Phylogeny and the Tree of Life

Transcription

1 Chapter 26 Phylgeny and the Tree f Life Lecture Outline Overview: Investigating the Tree f Life Evlutinary bilgy is abut bth prcess and pattern. The prcesses f evlutin are natural selectin and ther mechanisms that change the genetic cmpsitin f ppulatins. These prcesses lead t evlutinary patterns, the prducts that evlutin has prduced ver time. Phylgeny is the evlutinary histry f a species r grup f species. T recnstruct phylgeny, scientists use systematics, an analytical apprach t classifying the diversity and determining the evlutinary relatinships f living and extinct rganisms. Evidence used t recnstruct phylgenies can be btained frm the fssil recrd and frm mrphlgical, bichemical, and genetic similarities between rganisms. Scientists are wrking t cnstruct a universal tree f all life, which will be refined as new data are cllected. Cncept 26.1 Phylgenies shw evlutinary relatinships. Organisms share hmlgus characteristics due t cmmn descent. Organisms share genes, metablic pathways, and structural pathways with their clse relatives. The scientific discipline f taxnmy determines hw rganisms are named and classified. Taxnmy emplys a hierarchical system f classificatin. The Linnaean system, first frmally prpsed by Carlus Linnaeus in the 18 th century, has tw main characteristics. 1. Each species has a tw-part name. 2. Species are rganized hierarchically int brader and brader grups f rganisms. Under the binmial system, each species is assigned a tw-part Latinized name, a binmial. The first part f the name, the genus, is the clsest grup t which a species belngs. The secnd part, the specific epithet, refers t ne species within each genus. The first letter f the genus name is capitalized, and bth the genus and species names are italicized and Latinized. Fr example, Linnaeus assigned t humans the ptimistic scientific name Hm sapiens, which means wise man. A hierarchical classificatin grups species int increasingly inclusive taxnmic categries. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 26-1

2 Species that appear t be clsely related are gruped int the same genus. Fr example, the lepard, Panthera pardus, belngs t a genus that includes the African lin (Panthera le) and the tiger (Panthera tigris). Genera are gruped int prgressively brader categries: family, rder, class, phylum, kingdm, and dmain. Each taxnmic level is mre cmprehensive than the previus ne. As an example, all species f cats are mammals, but nt all mammals are cats. The named taxnmic unit at any level is called a taxn. Fr example, Panthera is a taxn at the genus level, and Mammalia is a taxn at the class level that includes all f the many rders f mammals. Higher classificatin levels are nt defined by sme measurable characteristic, such as the reprductive islatin that separates bilgical species. As a result, the larger categries are nt cmparable between lineages. An rder f snails des nt necessarily exhibit the same degree f mrphlgical r genetic diversity as an rder f mammals. Classificatin and phylgeny are linked. The evlutinary histry f a grup f rganisms can be represented in a diagram called a phylgenetic tree. The branching f the tree may match the hierarchical classificatin f grups nested within mre inclusive grups. In ther cases, similarities between rganisms may have led taxnmists t place a species in ther than the grup t which it is mst clsely related. Such species may be renamed s that their classificatin accurately reflects evlutinary histry. The difficulty f aligning Linnaean classificatin with phylgeny has led systematists t prpse that classificatin be based entirely n evlutinary relatinships. Phylcde nly names grups that include a cmmn ancestr and all its descendents. If this system were adpted, mst taxnmic names wuld be unchanged. Hwever, ranks such as family, rder, and class wuld be abandned. Sme cmmnly recgnized grups wuld becme part f ther grups previusly f the same rank. Because birds descended frm a taxn f reptiles, Aves (a Linnaean class) wuld be cnsidered a subgrup f the Reptilia (anther Linnaean class). A phylgenetic tree represents a hypthesis abut evlutinary relatinships. Evlutinary relatinships are ften represented as a series f dichtmies, r tw-way branch pints. Each branch pint represents the divergence f tw evlutinary lineages frm a cmmn ancestr. Sister taxa are grups f rganisms that share an immediate cmmn ancestr and are each ther s clsest relatives. A rted tree includes the mst recent cmmn ancestr t all taxa in the tree. The term basal taxn refers t a lineage that diverges early in the histry f a grup, lying n a branch that riginates near the cmmn ancestr f the grup. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 26-2

3 A plytmy, r branch pint frm which mre than tw descendent grups emerge, indicates ur limited understanding f certain evlutinary relatinships. Three key pints abut phylgenetic trees shuld be emphasized. 1. Phylgenetic trees are intended t shw patterns f descent, nt phentypic similarity. Clsely related rganisms may resemble ne anther due t cmmn ancestry, but may nt if their lineages have evlved at different rates r faced very different envirnmental cnditins. 2. The sequence f branching in a tree reflects patterns f descent and des nt indicate the abslute ages f particular species. 3. A taxn in a phylgenetic tree did nt evlve frm an adjacent taxn. Rather, bth taxa evlved frm a cmmn ancestr. A species phylgeny can prvide useful infrmatin. Frm a phylgeny f crn based n DNA data, researchers have identified tw clsely related species f wild grasses that may serve as reservirs f beneficial alleles. These alleles may be transferred t cultivated crn by plant breeding r genetic engineering. Phylgenetic trees played a rle in demnstrating that whale meat sld in Japan was illegally harvested frm prtected species. Cncept 26.2 Phylgenies are inferred frm mrphlgical and mlecular data. Phentypic and genetic similarities due t shared ancestry are called hmlgies. Organisms that share similar mrphlgies r DNA sequences are likely t be mre clsely related than rganisms withut such similarities. Mrphlgical divergence between clsely related species can be small r great, because mrphlgical diversity may be cntrlled by relatively few genetic differences. Cnsider the Hawaiian silverswrd plants, which vary frm tall, twiggy trees t dense, grundhugging shrubs. These phentypic differences are based n small mlecular divergences that arse ver the last 5 millin years, when the ldest f the Hawaiian Islands frmed. Similarity due t cnvergent evlutin is called analgy. When tw rganisms frm different evlutinary lineages experience similar envirnmental pressures, natural selectin may result in cnvergent evlutin. Fr example, marsupial and eutherian mles are very similar in external appearance. Hwever, they last shared a cmmn ancestr 140 millin years ag, when marsupial and eutherian mammals diverged. This cmmn ancestr was nt mle-like. Analgus similarities evlved independently in these tw mle lineages as they adapted t similar lifestyles. Distinguishing hmlgy frm analgy is critical in the recnstructin f phylgeny. Fr example, bth birds and bats have adaptatins that allw them t fly. A clse examinatin f a bat s wing shws a greater similarity t a cat s frelimb than t a bird s wing. Fssil evidence als dcuments that bat and bird wings arse independently frm walking frelimbs f different ancestrs. Thus, a bat s wing is hmlgus t ther mammalian frelimbs but is analgus in functin t a bird s wing. Analgus structures that have evlved independently are als called hmplasies. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 26-3

4 In general, the mre elements that are similar in tw cmplex structures, the mre likely it is that they evlved frm a cmmn ancestr. Fr example, the skulls f an adult human and an adult chimpanzee are frmed by the fusin f many bnes. The tw skulls match almst perfectly, bne fr bne. It is highly unlikely that such cmplex structures have separate rigins. Mre likely, the genes invlved in the develpment f bth skulls were inherited frm a cmmn ancestr. The same argument applies t cmparing genes, which are sequences f nucletides. If the genes in tw rganisms have very similar nucletide sequences, it is highly likely that the genes are hmlgus. It may be difficult t carry ut mlecular cmparisns f nucleic acids. The first step in mlecular cmparisns is t align nucleic acid sequences frm the tw species being studied. In clsely related species, sequences may differ at nly ne r a few sites. Distantly related species may have many differences r sequences f different length. Over evlutinary time, insertins and deletins may accumulate, altering the lengths f the gene sequences. Deletins r insertins may shift the remaining sequences, making it difficult t recgnize clsely matching nucletide sequences. T deal with this, researchers use cmputer prgrams t analyze cmparable DNA sequences f differing lengths and align them apprpriately. The fact that mlecules have diverged between species des nt tell scientists hw lng ag their cmmn ancestr lived. Mlecular divergences between lineages with reasnably cmplete fssil recrds can serve as a mlecular yardstick t measure the apprpriate time span f varius degrees f divergence. As with mrphlgical characters, it is necessary t distinguish hmlgy frm analgy t determine the usefulness f mlecular similarities fr recnstructin f phylgenies. Very similar sequences are mst likely hmlgies. In distantly related rganisms, identical bases in therwise different sequences may simply be cincidental matches r mlecular hmplasies. Scientists have develped mathematical tls that can distinguish distant hmlgies frm cincidental matches in extremely divergent sequences. Mlecular systematics uses DNA and ther mlecular data t determine evlutinary relatinships. Scientists have sequenced mre than 110 billin bases wrth f nucleic acid data frm thusands f species. The new data have supprted hyptheses abut many evlutinary relatinships and have clarified thers. Cncept 26.3 Shared characters are used t cnstruct phylgenetic trees. In an apprach t systematics called cladistics, cmmn descent is the primary criterin used t classify rganisms. Bilgists place species int grups called clades, each f which includes an ancestral species and all f its descendents. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 26-4

5 Clades are nested within larger clades. A valid clade is mnphyletic, cnsisting f an ancestral species and all its descendents. When bilgists lack infrmatin abut sme members f a clade, the result may be a paraphyletic gruping that cnsists f sme, but nt all, f the descendents. The result may als be several plyphyletic grupings that include taxa with different ancestrs. Such situatins call fr further recnstructin t uncver species that tie these grupings tgether int mnphyletic clades. Due t descent with mdificatin, rganisms share sme, but nt all, characteristics with their ancestrs. Systematists must srt thrugh hmlgus features, r characters, t separate shared derived characters frm shared ancestral characters. A character is any feature that a particular taxn pssesses. A shared derived character is an evlutinary nvelty unique t a particular clade. A shared ancestral character riginated in an ancestr f the clade. Fr example, the presence f hair is a gd character t use t distinguish the clade f mammals frm ther vertebrates. Hair is a shared derived character that uniquely identifies mammals. The presence f a backbne can qualify as a shared derived character, but at a deeper branch pint that distinguishes all vertebrates frm ther mammals. Amng vertebrates, the backbne is a shared ancestral character because it evlved in the ancestr cmmn t all vertebrates. Shared derived characters are useful in establishing a phylgeny, but shared ancestral characters are nt. The status f a shared derived character versus a shared ancestral character is relative, depending n the level at which the analysis is being perfrmed. Phylgenies can be inferred frm infrmatin abut shared derived characters. A key step in cladistic analysis is utgrup cmparisn, which is used t differentiate shared ancestral characters frm shared derived characters. T d utgrup cmparisn, scientists need t identify an utgrup, a species r grup f species frm an evlutinary lineage that is knwn t have diverged befre the lineage that includes the species we are studying (the ingrup). A suitable utgrup can be determined based n evidence frm mrphlgy, palentlgy, embrynic develpment, and gene sequences. T study the relatinships amng an ingrup f five vertebrates (a lepard, a turtle, a salamander, a tuna, and a lamprey) n a cladgram, an animal called the lancelet is a gd chice. The lancelet is a small member f the phylum Chrdata that lacks a backbne. The presence f a backbne, shared by all members f the ingrup but nt the utgrup, is a useful character fr the whle ingrup. The presence f hinged jaws, absent in lampreys and present in the ther ingrup taxa, helps t identify the earliest branch in the vertebrate cladgram. In sme phylgenetic trees, branch lengths are prprtinal t the rates f phylgenetic change r the times at which given events ccurred. In many phylgenetic trees, the branch lengths f the tree d nt have specific meaning. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 26-5

6 The chrnlgy represented by the branching pattern f the tree is relative (earlier vs. later) instead f abslute (hw many millins f years ag). In ther trees, the branch length reflects the number f changes that have taken place in a particular DNA sequence in that lineage. Fr example, if the ttal length f the hrizntal lines frm the base f such a tree t a muse is shrter than the length f the line leading t the fruit fly Drsphila, mre genetic changes have ccurred in the Drsphila lineage than in the muse lineage since the time the lineages leading t muse and fly diverged frm a cmmn ancestr. All the different lineages that have descended frm the cmmn ancestr t all living things have survived fr the same number f years. Humans and bacteria share a cmmn ancestr that lived mre than 3 billin years ag. Fssils and genetic evidence indicate that this ancestr was a single-celled prkaryte. Even thugh bacteria have changed little in their mrphlgy since that cmmn ancestr, there has been 3 billin years f change in bth the bacterial lineage and the eukarytic lineage that includes humans. The principles f maximum parsimny and maximum likelihd help systematists recnstruct phylgeny. As available data abut DNA sequences increase, it becmes mre difficult t draw the phylgenetic tree that best describes evlutinary histry. If yu are analyzing data fr 50 species, there are different ways t frm a tree. Accrding t the principle f maximum parsimny, scientists shuld lk fr the simplest explanatin that is cnsistent with the facts. In the case f a tree based n mrphlgical characters, the mst parsimnius tree is the ne that requires the fewest evlutinary events t have ccurred in the frm f shared derived characters. Fr phylgenies based n DNA sequences, the mst parsimnius tree requires the fewest base changes. The principle f maximum likelihd states that, given certain prbability rules abut hw DNA sequences change ver time, a tree shuld reflect the mst likely sequence f evlutinary events. Maximum-likelihd methds are cmplex and designed t use as much infrmatin as pssible. Cnsider tw pssible, equally parsimnius trees. A tree that assumes that DNA changes have ccurred at equal rates alng all branches f the tree frm the cmmn ancestr is mre likely than a tree that assumes widely different rates f evlutin in different branches. Many cmputer prgrams have been develped t search fr trees that are parsimnius and likely. Althugh we can never be certain precisely which tree truly reflects phylgeny, if the trees are based n a large amunt f accurate data, the varius methds usually yield similar trees. Phylgenetic trees are hyptheses. Any phylgenetic tree represents a hypthesis abut hw the rganisms in the tree are related. The best hypthesis is the ne that best fits all the available data. Scientists can make and test predictins based n the assumptin that a phylgeny is crrect. Fr example, in an apprach knwn as phylgenetic bracketing, scientists can predict by parsimny that features shared by tw grups f clsely related ancestrs are present in their cmmn ancestr and all f its descendents. Evidence suggests that birds descended frm a grup f bipedal Saurischian therpd dinsaurs. The clsest living relatives f birds are crcdiles. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 26-6

7 Birds and crcdiles share a number f features: Bth have fur-chambered hearts. Bth sing t defend territries and attract mates. Bth brd their eggs, birds by sitting n them and crcdiles by cvering their eggs with their neck. Based n these bservatins, bilgists predict that dinsaurs had fur-chambered hearts, sang, and brded eggs in nests. The fssil recrd des nt prvide evidence f dinsaur heart structure r singing behavir. Hwever, fssilized dinsaur nests have been fund with fssilized adults cruched ver the eggs in a brding psture. This evidence prvides independent data supprting the hypthesis that birds descended frm dinsaurs. Cncept 26.4 An rganism s evlutinary histry is dcumented in its genme. Mlecular systematics is a valuable apprach fr tracing an rganism s evlutinary histry. The mlecular apprach helps scientists understand phylgenetic relatinships that cannt be measured by cmparative anatmy and ther nnmlecular methds. Fr example, mlecular systematics helps bilgists uncver evlutinary relatinships between grups that have little grund fr mrphlgical cmparisn, such as animals and fungi. This apprach allws bilgists t cnstruct phylgenies amng grups f living rganisms with n fssil recrd. Mlecular bilgy has helped t extend systematics t evlutinary relatinships far abve and belw the species level. The ability f mlecular trees t encmpass bth shrt and lng perids f time is based n the fact that different genes evlve at different rates, even in the same evlutinary lineage. Fr example, the DNA that cdes fr ribsmal RNA (rrna) changes relatively slwly, s DNA sequences in these genes can be cmpared t srt ut relatinships between taxa that diverged hundreds f millins f years ag. Studies f rrna sequences indicate that fungi are mre clsely related t animals than t plants. In cntrast, mitchndrial DNA (mtdna) evlved relatively recently and can be used t explre recent evlutinary events, such as relatinships between grups within a species. One research team has used mtdna t trace the relatinships between Native American grups. Gene duplicatin has prvided pprtunities fr evlutinary change. Gene duplicatin increases the number f genes in the genme, prviding pprtunities fr further evlutinary change. Repeated duplicatins have resulted in gene families, grups f related genes within an rganism s genme. There are tw types f hmlgus genes: rthlgus genes and paralgus genes. Orthlgus genes are hmlgus genes fund in different species because f speciatin. The cytchrme c genes (which cde fr an electrn transprt chain prtein) in humans and dgs are rthlgus. Paralgus genes result frm gene duplicatin and are fund in mre than ne cpy in the same genme. Olfactry receptr genes have undergne many gene duplicatins in vertebrates. Humans and mice each have huge families f mre than 1,000 f these paralgus genes. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 26-7

8 Orthlgus genes diverge after speciatin and are fund in separate gene pls. Fr example, althugh the cytchrme c genes in humans and dgs serve a similar functin, the gene s sequence in humans has diverged frm that in dgs in the time since these species last shared a cmmn ancestr. Paralgus genes can diverge within a species because they are present in mre than ne cpy in the genme. The paralgus genes f the human lfactry receptr gene family have diverged frm each ther during ur lng evlutinary histry. They nw specify prteins that cnfer sensitivity t a wide variety f mlecules, ranging frm fd drs t sex phermnes. Cmparisn f genmes f different rganisms prvides infrmatin abut genme evlutin. Nw that we have cmpared entire genmes f different rganisms, tw remarkable patterns have emerged. Lineages that diverged lng ag can share rthlgus genes. Apprximately 99% f the genes f humans and mice are detectably rthlgus, and 50% f human genes are rthlgus with thse f yeast. All living things share many bichemical and develpment pathways. The number f genes seems nt t have increased at the same rate as phentypic cmplexity. Humans have nly fur times as many genes as yeast, a simple unicellular eukaryte, althugh we have a large, cmplex brain and a bdy that cntains mre than 200 different types f tissues. Many human genes are mre versatile than thse f yeast and can carry ut a wide variety f tasks in varius bdy tissues. Cncept 26.5 Mlecular clcks help track evlutinary time. One f the gals f evlutinary bilgy is t understand the relatinships amng all rganisms, including thse fr which there is n fssil recrd. Mlecular clcks serve as yardsticks fr measuring the abslute time f evlutinary change. Mlecular clcks are based n the bservatin that sme genes and ther regins f the genme evlve at cnstant rates. Fr these regins, the number f nucletide substitutins in rthlgus genes is prprtinal t the divergence time, the time that has elapsed since the tw species shared a cmmn ancestr. In the case f paralgus genes, the number f substitutins is prprtinal t the time since the genes became duplicated. Scientists calibrate the mlecular clck f a gene by graphing the number f genetic differences (nucletide, cdn, r amin acid differences) against the dates f evlutinary branch pints that are knwn frm the fssil recrd. The average rate f genetic change inferred frm such a graph can be used t estimate the abslute date f evlutinary events that have n fssil recrd. N mlecular clck is cmpletely accurate. Genes that make gd mlecular clcks have fairly smth average rates f change. Over time, there may be chance deviatins abve and belw the average rate. Rates f change f varius genes vary greatly. Sme genes evlve a millin times faster than thers. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 26-8

9 Many f the changes in genes used as mlecular clcks are assumed t be selectively neutral, fixed in a ppulatin by genetic drift. The mlecular clck apprach assumes that much f the change in DNA sequences is due t genetic drift and is selectively neutral. The neutral thery suggests that much evlutinary change in genes and prteins has n effect n fitness and, therefre, is nt influenced by natural selectin. Supprters f this thery pint ut that many new mutatins are harmful and are remved quickly. Hwever, if mst ther mutatins are neutral and have little r n effect n fitness, then the rate f mlecular change shuld be clcklike in its regularity. Differences in the rates f change f specific genes are a functin f the imprtance f the gene. If the exact sequence f amin acids specified by a gene is essential t survival, mst mutatins will be harmful and will be remved by natural selectin. If the sequence f genes is less critical, mre mutatins will be neutral, and mutatins will accumulate mre rapidly. Sme DNA changes are favred by natural selectin, which leads sme scientists t questin the accuracy and utility f mlecular clcks fr timing evlutin. Evidence suggests that almst 50% f the amin acid differences in prteins f tw Drsphila species have resulted frm directinal natural selectin. Over very lng perids f time, fluctuatins in the rate f accumulatin f mutatins due t natural selectin may even ut. Even genes with irregular clcks can serve as apprximate markers f elapsed time. Bilgists are skeptical f cnclusins derived frm mlecular clcks that have been extraplated t time spans beynd the calibratin in the fssil recrd Few fssils are lder than 550 millin years ld. Estimates fr evlutinary divergences prir t that time may assume that mlecular clcks have been cnstant ver billins f years. Such estimates have a high degree f uncertainty. Mlecular clcks may be calibrated with many genes, rather than ne r a few. When many genes are used, fluctuatins in the evlutinary rate due t natural selectin r ther factrs that vary ver time may average ut. One grup f researchers cnstructed mlecular clcks f vertebrate evlutin frm published data fr 658 nuclear genes. The mlecular and fssil-based estimates f divergence times agreed clsely. The mlecular clck apprach has been used t date the jump f the HIV virus frm related viruses that infect chimpanzees and ther primates t humans. The virus has spread t humans mre than nce. The multiple rigins f HIV are reflected in the variety f strains f the virus. HIV s genetic material is RNA, which evlves quickly. HIV-1 M is the mst cmmn HIV strain. Investigatrs have calibrated the mlecular clck fr the virus by cmparing samples f the virus cllected at varius times, including a sample frm Frm their analysis, researchers prject that the HIV-1 M strain invaded humans in the 1930s. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 26-9

10 Cncept 26.6 New infrmatin cntinues t revise ur understanding f the tree f life. Early taxnmists classified all knwn species int tw kingdms: plants and animals. In the late 1960s, a five-kingdm system was widely adpted. The kingdms were Mnera (prkarytes), Prtista (primarily unicellular eukarytes), Plantae, Fungi, and Animalia. Mnera included all prkarytic cells. Eukarytic cells were divided amng the ther five kingdms. Phylgenies based n genetic data revealed a fundamental prblem with the five-kingdm system: Sme prkarytes differed as much frm each ther as they did frm eukarytes. Bilgists have nw adpted a three-dmain system. The three dmains Bacteria, Archaea, and Eukarya are at a taxnmic level higher than the kingdm level. Analyses f nearly 100 cmpletely sequenced genmes supprt the validity f these dmains. The dmain Bacteria cntains mst f the currently knwn prkarytes, including the bacteria mst clsely related t chlrplasts and mitchndria. The dmain Archaea cnsists f a diverse grup f prkarytes inhabiting a wide variety f envirnments. Bacteria and archaea differ in many key structural, bichemical, and physilgical characteristics. Sme archaea can use hydrgen as an energy surce, and thers were the chief surce f the natural gas depsits that are fund thrughut Earth s crust. The dmain Eukarya includes all rganisms with cells cntaining true nuclei. Of the five kingdms previusly recgnized by taxnmists, Plantae, Fungi, and Animalia are used. The kingdm Mnera, which included rganisms in tw different dmains, is bslete. The kingdm Prtista is plyphyletic, including rganisms that are mre clsely related t plants, animals, r fungi than t ther prtists. Much f the histry f life has been abut single-celled rganisms. Mst evlutinary branches n the tree f life are dminated by single-celled rganisms. The first majr split in the histry f life ccurred when the bacteria diverged frm the ther tw dmains. Eukarytes and archaea are mre clsely related t each ther than either is t bacteria. The tree f all life is based largely n rrna genes, which cde fr the RNA within ribsmes. These genes have evlved s slwly that hmlgies between distantly related rganisms can be detected. Other genes reveal a different set f relatinships. Many f the metablic genes in yeast are mre similar t bacterial genes than t archaea genes. This finding suggests a mre recent cmmn ancestr between eukarytes and bacteria. Early in the histry f life, there were many interchanges f genes between rganisms in different dmains. These interchanges tk place thrugh hrizntal gene transfer, in which genes are transferred frm ne genme t anther by mechanisms such as transpsable elements, plasmid exchange, and viral infectin. In sme cases, different rganisms fused t prduce new, hybrid rganisms. Recent research reinfrces the view that hrizntal gene transfer is imprtant. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc

11 A 2008 analysis indicated that, n average, 80% f the genes in 181 prkarytic genmes had mved between species at sme pint during the curse f evlutin. The ccurrence f hrizntal gene transfers helps explain why universal genes built frm different genes give incnsistent results. Sme scientists suggest that hrizntal gene transfers were s cmmn that the early histry f life shuld be represented as a tangled netwrk instead f a dichtmusly branched tree. Other scientists have suggested that the histry f life shuld be represented by a ring, nt a tree. An analysis based n hundreds f genes supprts the hypthesis that eukarytes arse as a fusin between an early bacterium and an early archaean. If eukarytes arse as an endsymbisis between an early bacterium and an early archaean, then eukarytes are related equally clsely t bacteria and archaea. Such an evlutinary relatinship cannt be depicted in a tree f life, but can be depicted in a ring f life. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc