OVERCOMING OBSTACLES TO EVOLUTION EDUCATION

Transforming Our Thinking about Transitional Forms

Louise S Mead

Published online 16 April 2009 Springer Science + Business Media LLC 2009

Abstract A common misconception of evolutionary biol-ogy is that it involves a search for ldquomissing linksrdquo in thehistory of life Relying on this misconception antievolu-tionists present the supposed absence of transitional formsfrom the fossil record as evidence against evolutionStudents of biology need to understand that evolution is abranching process paleontologists do not expect to findldquomissing linksrdquo and evolutionary research uses indepen-dent lines of evidence to test hypotheses and makeconclusions about the history of life Teachers can facilitatesuch learning by incorporating cladistics and tree-thinkinginto the curriculum and by using evograms to focus onimportant evolutionary transitions

Keywords Macroevolution Transitional features

Creationism Tree-thinking Cladistics Evograms

The inadequacy of the fossil record to document fully allmajor evolutionary transitions continues to provide a basis forcreationists to undermine evolution education This wasapparent in a recent proposed amendment to the TexasEssential Knowledge and Skills document that asked studentsto ldquoanalyze and evaluate the sufficiency and insufficiency ofcommon ancestry to explain the sudden appearancehellipofgroups in the fossil recordrdquo (Texas State Board of EducationMeeting 22 January 2009) Even when confronted with anexemplary extensive fossil record showing step-by-steptransitions as is now available for the evolutionary transi-tion from land mammals to whales creationists commonly

respond that such an array of fossils corresponds to speciesthat are not along a direct line of descent and therefore notevidence of macroevolutionary change

A reluctance adequately to address macroevolution alsocharacterizes textbooks as noted by Padian (2008) Eventhe best high school biology textbooks omit a comprehensivepresentation of macroevolution Clearly the problem withldquomissing linksrdquo as evidence against evolution is ldquoinsolublewhen dealing with anyone who does not want to accept thatthere could be transitional formsrdquo (Padian and Angielczyk2007204) High school teachers can however redirectstudents toward a better understanding of macroevolutionas well as current paleontological practice by (1) focusing ontransitional features rather than the ldquomissing linkrdquo ortransitional forms (2) incorporating tree-thinking and intro-ducing cladistics into the life science and biology curricu-lum and (3) presenting evograms to depict major transitionsin evolution

The Missing Link

The concept of a ldquomissing linkrdquo is an ldquoarchaic expressionrdquo(Padian and Angielczyk 2007) tracing back to the GreatChain of Being a view of the physical and metaphysicalworld as an unbroken chain It was later temporalized by theevolutionary thought of the eighteenth and nineteenth centuryto the idea of evolution as a progressive climb up a ladder(Ruse 1997) These views of evolution create the falseexpectation that there should be fossil evidence showing ldquoacomplete chain of life from simple to complexrdquo (Vardiman2003328) Creationists rely on such views to support theirarguments against macroevolution in particular by pointingout the ldquoconspicuousrdquo absence of ldquolarge numbers of inter-mediate fossil organismsrdquo (Changing lives 2003) using what

Evo Edu Outreach (2009) 2310ndash314DOI 101007s12052-009-0126-3

L S Mead ()National Center for Science EducationPO Box 9477 Berkeley CA 94709-0477 USAe-mail meadncseweborg

is still unknown to question whether evolution has occurredI will deal with the misconception of evolution as a ladder-like progression shortly but should the fossil record beexpected to reveal all species that have ever lived Clearlynot Knowledge of the fossil record will never be compre-hensive (Darwin 1859 Padian and Angielczyk 2007Prothero 2007) First there is too much of the Earth toexplore and paleontologists have to be content with samplesSecond given our knowledge of geology we understand thatnot all organisms will be fossilized and that there will besystematic biases in what organisms are fossilized (Kidwelland Holland 2002) Therefore any statements rest on afallible if informed assessment of the necessarily incom-plete evidence

Even if one does not expect that the whole Great Chainof Being should be found in the fossil record there is yetanother outmoded view of evolution driving creationistantievolution propaganda Expectations of ldquointermediaterdquoforms reflect neither Darwinrsquos original thoughts nor currentthinking and practices in evolutionary biology and paleon-tology Darwin (1859) was very clear on this The ancestorof two living forms is unlikely to be found alive because itwould have been outcompeted in most cases by newlyadapting forms and an extinct ancestor of two living formswould not be expected to look intermediate between themToday evolutionary biologists and paleontologists do notfocus on finding ldquointermediatesrdquo but rather on reconstruct-ing evolutionary relationships and history using sharedderived characters or synapomorphies Willi Hennig revo-lutionized systematics in the 1960s with the introduction ofcladistics which ushered in a new method of phylogeneticanalysis and a new approach to systematics Instead ofrelying on a Linnaean system of classification cladisticsplaced the focus on evolutionary history specificallyidentifying features as ancestral (general) or derived

(evolved after the lineage split from the ancestor) If ashared derived character or synapomorphy is found in twoor more related organisms it is inferred to have beenpresent in their common ancestor irrespective of whether ornot there is a fossil record for that ancestor (Hennig 1966)Rather than trying to find the actual fossil corresponding tothe ldquomissing linkrdquo between lobe-fins and tetrapods pale-ontologists instead look for fossils with characters or fea-tures important for an adaptive transition from life in anaquatic environment to life on land and that are shared asthe result of common ancestry

Shoots vs Ladders

Thinking of evolution as a progression from simple tocomplex or ladder-like furthers the idea that evolution islineal and that it should be possible to reconstruct a directline of ancestors However the evolution of life instead ofresembling a ladder is more similar to a branching bushDarwinrsquos (1859) contribution to phylogenetic analysisindeed was to introduce the concept of a branching tree oflife with organisms related through common ancestry(Fig 1) Each branch on the tree represents a distinctlineage multiple branches can extend from a commonpoint joined by a set of characters present in the commonancestor lineages can also acquire characters that are notshared in a common ancestor Finally as a result of extinc-tion not all lineages persist into the future as can clearly beseen in Fig 1 Tree-thinking shifts the focus from lookingfor fossils of lineal (direct) ancestors to looking forsynapomorphies that link collateral (side-branch) ancestorsYour grandmother is a lineal ancestor your great-aunt acollateral ancestor (Fig 2) but their lives and times wereprobably not that different (Padian and Angielczyk 2007)

Fig 1 The diagram ofdivergence of taxa presentedby Charles Darwin in On theorigin of species (1859)

Evo Edu Outreach (2009) 2310ndash314 311

Fig 2 Sample family tree foran individual (you) showingyour collateral (indicated bydashed lines) and direct or lineal(indicated by solid lines)ancestors

Fig 3 ldquoEvogramrdquo by Brian Swartz and Josh Frankel of UCMP (Padian 2008) printed with permission of Oxford University Press and the Societyfor Integrative and Comparative Biology

312 Evo Edu Outreach (2009) 2310ndash314

which means that information about one provides informa-tion about the other Paleontologists do not expect to findthe direct lineal ancestor of an extant species (Padian andAngielczyk 2007) nor do they expect to recognize a directancestor as ldquothe ancestorrdquo even if they did find it Howeverby understanding the lives and times of the species in itsfamily tree they can understand what its ancestor wouldhave been like

Figure on Adaptive Change

David Attenborough recently declared in a program aboutDarwin that ldquoyoursquod have to be extraordinarily blinkered ifyou didnrsquot stub your toe against the theory of evolution veryearly on in your liferdquo particularly if as a child you collectedfossils (Charles Darwin and the tree of life 2009) The data tosubstantiate many of the major transitions in evolution havebeen amassed Lines of evidence include cladograms fromindependent datasets (ie fossils morphology and mole-cules) stratigraphy and radiometric dating to establishevents in time and paleoecology to interpret ancient envi-ronments Embryology behavior histology geochemistryfunctional morphology physiology and biochemistry alladd to the array of independent lines of evidence used totest evolutionary hypotheses

Padian (2008) suggests depicting these various linesof evidence particularly the ones that are important inmajor evolutionary transitions in a single diagram calledan ldquoevogramrdquo (Fig 3) to help students to understand thatevolutionary research is conducted in an integrative wayand in a way that relies on testing independent lines ofevidence against each other to demonstrate concordanceEvograms have a cladogram as the backbone of thediagram generated using an entirely separate base ofinformation Because a cladogram is basic to understandingevolutionary relationships other types of noncladogramevolutionary diagrams will only confuse students (Catleyand Novick 2008) In Fig 3 fossil evidence of limbstructure is depicted in one row Structures of the fossilsshowing corresponding bones in corresponding colors areinterpreted in the next row Important clade names andimportant synapomorphies can also be added to the nodesof the cladogram (Padian 2008) Evograms integrate thevarious independent lines of evidence providing a moreunited picture of evolution

The challenge is that teachers are generally neithertrained nor equipped to teach this way Indeed tree-thinking has only recently trickled down to high schoolbiology (Goldsmith 2003 Catley 2006 Baum and Offner2008) Most high school and many college introductorybiology texts are woefully lacking in their presentation ofcomprehensive figures integrating phylogenetic analyses

with hypotheses about evolutionary process In fact itappears that many of the diagrams may actually be mis-leading (Catley and Novick 2008) In a recent analysis ofevolutionary diagrams in biology textbooks Catley andNovick (2008) found that even though 72 of the diagramsin their survey of 31 textbooks were cladograms there werestriking differences across grade levels Middle school textshad the fewest cladograms with almost twice as manynoncladograms High school texts included more clado-grams but the ratio of cladograms to noncladograms wasapproximately equal Such high proportions of nonclado-grams are disturbing if these support misconceptions aboutevolutionary processes (Catley and Novick 2008)

I urge teachers to embrace cladistics and tree-thinking as away of examining the natural world and to encourage studentsto make such thinking a habit For a first step the followingwebsites offer basic exercises on cladistics and tree-thinkingEvolution and the Nature of Science Institutes (httpwwwindianaedu~ensiweb) Understanding Evolution (httpevolutionberkeleyedu) What did T rex taste like (httpwwwucmpberkeleyedueducationexplorationstoursTrex)Also recommended are ways of teaching about evolutionconsistent with modern practice in systematics using ldquoances-tralrdquo and ldquoderivedrdquo instead of ldquoprimitiverdquo and ldquoadvancedrdquowhen referring to organisms and characters which promotesthinking about evolution as a tree rather than a ladder (andalso follows the good example of Darwin who admonishedhimself ldquoNever use the words higher or lowerrdquo) includingexercises that define clades by synapomorphies and map suchcharacters onto phylogenies (see Staub et al 2006 and ENSIMaking Cladograms) and clarifying that a cladogram orphylogenetic tree is a hypothesis given the data and that anode on a cladogram is a hypothetical ancestor not an actualancestor for which we should expect to find a fossil repre-sentative To attain scientific literacy students need to learnthat evolution is a branching process that paleontologists arenot searching for missing links but for transitional featuresand that evolutionary research is conducted in a way thatenables scientists to use independent lines of evidence toconverge on robust conclusions about the history of lifeTeachers need to learn these lessons in order to be able toimpart them

References

Baum D Offner S Phylogenies and tree-thinking Am Biol Teach 200870(4)222ndash9 doi1016620002-7685(2008)70[222PT]20CO2

Catley KM Darwinrsquos missing linkmdasha novel paradigm for evolutioneducation Sci Educ 200690767ndash83 doi101002sce20152

Catley KM Novick L Seeing the wood for the trees an analysis ofevolutionary diagrams in biology textbooks Bioscience200858969ndash87 doi101641B581011

Changing lives (2003) httpwwwchanginglivesonlineorgevolutionhtml

Evo Edu Outreach (2009) 2310ndash314 313

is still unknown to question whether evolution has occurredI will deal with the misconception of evolution as a ladder-like progression shortly but should the fossil record beexpected to reveal all species that have ever lived Clearlynot Knowledge of the fossil record will never be compre-hensive (Darwin 1859 Padian and Angielczyk 2007Prothero 2007) First there is too much of the Earth toexplore and paleontologists have to be content with samplesSecond given our knowledge of geology we understand thatnot all organisms will be fossilized and that there will besystematic biases in what organisms are fossilized (Kidwelland Holland 2002) Therefore any statements rest on afallible if informed assessment of the necessarily incom-plete evidence

Even if one does not expect that the whole Great Chainof Being should be found in the fossil record there is yetanother outmoded view of evolution driving creationistantievolution propaganda Expectations of ldquointermediaterdquoforms reflect neither Darwinrsquos original thoughts nor currentthinking and practices in evolutionary biology and paleon-tology Darwin (1859) was very clear on this The ancestorof two living forms is unlikely to be found alive because itwould have been outcompeted in most cases by newlyadapting forms and an extinct ancestor of two living formswould not be expected to look intermediate between themToday evolutionary biologists and paleontologists do notfocus on finding ldquointermediatesrdquo but rather on reconstruct-ing evolutionary relationships and history using sharedderived characters or synapomorphies Willi Hennig revo-lutionized systematics in the 1960s with the introduction ofcladistics which ushered in a new method of phylogeneticanalysis and a new approach to systematics Instead ofrelying on a Linnaean system of classification cladisticsplaced the focus on evolutionary history specificallyidentifying features as ancestral (general) or derived

(evolved after the lineage split from the ancestor) If ashared derived character or synapomorphy is found in twoor more related organisms it is inferred to have beenpresent in their common ancestor irrespective of whether ornot there is a fossil record for that ancestor (Hennig 1966)Rather than trying to find the actual fossil corresponding tothe ldquomissing linkrdquo between lobe-fins and tetrapods pale-ontologists instead look for fossils with characters or fea-tures important for an adaptive transition from life in anaquatic environment to life on land and that are shared asthe result of common ancestry

Shoots vs Ladders

Thinking of evolution as a progression from simple tocomplex or ladder-like furthers the idea that evolution islineal and that it should be possible to reconstruct a directline of ancestors However the evolution of life instead ofresembling a ladder is more similar to a branching bushDarwinrsquos (1859) contribution to phylogenetic analysisindeed was to introduce the concept of a branching tree oflife with organisms related through common ancestry(Fig 1) Each branch on the tree represents a distinctlineage multiple branches can extend from a commonpoint joined by a set of characters present in the commonancestor lineages can also acquire characters that are notshared in a common ancestor Finally as a result of extinc-tion not all lineages persist into the future as can clearly beseen in Fig 1 Tree-thinking shifts the focus from lookingfor fossils of lineal (direct) ancestors to looking forsynapomorphies that link collateral (side-branch) ancestorsYour grandmother is a lineal ancestor your great-aunt acollateral ancestor (Fig 2) but their lives and times wereprobably not that different (Padian and Angielczyk 2007)

Fig 1 The diagram ofdivergence of taxa presentedby Charles Darwin in On theorigin of species (1859)

Evo Edu Outreach (2009) 2310ndash314 311

Fig 2 Sample family tree foran individual (you) showingyour collateral (indicated bydashed lines) and direct or lineal(indicated by solid lines)ancestors

Fig 3 ldquoEvogramrdquo by Brian Swartz and Josh Frankel of UCMP (Padian 2008) printed with permission of Oxford University Press and the Societyfor Integrative and Comparative Biology

312 Evo Edu Outreach (2009) 2310ndash314

which means that information about one provides informa-tion about the other Paleontologists do not expect to findthe direct lineal ancestor of an extant species (Padian andAngielczyk 2007) nor do they expect to recognize a directancestor as ldquothe ancestorrdquo even if they did find it Howeverby understanding the lives and times of the species in itsfamily tree they can understand what its ancestor wouldhave been like

Figure on Adaptive Change

David Attenborough recently declared in a program aboutDarwin that ldquoyoursquod have to be extraordinarily blinkered ifyou didnrsquot stub your toe against the theory of evolution veryearly on in your liferdquo particularly if as a child you collectedfossils (Charles Darwin and the tree of life 2009) The data tosubstantiate many of the major transitions in evolution havebeen amassed Lines of evidence include cladograms fromindependent datasets (ie fossils morphology and mole-cules) stratigraphy and radiometric dating to establishevents in time and paleoecology to interpret ancient envi-ronments Embryology behavior histology geochemistryfunctional morphology physiology and biochemistry alladd to the array of independent lines of evidence used totest evolutionary hypotheses

Padian (2008) suggests depicting these various linesof evidence particularly the ones that are important inmajor evolutionary transitions in a single diagram calledan ldquoevogramrdquo (Fig 3) to help students to understand thatevolutionary research is conducted in an integrative wayand in a way that relies on testing independent lines ofevidence against each other to demonstrate concordanceEvograms have a cladogram as the backbone of thediagram generated using an entirely separate base ofinformation Because a cladogram is basic to understandingevolutionary relationships other types of noncladogramevolutionary diagrams will only confuse students (Catleyand Novick 2008) In Fig 3 fossil evidence of limbstructure is depicted in one row Structures of the fossilsshowing corresponding bones in corresponding colors areinterpreted in the next row Important clade names andimportant synapomorphies can also be added to the nodesof the cladogram (Padian 2008) Evograms integrate thevarious independent lines of evidence providing a moreunited picture of evolution

The challenge is that teachers are generally neithertrained nor equipped to teach this way Indeed tree-thinking has only recently trickled down to high schoolbiology (Goldsmith 2003 Catley 2006 Baum and Offner2008) Most high school and many college introductorybiology texts are woefully lacking in their presentation ofcomprehensive figures integrating phylogenetic analyses

with hypotheses about evolutionary process In fact itappears that many of the diagrams may actually be mis-leading (Catley and Novick 2008) In a recent analysis ofevolutionary diagrams in biology textbooks Catley andNovick (2008) found that even though 72 of the diagramsin their survey of 31 textbooks were cladograms there werestriking differences across grade levels Middle school textshad the fewest cladograms with almost twice as manynoncladograms High school texts included more clado-grams but the ratio of cladograms to noncladograms wasapproximately equal Such high proportions of nonclado-grams are disturbing if these support misconceptions aboutevolutionary processes (Catley and Novick 2008)

I urge teachers to embrace cladistics and tree-thinking as away of examining the natural world and to encourage studentsto make such thinking a habit For a first step the followingwebsites offer basic exercises on cladistics and tree-thinkingEvolution and the Nature of Science Institutes (httpwwwindianaedu~ensiweb) Understanding Evolution (httpevolutionberkeleyedu) What did T rex taste like (httpwwwucmpberkeleyedueducationexplorationstoursTrex)Also recommended are ways of teaching about evolutionconsistent with modern practice in systematics using ldquoances-tralrdquo and ldquoderivedrdquo instead of ldquoprimitiverdquo and ldquoadvancedrdquowhen referring to organisms and characters which promotesthinking about evolution as a tree rather than a ladder (andalso follows the good example of Darwin who admonishedhimself ldquoNever use the words higher or lowerrdquo) includingexercises that define clades by synapomorphies and map suchcharacters onto phylogenies (see Staub et al 2006 and ENSIMaking Cladograms) and clarifying that a cladogram orphylogenetic tree is a hypothesis given the data and that anode on a cladogram is a hypothetical ancestor not an actualancestor for which we should expect to find a fossil repre-sentative To attain scientific literacy students need to learnthat evolution is a branching process that paleontologists arenot searching for missing links but for transitional featuresand that evolutionary research is conducted in a way thatenables scientists to use independent lines of evidence toconverge on robust conclusions about the history of lifeTeachers need to learn these lessons in order to be able toimpart them

References

Baum D Offner S Phylogenies and tree-thinking Am Biol Teach 200870(4)222ndash9 doi1016620002-7685(2008)70[222PT]20CO2

Catley KM Darwinrsquos missing linkmdasha novel paradigm for evolutioneducation Sci Educ 200690767ndash83 doi101002sce20152

Catley KM Novick L Seeing the wood for the trees an analysis ofevolutionary diagrams in biology textbooks Bioscience200858969ndash87 doi101641B581011

Changing lives (2003) httpwwwchanginglivesonlineorgevolutionhtml

Evo Edu Outreach (2009) 2310ndash314 313

Fig 2 Sample family tree foran individual (you) showingyour collateral (indicated bydashed lines) and direct or lineal(indicated by solid lines)ancestors

Fig 3 ldquoEvogramrdquo by Brian Swartz and Josh Frankel of UCMP (Padian 2008) printed with permission of Oxford University Press and the Societyfor Integrative and Comparative Biology

312 Evo Edu Outreach (2009) 2310ndash314

which means that information about one provides informa-tion about the other Paleontologists do not expect to findthe direct lineal ancestor of an extant species (Padian andAngielczyk 2007) nor do they expect to recognize a directancestor as ldquothe ancestorrdquo even if they did find it Howeverby understanding the lives and times of the species in itsfamily tree they can understand what its ancestor wouldhave been like

Figure on Adaptive Change

David Attenborough recently declared in a program aboutDarwin that ldquoyoursquod have to be extraordinarily blinkered ifyou didnrsquot stub your toe against the theory of evolution veryearly on in your liferdquo particularly if as a child you collectedfossils (Charles Darwin and the tree of life 2009) The data tosubstantiate many of the major transitions in evolution havebeen amassed Lines of evidence include cladograms fromindependent datasets (ie fossils morphology and mole-cules) stratigraphy and radiometric dating to establishevents in time and paleoecology to interpret ancient envi-ronments Embryology behavior histology geochemistryfunctional morphology physiology and biochemistry alladd to the array of independent lines of evidence used totest evolutionary hypotheses

Padian (2008) suggests depicting these various linesof evidence particularly the ones that are important inmajor evolutionary transitions in a single diagram calledan ldquoevogramrdquo (Fig 3) to help students to understand thatevolutionary research is conducted in an integrative wayand in a way that relies on testing independent lines ofevidence against each other to demonstrate concordanceEvograms have a cladogram as the backbone of thediagram generated using an entirely separate base ofinformation Because a cladogram is basic to understandingevolutionary relationships other types of noncladogramevolutionary diagrams will only confuse students (Catleyand Novick 2008) In Fig 3 fossil evidence of limbstructure is depicted in one row Structures of the fossilsshowing corresponding bones in corresponding colors areinterpreted in the next row Important clade names andimportant synapomorphies can also be added to the nodesof the cladogram (Padian 2008) Evograms integrate thevarious independent lines of evidence providing a moreunited picture of evolution

The challenge is that teachers are generally neithertrained nor equipped to teach this way Indeed tree-thinking has only recently trickled down to high schoolbiology (Goldsmith 2003 Catley 2006 Baum and Offner2008) Most high school and many college introductorybiology texts are woefully lacking in their presentation ofcomprehensive figures integrating phylogenetic analyses

with hypotheses about evolutionary process In fact itappears that many of the diagrams may actually be mis-leading (Catley and Novick 2008) In a recent analysis ofevolutionary diagrams in biology textbooks Catley andNovick (2008) found that even though 72 of the diagramsin their survey of 31 textbooks were cladograms there werestriking differences across grade levels Middle school textshad the fewest cladograms with almost twice as manynoncladograms High school texts included more clado-grams but the ratio of cladograms to noncladograms wasapproximately equal Such high proportions of nonclado-grams are disturbing if these support misconceptions aboutevolutionary processes (Catley and Novick 2008)

I urge teachers to embrace cladistics and tree-thinking as away of examining the natural world and to encourage studentsto make such thinking a habit For a first step the followingwebsites offer basic exercises on cladistics and tree-thinkingEvolution and the Nature of Science Institutes (httpwwwindianaedu~ensiweb) Understanding Evolution (httpevolutionberkeleyedu) What did T rex taste like (httpwwwucmpberkeleyedueducationexplorationstoursTrex)Also recommended are ways of teaching about evolutionconsistent with modern practice in systematics using ldquoances-tralrdquo and ldquoderivedrdquo instead of ldquoprimitiverdquo and ldquoadvancedrdquowhen referring to organisms and characters which promotesthinking about evolution as a tree rather than a ladder (andalso follows the good example of Darwin who admonishedhimself ldquoNever use the words higher or lowerrdquo) includingexercises that define clades by synapomorphies and map suchcharacters onto phylogenies (see Staub et al 2006 and ENSIMaking Cladograms) and clarifying that a cladogram orphylogenetic tree is a hypothesis given the data and that anode on a cladogram is a hypothetical ancestor not an actualancestor for which we should expect to find a fossil repre-sentative To attain scientific literacy students need to learnthat evolution is a branching process that paleontologists arenot searching for missing links but for transitional featuresand that evolutionary research is conducted in a way thatenables scientists to use independent lines of evidence toconverge on robust conclusions about the history of lifeTeachers need to learn these lessons in order to be able toimpart them

References

Baum D Offner S Phylogenies and tree-thinking Am Biol Teach 200870(4)222ndash9 doi1016620002-7685(2008)70[222PT]20CO2

Catley KM Darwinrsquos missing linkmdasha novel paradigm for evolutioneducation Sci Educ 200690767ndash83 doi101002sce20152

Catley KM Novick L Seeing the wood for the trees an analysis ofevolutionary diagrams in biology textbooks Bioscience200858969ndash87 doi101641B581011

Changing lives (2003) httpwwwchanginglivesonlineorgevolutionhtml

Evo Edu Outreach (2009) 2310ndash314 313

which means that information about one provides informa-tion about the other Paleontologists do not expect to findthe direct lineal ancestor of an extant species (Padian andAngielczyk 2007) nor do they expect to recognize a directancestor as ldquothe ancestorrdquo even if they did find it Howeverby understanding the lives and times of the species in itsfamily tree they can understand what its ancestor wouldhave been like

Figure on Adaptive Change

David Attenborough recently declared in a program aboutDarwin that ldquoyoursquod have to be extraordinarily blinkered ifyou didnrsquot stub your toe against the theory of evolution veryearly on in your liferdquo particularly if as a child you collectedfossils (Charles Darwin and the tree of life 2009) The data tosubstantiate many of the major transitions in evolution havebeen amassed Lines of evidence include cladograms fromindependent datasets (ie fossils morphology and mole-cules) stratigraphy and radiometric dating to establishevents in time and paleoecology to interpret ancient envi-ronments Embryology behavior histology geochemistryfunctional morphology physiology and biochemistry alladd to the array of independent lines of evidence used totest evolutionary hypotheses

Padian (2008) suggests depicting these various linesof evidence particularly the ones that are important inmajor evolutionary transitions in a single diagram calledan ldquoevogramrdquo (Fig 3) to help students to understand thatevolutionary research is conducted in an integrative wayand in a way that relies on testing independent lines ofevidence against each other to demonstrate concordanceEvograms have a cladogram as the backbone of thediagram generated using an entirely separate base ofinformation Because a cladogram is basic to understandingevolutionary relationships other types of noncladogramevolutionary diagrams will only confuse students (Catleyand Novick 2008) In Fig 3 fossil evidence of limbstructure is depicted in one row Structures of the fossilsshowing corresponding bones in corresponding colors areinterpreted in the next row Important clade names andimportant synapomorphies can also be added to the nodesof the cladogram (Padian 2008) Evograms integrate thevarious independent lines of evidence providing a moreunited picture of evolution

The challenge is that teachers are generally neithertrained nor equipped to teach this way Indeed tree-thinking has only recently trickled down to high schoolbiology (Goldsmith 2003 Catley 2006 Baum and Offner2008) Most high school and many college introductorybiology texts are woefully lacking in their presentation ofcomprehensive figures integrating phylogenetic analyses

with hypotheses about evolutionary process In fact itappears that many of the diagrams may actually be mis-leading (Catley and Novick 2008) In a recent analysis ofevolutionary diagrams in biology textbooks Catley andNovick (2008) found that even though 72 of the diagramsin their survey of 31 textbooks were cladograms there werestriking differences across grade levels Middle school textshad the fewest cladograms with almost twice as manynoncladograms High school texts included more clado-grams but the ratio of cladograms to noncladograms wasapproximately equal Such high proportions of nonclado-grams are disturbing if these support misconceptions aboutevolutionary processes (Catley and Novick 2008)

I urge teachers to embrace cladistics and tree-thinking as away of examining the natural world and to encourage studentsto make such thinking a habit For a first step the followingwebsites offer basic exercises on cladistics and tree-thinkingEvolution and the Nature of Science Institutes (httpwwwindianaedu~ensiweb) Understanding Evolution (httpevolutionberkeleyedu) What did T rex taste like (httpwwwucmpberkeleyedueducationexplorationstoursTrex)Also recommended are ways of teaching about evolutionconsistent with modern practice in systematics using ldquoances-tralrdquo and ldquoderivedrdquo instead of ldquoprimitiverdquo and ldquoadvancedrdquowhen referring to organisms and characters which promotesthinking about evolution as a tree rather than a ladder (andalso follows the good example of Darwin who admonishedhimself ldquoNever use the words higher or lowerrdquo) includingexercises that define clades by synapomorphies and map suchcharacters onto phylogenies (see Staub et al 2006 and ENSIMaking Cladograms) and clarifying that a cladogram orphylogenetic tree is a hypothesis given the data and that anode on a cladogram is a hypothetical ancestor not an actualancestor for which we should expect to find a fossil repre-sentative To attain scientific literacy students need to learnthat evolution is a branching process that paleontologists arenot searching for missing links but for transitional featuresand that evolutionary research is conducted in a way thatenables scientists to use independent lines of evidence toconverge on robust conclusions about the history of lifeTeachers need to learn these lessons in order to be able toimpart them

References

Baum D Offner S Phylogenies and tree-thinking Am Biol Teach 200870(4)222ndash9 doi1016620002-7685(2008)70[222PT]20CO2

Catley KM Darwinrsquos missing linkmdasha novel paradigm for evolutioneducation Sci Educ 200690767ndash83 doi101002sce20152

Catley KM Novick L Seeing the wood for the trees an analysis ofevolutionary diagrams in biology textbooks Bioscience200858969ndash87 doi101641B581011

Changing lives (2003) httpwwwchanginglivesonlineorgevolutionhtml

Evo Edu Outreach (2009) 2310ndash314 313

Charles Darwin and the tree of life Broadcast on BBC One Sunday 1February 2009 httpwwwbbccoukprogrammesb00hd5mf

Darwin C On the origin of species by means of natural selectionLondon Murray 1859

Goldsmith DW Presenting cladistic thinking to biology majors andgeneral science students Am Biol Teach 200365(9)679ndash92

Hennig W Phylogenetic systematics (trans DD Davis and RZangerl) Urbana University of Illinois Press 1966

Kidwell SM Holland SM The quality of the fossil record implicationsfor evolutionary analyses Annu Rev Ecol Syst 200233561ndash88doi101146annurevecolsys33030602152151

Padian K Trickle-down evolution an approach to getting majorevolutionary adaptive changes into textbooks and curriculaIntegr Comp Biol 200848175ndash88 doi101093icbicn023

Padian K Angielczyk KD ldquoTransitionalrdquo forms versus transitionalfeatures In Petto AJ Godfrey LR editors Scientists confront

intelligent design and creationism New York Norton 2007p 197ndash230

Prothero DR Evolution what the fossils say and why it matters NewYork Columbia University Press 2007

Ruse M Monad to man the concept of progress evolutionary biologyCambridge Harvard University Press 1997

Staub NL Pauw PG Pauw D Seeing the forest through the treeshelping students appreciate lifersquos diversity by building the tree oflife Am Biol Teach 200668(3)49ndash51 doi1016620002-7685(2006)68[149STFTTT]20CO2

Texas State Board of EducationMeeting 22 January 2009 Amendmentsof Don McLeroy httpwwwtexscienceorgpdfmcleroy-biology-amendmentspdf

Vardiman L In Ashton JF editor In six days why 50 scientists chooseto believe in creationism Green Forest AR Master Books 2003p 327ndash328

314 Evo Edu Outreach (2009) 2310ndash314