An Introduction to Animal Diversity

Transcription

1 Chapter 32 An Intrductin t Animal Diversity Lecture Outline Overview: Welcme t Yur Kingdm Bilgists have identified 1.3 millin living species f animals. Estimates f the ttal number f animal species run far higher. This diversity encmpasses a huge range f mrphlgical diversity. Cncept 32.1 Animals are multicellular, hetertrphic eukarytes with tissues that develp frm embrynic layers There are exceptins t nearly every criterin fr distinguishing an animal frm ther life frms. Several characteristics taken tgether make up a reasnable definitin f an animal. Animals are multicellular, ingestive hetertrphs. Unlike fungi, mst animals d nt feed by absrptin. Instead, animals ingest their fd and then use enzymes t digest it within their bdies. In cntrast, plants are auttrphic eukarytes that generate rganic mlecules by phtsynthesis, while fungi are absrptive hetertrphs. Animal cells lack cell walls that prvide structural supprt fr plants and fungi. The multicellular bdies f animals are held tgether by extracellular structural prteins, especially cllagen, which is unique t animals. Animals have tw specialized types f cells fund nly in animals: nerve cells fr impulse cnductin and muscle cells fr mvement. These cells are rganized int nervus tissue and muscle tissue. Mst animals reprduce sexually, with the diplid stage usually dminating the life cycle. In mst animals, a small flagellated sperm fertilizes a larger, nnmtile egg. The zygte underges cleavage, a successin f mittic cell divisins, leading t the frmatin f a multicellular stage called the blastula, which takes the shape f a hllw ball in many animals. During gastrulatin, layers f embrynic tissues that will develp int adult bdy parts are prduced. The resulting develpment stage is called a gastrula. Sme animals develp directly thrugh transient stages int adults, but mst have a distinct larval stage r stages. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 32-1

2 A larva is a sexually immature stage that is mrphlgically distinct frm the adult stage, usually eats different fds, and may live in a different habitat frm the adult. Animal larvae eventually underg metamrphsis, transfrming the animal int an adult. Animals share a unique hmebx-cntaining family f genes knwn as Hx genes. Hx genes play imprtant rles in the develpment f animal embrys, cntrlling the expressin f dzens r even hundreds f ther genes that influence animal mrphlgy. Spnges, which are amng the simplest living animals, lack Hx genes. Hwever, spnges have ther hmebx genes that influence their shape, such as thse that regulate the frmatin f water channels in the bdy wall. In the ancestrs f mre cmplex animals, the Hx gene family arse via the duplicatin f earlier hmebx genes. Over time, the Hx gene family underwent a series f duplicatins. In vertebrates, insects, and mst ther animals, Hx genes regulate the frmatin f the anterir-psterir (frnt-t-back) axis, as well as ther aspects f develpment. Similar sets f cnserved genes gvern the develpment f bth a fly and a human, despite their bvius differences and hundreds f millins f years f divergent evlutin. Cncept 32.2 The histry f animals spans mre than half a billin years The animal kingdm includes nt nly a great diversity f living species but als the even greater diversity f extinct nes. Sme palentlgists have estimated that ver 99% f all animal species are extinct. Varius studies suggest that animal diversity riginated during the last billin years. Estimates based n mlecular clcks suggest that the ancestrs f animals diverged frm the ancestrs f fungi abut a billin years ag. Similar studies suggest that the cmmn ancestr f living animals may have lived 675 millin t 800 millin years ag. A cmbinatin f mrphlgical and mlecular evidence indicates that the cmmn ancestrs f living animals may have been a suspensin feeder similar t present-day chanflagellates. Neprterzic Era (1 billin 542 millin years ag) Despite mlecular data that indicate an earlier rigin f animals, the ldest generally accepted animal fssils range in age frm 565 millin t 550 millin years ld. These fssils are knwn as the Ediacara bita, named fr the Ediacara Hills f Australia. Sme f the Ediacara fauna are spnges, while thers may be related t varius grups f living cnidarians. Sme d nt appear t be clsely related t any living alga r animals. Neprterzic rcks have als yielded micrscpic signs f early animals. 575-millin-year-ld micrfssils discvered in China exhibit the basic structural rganizatin f present-day animal embrys. Debate cntinues abut whether the fssil embrys are animals r are members f extinct grups that are clsely related t animals (but are nt actually animals). Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 32-2

3 Palezic Era ( millin years ag) Animals underwent cnsiderable diversificatin between millin years ag, during the Cambrian perid f the Palezic Era. During this perid, knwn as the Cambrian explsin, abut half f extant animal phyla arse, including the first arthrpds, chrdates, and echinderms. Fssils f Cambrian animals include the first animals with hard, mineralized skeletns. The increase in the diversity f bilaterian phyla during the Cambrian was accmpanied by a decline in the diversity f Ediacaran life. What caused these events? There are several current hyptheses regarding the reasn fr the Cambrian explsin: new predatr-prey relatinships, atmspheric changes, and develpmental flexibility. During the Cambrian perid, predatrs acquired nvel adaptatins, such as frms f lcmtin that helped them catch prey, while prey species acquired new defenses, such as prtective shells. As new predatr-prey relatinships emerged, natural selectin may have led t the decline f sme grups and the rise f thers. Increased levels f atmspheric xygen preceded the Cambrian explsin. Mre xygen may have prvided pprtunities fr animals with higher metablic rates and larger bdy sizes. The rigin f Hx genes and ther genetic changes affecting the regulatin f develpmental genes may have prvided the develpmental flexibility that prduced new bdy frms. These hyptheses are nt mutually exclusive: All may have played a rle. In the Ordvician, Silurian, and Devnian perids, animal diversity cntinued t increase, punctuated by episdes f mass extinctin. Vertebrates (fishes) became the tp predatrs f marine fd webs. By 460 millin years ag, arthrpds began t adapt t terrestrial habitats. Millipedes and centipedes appeared n land. Fern galls enlarged cavities that resident insects stimulate fern plants t frm, prviding prtectin fr the insects date back t at least 302 millin years ag, suggesting that insects and plants were influencing each ther s evlutin by that time. Vertebrates mved t land abut 365 millin years ag and diversified int many lineages. Tw f these survive tday: amphibians and amnites. Meszic Era ( millin years ag) Animal phyla began t spread int new eclgical habitats. In the ceans, the first cral reefs frmed, prviding marine animals with new habitats. Sme reptiles returned t the water and succeeded as large aquatic predatrs. On land, flight evlved in ptersaurs and birds. Large and small herbivrus and carnivrus dinsaurs evlved. The first mammals tiny ncturnal insect-eaters arse. During the late Meszic, bth insects and flwering plants underwent dramatic radiatins. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 32-3

4 Cenzic Era (65.5 millin years ag t the present) This era began with mass extinctins f terrestrial and marine animals. Amng the grups f species that disappeared were large, nnflying dinsaurs and marine reptiles. Large mammalian herbivres and carnivres diversified as mammals explited vacated eclgical niches. Sme primate species in Africa adapted t pen wdlands and savannas as glbal climates cled. Our ancestrs were amng these grassland apes. Cncept 32.3 Animals can be characterized by bdy plans Althugh animal species shw great mrphlgical variatin, this diversity can be categrized int a relatively small number f majr bdy plans. A bdy plan is a set f mrphlgical and develpmental traits integrated int a functinal whle the living animal. Bdy plans are f interest in the study f ev-dev, the interface between evlutin and develpment. Like all rganismal features, animal bdy plans have evlved ver time. Sme f the evlutinary changes in bdy plans appear t have ccurred early in the histry f animal life. Fr example, a key step in the mlecular cntrl f gastrulatin has remained unchanged fr mre than 500 millin years. This early evlutinary innvatin was f fundamental imprtance: Gastrulatin helps t explain why mst animals are nt a hllw ball f cells. Other aspects f animal bdy plans have changed multiple times as varius animal lineages evlved and diversified. Similar bdy frms have smetimes evlved independently in tw different lineages. Animals can be categrized by the symmetry (r absence f symmetry) f their bdies. Many spnges lack symmetry. Sme animals, such as sea anemnes, have radial symmetry. These animals have tp (muth) and bttm sides, but nt frnt and back ends and n left and right sides. Many animals have bilateral symmetry. A bilateral animal has tw axes f rientatin: frnt t back and tp t bttm. Bilaterally symmetrical animals have drsal (tp) and ventral (bttm) sides, left and right sides, and anterir (frnt) and psterir (back) ends. Linked with bilateral symmetry is cephalizatin, an evlutinary trend tward the cncentratin f nervus and sensry equipment n the anterir end. The symmetry f an animal generally fits its lifestyle. Many radial animals are sessile r planktnic and need t meet the envirnment equally well frm all sides. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 32-4

5 Animals that mve actively frm place t place are generally bilaterally symmetrical. The central nervus system f bilateral animals allws them t crdinate cmplex mvements invlved in crawling, burrwing, flying, and swimming. Fssil evidence indicates that these tw fundamentally different kinds f symmetry radial and bilateral have been present fr at least 550 millin years. Animal bdy plans vary accrding t the rganizatin f the animal s tissues. True tissues are cllectins f specialized cells islated frm ther tissues by membranus layers. Spnges and a few ther grups lack true tissues. In all ther animals, the embry becmes layered thrugh the prcess f gastrulatin. As develpment prgresses, germ layers, cncentric layers f embrynic tissue, frm varius tissues and rgans. Ectderm, the germ layer cvering the surface f the embry, gives rise t the uter cvering and, in sme phyla, t the central nervus system. Endderm, the innermst germ layer, lines the puch that frms during gastrulatin (the archentern) and gives rise t the lining f the digestive tract r cavity and the rgans derived frm it, such as the liver and lungs f vertebrates. Animals with nly tw germ layers, such as cnidarians and cmb jellies, are diplblastic. Other animals are triplblastic and have three germ layers. In these animals, a third germ layer, the mesderm, lies between the endderm and the ectderm. The mesderm develps int the muscles and mst ther rgans between the digestive tube and the uter cvering f the animal. Triplblasts include a brad range f animals, frm arthrpds t vertebrates. The bdy cavities f triplblastic animals vary. Mst triplblastic animals pssess a bdy cavity (a fluid- r air-filled space lcated between the digestive tract and the uter bdy wall) knwn as a celm. A true celm frms frm tissue derived frm mesderm. The inner and uter layers f tissue that surrund the celm cnnect and frm structures that suspend the internal rgans. Animals that pssess a true celm are knwn as celmates. Sme triplblastic animals have a cavity frmed frm mesderm and endderm. Such a cavity is a pseudcelm, and animals that have ne are called pseudcelmates. A pseudcelm is a fully functinal bdy cavity, despite its name. Sme triplblastic animals lack a celm. These animals, knwn as acelmates, have a slid bdy withut a bdy cavity. A bdy cavity has many functins. Its fluid cushins the suspended rgans, helping t prevent internal injury. In sft-bdied celemates, the nncmpressible fluid f the bdy cavity can functin as a hydrstatic skeletn against which muscles can wrk. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 32-5

6 The presence f a bdy cavity enables the internal rgans t grw and mve independently f the uter bdy wall. A grup f rganisms with similar bdy plans belng t the same grade. Fr instance, celmates and pseudcelmates represent tw animal grades. Phylgenetic studies suggest that true celms and pseudcelms have evlved many times in the curse f animal evlutin. Thus, the terms celmate and pseudcelmate refer t grades, nt clades. As illustrated by this example, a grade is nt necessarily equivalent t a clade (a grup that includes an ancestral species and all f its descendants). Describing an rganism as celmate r pseudcelmate can be helpful in describing certain f its features, these terms may nt help t understand evlutinary histry. The develpmental mdes f animals vary. Mst animals can be categrized as having ne f tw develpmental mdes: prtstme develpment r deuterstme develpment. The differences between these mdes f develpment center n cleavage pattern, celm frmatin, and blastpre fate. Many prtstmes underg spiral cleavage, in which the planes f cell divisin are diagnal t the vertical axis f the embry. Mst prtstmes als shw determinate cleavage, whereby the develpmental fate f each embrynic cell is determined very early. Mst deuterstmes underg radial cleavage, in which the cleavage planes are parallel r perpendicular t the vertical axis f the embry. Mst deuterstmes shw indeterminate cleavage, whereby each cell in the early embry retains the capacity t develp int a cmplete embry. The indeterminate cleavage f the human zygte makes identical twins pssible. In gastrulatin, the develping digestive tube f an embry initially frms as a blind puch, the archentern, which becmes the gut. As the archentern frms in a prtstme, initially slid masses f mesderm split t frm the celm. In deuterstmes, mesderm buds ff frm the wall f the archentern and hllws t becme the celm. Prtstme and deuterstme develpment differ in the fate f the blastpre, the indentatin during gastrulatin that leads t the frmatin f the archentern. In mst bilaterians, a secnd pening frms at the ppsite end f the gastrula frm the archentern. The blastpre and the secnd pening becme the tw penings f the digestive tube (the muth and anus). In prtstme develpment, the muth generally develps frm the first pening, the blastpre. This is the rigin f the term prtstme ( first muth ). In deuterstme ( secnd muth ) develpment, the blastpre usually develps int the anus, and the muth is derived frm the secndary pening. Cncept 32.4 New views f animal phylgeny are emerging frm mlecular data Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 32-6

7 Zlgists currently recgnize abut three dzen animal phyla. The bundaries and relatinships amng these phyla cntinue t be debated. Traditinally, zlgists based hyptheses abut animal phylgeny n mrphlgical data. Nw bilgists als estimate phylgenies using mlecular data. New studies f lesser-knwn phyla and fssil data als help clarify when key mrphlgical traits arse in varius grups. Mdern phylgenetic systematics seeks t place rganisms int clades, each f which includes an ancestral species and all f its descendants. Based n cladistic methds, a phylgenetic tree takes shape as a hierarchy f clades nested within larger clades. Clades are inferred frm shared derived characters that are unique t members f the clade. Fr example, a clade might be inferred frm key anatmical and embrylgical similarities that researchers cnclude are hmlgus. Mlecular data such as DNA sequences are anther surce f infrmatin fr inferring cmmn ancestry. Whether the data used are traditinal mrphlgical characters, mlecular sequences, r sme cmbinatin, the gal is the same: t recnstruct evlutinary histry. Tw current phylgenetic hyptheses can be cmpared: ne based n mrphlgical data and the ther based n mlecular data. The hyptheses agree n several majr features f animal phylgeny. 1. All animals share a cmmn ancestr. Bth animal trees indicate that animals are mnphyletic, frming a clade called Metaza. 2. Spnges are basal animals. Spnges branch frm the base f bth animal trees. Mrphlgical and mlecular analyses published in 2009 indicate that spnges are mnphyletic; ther recent studies had suggested that spnges are paraphyletic. 3. Eumetaza is a clade f animals with true tissues. All animals except spnges and a few thers belng t a clade f eumetazans. The cmmn ancestr f living eumetazans acquired true tissues. Basal members f the eumetazan clade include the phyla Ctenphra and Cnidaria. These basal eumetaza are diplblastic and generally have radial symmetry. 4. Mst animal phyla belng t the clade Bilateria. Bilateral symmetry and the presence f three germ layers are shared derived characteristics that help t define the clade Bilateria. 5. Chrdates and sme ther phyla belng t the clade Deuterstmia. The name deuterstme refers t a mde f animal develpment and als t a clade that includes vertebrates and ther chrdates. Nte that the traditinal and mlecular views f animal phylgeny disagree as t which ther phyla are als deuterstmes. The hyptheses disagree n sme significant pints, including the relatinships amng the bilaterians. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 32-7

8 The mrphlgy-based tree divides the bilaterians int deuterstmes and prtstmes. This divisin assumes that these tw mdes f develpment reflect a phylgenetic pattern. Within the prtstmes, arthrpds are gruped with annelids because bth grups have segmented bdies. Mlecular phylgenies based n mitchndrial genes, ribsmal genes, Hx genes, and dzens f prtein-encding genes suggest a different view. These studies indicate that there are three majr clades f bilaterally symmetrical animals: Deuterstmia, Lphtrchza, and Ecdysza. In this phylgeny, arthrpds and annelids are nt clsely related. Traditinally, acelmate flatwrms (the Acela) were classified with ther flatwrms in the phylum Platyhelminthes. Recent studies indicate that acel flatwrms are basal bilaterians, nt members f the phylum Platyhelminthes. The basal psitin f Acela suggests that bilaterians descended frm a cmmn ancestr that resembled living acel flatwrms, with a simple nervus system, a saclike gut with a single pening, and n excretry system. The mlecular phylgeny assigns animal phyla nt in Deuterstmia t tw taxa: the ecdyszans and the lphtrchzans. The clade name Ecdysza refers t a characteristic shared by nematdes and arthrpds, amng ther taxa. These animals secrete external skeletns (exskeletns). As the animal grws, it mlts the ld exskeletn and secretes a new, larger ne, a prcess called ecdysis. Thugh named fr this prcess, the clade is actually defined mainly by mlecular evidence f the cmmn ancestry f its members. The name Lphtrchza refers t tw characteristic features f animals in this clade. Sme animals, such as brachipds, develp a lphphre, a crwn f ciliated tentacles used fr feeding. Other phyla, including annelids and mlluscs, have a distinctive larval stage called a trchphre larva. Systematists cntinue t cnduct large-scale analyses f multiple genes and mrphlgical traits acrss a wide range f animal phyla in an effrt t gain a clearer picture f hw the diversity f animal bdy plans arse. Lecture Outline fr Campbell/Reece Bilgy, 9 th Editin, Pearsn Educatin, Inc. 32-8