs, J. S. Armstrong, A. J. Gibbs. Recombination in the hemaglutinin gene of the 1918 "Spanish flu".Science 293, 1842 (2001).

7) The trees show the phylogeny estimated for the same mammalian H1 influenzas based on portions of thehaemaglutin genes. Tree A was estimated from bases 310-870 whereas tree B was estimated from bases 1070-1650.Under the assumption that these trees are correctly rooted, how does this result support the inference that the SouthCarolina 1918 strain (the only exemplar from the 1918 flu pandemic) arose from recombination between a humanand a swine virus?

a) The fact that the South Carolina 1918 strain is near the root of both trees suggests that it is of mixedidentity

b) The fact that the South Carolina 1918 strain is more closely related to the swine strains in tree A but to thehuman strains in tree B

c) The fact that the South Carolina 1918 strain is a direct ancestor of all the swine strains in tree A but wasisolated from a human

d) The fact that the South Carolina 1918 strain is more closely related to the Iowa 30 swine strain than to theScotland 94 human strain in both trees

F. Bossuyt et al. Local endemism within the Western Ghats-Sri Lanka biodiversity hotspot. Science 306, 479 (2004).

8) These trees for six different groups of organisms for (A) tree frogs, (B) caecilians, (C) uropeltid snakes, (D)freshwater fishes, (E) freshwater shrimps, and (F) freshwater crabs are all colored based on whether the speciescome from mainland India or Sri Lanka. Assuming these trees and the reconstruction of ancestral geographicdistributions are accurate, which of the trees includes a single well-supported migration from Sri Lanka back tosouthern India?

a) A, Fb) B, Cc) Cd) A, B

C. R. Currie et al. Ancient tripartite coevolution in the attine ant-microbe symbiosis. Science 299, 325. (2003).

9) The three trees depict the relationships between leaf cutter ants (left), fungi cultivated by those ants (middle) andfungi parasitizing ant gardens (right). By comparing the phylogenies, we can distinguish whether fungi are usuallypassed on vertically (through generations of an ant lineage) or horizontally (e.g., among unrelated ants). Allowingthat differences between trees could reflect error in phylogenetic inference, what does the similarity between thethree phylogenies suggest?

a) Cultivars are passed horizontally and parasites verticallyb) Cultivars are passed vertically and parasites horizontallyc) Cultivars and parasites are passed verticallyd) Cultivars and parasites are passed horizontally

B. Asfaw et al. Australopithecus garhi: a new species of early hominid from Ethiopia. Science 284, 629 (1999).[The above is only a portion of the figure]

10) The above tree summarizes the inferred relationships among a number of hominid fossils as related to humans(Homo). The point where multiple lineage arise from a single node (a polytomy) is here intended to indicateuncertainty rather than evidence of a simultaneous divergence of an ancestral lineage into five descendant lineages.The fossil ages of each taxon are color-coded on the right. Note that fossils of species 7 predate those of its sistertaxon, species 8. If the tree is accurate representation of the evolutionary history of these species, what is theminimum age we could infer for lineage 8?

a) 2.25b) 2.5c) 3.0d) It cannot be determined from these data

Tree thinking quiz II - KeyD. A. Baum, S. D. Smith, and S. D. Donovan

1) A

2) D

3) D

4) B

5) D

6) C

7) B

8) A

9) C

10) B