Chater 18 Mass Extinctions, Opportunities and Adaptive Radiations：18章大滅絕機(jī)遇和適應(yīng)輻射

1、figure co: dinosaur fossil styve reineck/shutterstock, inc.extinctionoverview extinctions are as important in the history of life as are the evolution of new species explaining extinctions is just as challenging a scientific question as explaining the evolution of new species extinctions are opportu

2、nities for adaptive radiations because extinctions open or re-open niches for new species to invade and occupy to understand extinctions, we need to identify rates, patterns and causesextinction georges cuvier is credited with establishing the reality of extinction for the scientific community in a

3、lecture to the french institute in 1796 g.g. simpson and many other evolutionary biologists have estimated that 99% of all species are already extinct the only drawback to accepting that number is our lack of knowledge of how many species are actually living today, much less how many were alive in t

4、he past but not all species have gone extinct; there are some living fossilssurvivors lingula a marine organism (brachiopod) occupying vertical burrows in sand and mud has survived morphologically unchanged since the silurian survivors horseshoe crabs the horseshoe crab (limulus), an inhabitant of m

5、arine shores, has lived morphologically unchanged since the ordovician survivors cycads and horsetails survivors my favorite! periplaneta americana:the cockroachextinctions we know very little about natural extinctions, especially the precise causes fossil records demonstrate that extinctions have o

6、ccurred repeatedly in the past but physical evidence of causative agents are rarely preserved cause and effect is hard to establishextinctions habitat disruption volcanic eruptions asteroid impacts sea level change habitat modification climate change mountain-building sea level change precipitation

7、change toxic materials “exotic” species introductions continental driftco-evolution & nichesco-evolution & niches any species living in a niche has evolutionary relationships with other species; some casual, some crucial therefore, the extinction of a species will have repercussions in the n

8、iches of all species which have co-evolutionary relationships with the newly extinct speciesrates of extinction there is much debate about the degree and the importance of different rates of extinction once again, the incomplete fossil record makes answering the question far more difficult the simpl

9、e comparison is between a background rate of “uniform” extinctions, and the occasional episodes of “mass” extinctionsuniform/background extinctions species average survival time marine invertebrates 30 million years mammals 2 to 3 million years one estimate for all fossil species 4 million years rat

10、e of species extinction one estimate for the background extinction rate is for one to seven species to die each year the same source estimated mass extinction rates as being 3 to 4 times the background rate with 75 to 95% of species dying; this is a rate of perhaps 15 to 30 species per yearuniform/b

11、ackground extinctions a major problem in estimating the rate of fossil species extinction is the incomplete fossil record approximately one quarter of a million fossil species have been identified so far (in 35,000 genera from 4,000 families)uniform/background extinctions based on current biodiversi

12、ty (40 million living species?) and average species extinction rates, there may well have been 5 to 50 billion species present on earth since the origin of life on earth so all our conclusions should be considered very tentativeextinctions extinction is the converse of speciation; species arise and

13、species disappear extinction can be considered at levels of increasing severity and impact: extinction may be local and limited to demes extinction may eliminate an entire species extinction may eliminate most or all of the species in a region, habitat, or ecosystem extinction may be of much larger

14、scale, eliminating most of the species on a continent or on earth these are mass extinctions at least five mass extinctions have occurred in the history of life on earth each mass extinction has been followed by the successful adaptive radiations of new organisms source: raup, d.m. and j.j. sepkoski

15、, jr., science 231 (1986): 833835.table t01: details of the five major mass extinction events since the cambrianthe five major mass extinction events why are mass extinction events defined by the relatively abrupt disappearance of at least 75% of marine animal species? because over the 500 million y

16、ears of metazoan existence, the most complete fossil records are for marine animals geologists tend to divide that time into eras and periods by mass extinctions, which are followed by adaptive radiations of new forms; or simply by major adaptive radiations of new forms (index fossils)radiations of

17、the metazoans in the phanerozoicmass extinction eventsnote that this chart tracks all animal families,not just marine animals, and at the family level, not the species levelthe ordovician extinction event the second-largest of the five major extinction events in earths history in terms of percentage

18、 of genera that went extinct and the second largest overall in the overall loss of life between about 450 ma to 440 ma, two bursts of extinction occurred, separated by one million years this was the second biggest extinction of marine life, ranking only below the permian extinctionthe ordovician ext

19、inction event at the time, all known metazoan life was confined to the seas and oceans more than 60% of marine invertebrates died; brachiopods, bivalves, echinoderms, bryozoans and corals were particularly affected the immediate cause of extinction appears to have been the tectonic movement of gondw

20、ana into the south polar regionthe ordovician extinction event gondwanan drift south led to global cooling, glaciation and consequent sea level fall the falling sea levels disrupted or eliminated expansive shallow marine habitats along the continental shelves the event was preceded by a fall in atmo

21、spheric co2, a global cooling, and the newly forming appalachian mountains may have been the co2 sinkmiddle ordovicianthe devonian extinction event the third-largest of the five major extinction events in earths history in terms of percentage of genera that went extinct the timing is less well under

22、stood conflicting hypotheses propose from as few as two to as many as seven related bursts of extinction centered on 365 ma to 440 ma, over as little as one half to as many as 25 million years the extinction seems to have primarily affected marine lifethe devonian extinction event by the late devoni

23、an, there were massive reefs built by stromatolites and corals in the oceans, while the land had been colonized by plants and insects vascular plants were becoming tall and changing the soils as well as the co-evolving biota euramerica and gondwana were beginning to converge into what would become p

24、angea hard-hit groups include brachiopods, trilobites, and reef-building organisms; the latter almost completely disappeared, with coral reefs only returning upon the evolution of modern corals during the mesozoic surprisingly, jawed vertebrates seem to have been unaffected by the loss of reefs, whi

25、le agnathans were in decline long before the end of the devonianthe devonian extinction event the causes of the devonian extinctions are unclear the extinction of 20% of all animal families and 70-80% of all animal species leading theories include changes in sea level and ocean anoxia, possibly trig

26、gered by global cooling (glaciation on gondwana) or oceanic volcanism the widespread oceanic anoxia prohibited decay and allowed the preservation of sedimented organic matter as petroleum the impact of a comet or another extraterrestrial body has also been suggested, but the evidence is weaklate dev

27、onian / early carboniferous the permian extinction event the earths most severe mass extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct it is the only known mass extinction of insects some 57% of all families and 83% of all genera were k

28、illed because so much biodiversity was lost, the recovery of life on earth took significantly longer than after other extinction eventsthe permian extinction event there were from one to three distinct pulses of extinctions that occurred about 245-251 million years ago there are several proposed mec

29、hanisms for the extinctions the earlier phase was likely due to gradual environmental change, while the latter phase may has been due to a catastrophic eventthe permian extinction event suggested mechanisms for the latter catastrophic extinction pulse include: large or multiple bolide (meteor/comet)

30、 impact events increased volcanism and sudden release of methane clathrate from the sea floor gradual changes include sea-level change, anoxia, increasing aridity, and a shift in ocean circulation patterns driven by climate change excess dissolved co2 acidified the oceans, contributing to the declin

31、e of shelled organismsthe permian extinction event most fossil insect groups found after the permiantriassic boundary differ significantly from those that lived prior to the ptr extinction over two-thirds of terrestrial labyrinthodont amphibians, sauropsid (reptile) and therapsid (mammal-like reptil

32、e) families became extinct large herbivores suffered the heaviest losseslate permianthe triassic extinction event the first of the final two more modest of the five major extinction events the extinction occurred around 208 million years ago and happened in less than 10,000 years just before pangaea

33、 started to break apart this extinctions struck marine life and terrestrial life profoundly at least half of the species now known to have been living at that time went extinct in the seas, a whole class (conodonts) and 20% of all marine families disappeared conodonts were early eel-like chordatesth

34、e triassic extinction event on land, all large crurotarsans (non-dinosaurian archosaurs) other than the crocodilians, some remaining therapsids, and many of the large amphibians were wiped out this event vacated terrestrial ecological niches, allowing the dinosaurs to assume the dominant roles in th

35、e jurassic period statistical analysis of triassic marine losses suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctionsthe triassic extinction event several explanations for this event have been suggested, but all have unanswered challe

36、nges: gradual climate change or sea-level fluctuations during the late triassic; however, this does not explain the suddenness of the extinctions in the marine realm asteroid impact, but no impact crater has been dated to coincide with the triassicjurassic boundary; the largest late triassic impact

37、crater occurred about 12 million years before the extinction event massive volcanic eruptions (known from the central atlantic magmatic province - an event that triggered the opening of the atlantic ocean) that the would release co2 or sulfur dioxide and aerosols, which would cause either intense gl

38、obal warming (from the former) or cooling (from the latter)late triassic / early jurassicthe late cretaceous extinction event the second of two more modest extinction events, the fifth and final of the five major extinction events there is agreement that it was a relatively rapid extinction event da

39、ted to 65.5 million years widely known as the kt extinction event, it is associated with a geological signature known as the kt boundary, usually a thin band of iridium-rich sedimentation found in various parts of the worldthe late cretaceous extinction event the event marks the end of the mesozoic

40、era and the beginning of the cenozoic era essentially all non-avian dinosaurs, mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates became extinct stem mammalian clades passed through the boundary with few extinctions and began their remarkably successful adaptive radiatio

41、ns across the globethe late cretaceous extinction event scientists theorize that the kt extinctions were caused by one or more catastrophic events, such as massive asteroid impacts like the chicxulub impact, a 10km diameter meteorite, leaving a crater 200 km in diameter or increased volcanic activit

42、yimpact caused acid rain, ash that blocked out the sun for months, severe global cooling (nuclear winter). increase in atmospheric co2, resulting in global warming, the final blow to dinosaurs & many other cretaceous species.the late cretaceous extinction event several bolide impacts may have co

43、ntributed to massive volcanic activity, such as the deccan traps of west-central india, one of the largest volcanic features on earth, have been dated to the approximate time of the extinction eventthe late cretaceous extinction event these geological events may have reduced sunlight and hindered ph

44、otosynthesis, leading to a massive disruption in earths ecology other researchers believe the extinction was more gradual, resulting from slower changes in sea level or climatewhat happened to the dinosaurs? sediments were deposited by enormous tsunamis (tidal waves) along the coastline 70% of known

45、 fossils, including non-avian dinosaurs, were wiped outwhat happened to the dinosaurs?the late cretaceous extinction event before the end of the cretaceous, flight evolved independently three times: insects, flying reptiles, birds (avian dinosaurs) by the end of the cretaceous 65 mya, most dinosaurs

46、 along with other large marine reptiles and various invertebrates died out no land vertebrate larger than a large dog survived the kt boundary event the angiosperm radiation was well underway during the cretaceous, but the shift from gymnosperm to angiosperm dominated forests may have been triggered

47、 by the late cretaceous extinctionlate cretaceousthe five major mass extinction eventsnote in the left chart that global temperatures have fluctuated dramatically over the time of life on earth and those dramatic changes do not always correlate with mass extinction eventsthe five major mass extincti

48、on events plants are relatively immune to mass extinction, with the impact of all the major mass extinctions negligible at the family level even the reduction observed in species diversity (of 50%) may be mostly due to differential preservation of plant fossils however, a massive rearrangement of ec

49、osystems does occur, with climax communities and dominant plants, plant abundances and distributions changing profoundly after mass extinctions mass extinctionsfigure 01: percentage of marine animal extinctions adapted from fox, w. t., paleobiology 13 (1987): 257-271.it is important to remember that

50、 mass extinctions are just temporary increases in extinction rates that are significantly more severe than the average background extinctions rates (illustrated in green) which also fluctuate through timepercentage of species wiped outordovician-silurian - 85%late devonian - 82%permian-triassic - 96

51、%end triassic - 76%cretaceous-tertiary - 76%mass extinction probable causesto the degree that mass extinctions are real, rather than artifacts of a poor fossil record, the causes are probably complex and multifactorialwhat happened at the big five mass extinctions?abiotic causes for mass extinctions

52、1. plate tectonics1. intermingling of biotas / introduced species effects2. trophic stability3. changes in sea level4. volcanic activity changing atmospheric gases and dust levels5. ice ages with glaciations, sea levels fallings, increased tropical aridity2. planetary collisions3. cosmic forces &

53、; periodic galactic cyclestrophic stability when a landmass (a) is broken in two (b), this adds area along the perimeter where they split; this adds to the intertidal area which is a species and nutrient rich habitat when two landmasses (b) are brought together (a), this results in loss of available

54、 intertidal areatrophic stability the larger the land mass, the less climatic buffering from the oceans, which are heat sinks therefore, during the time of pangaeas stability, there may have been more extremes of hot and cold, and wet and dry, in the supercontinents interiorchanges in sea level move

55、ment of the earths crustal plates results in their slow collision with each other usually one plate over rides another, as shown here note how the ocean basin between them changes in size and depth, thereby changing sea level against the side of the continentsvolcanic activity locally, lava flows st

56、erilize and reform the surface, start fires, and their explosive blasts may also create damage within the surrounding habitatvolcanic activity large volumes of volcanic dust enter the atmosphere and become a possible cause for the cooling of the earth by blocking out the suns rays and reducing photo

57、synthesisice ages vulcanism or other forces may contribute to the cooling of the earth and the formation of glaciers that covers a part of the earths surface for periods of time formation of glaciers causes: ocean levels drop (due to water trapped in glaciers as ice) decrease in o2 levels increase i

58、n salt (mineral) content of oceans changes in natural environmentsice ages and extinctions ice ages (blue) are indicated along this geologic time line for comparison to five mass extinction episodes (red) there is no tight correlation between ice ages and mass extinctionsiceageiceageextraterrestrial

59、 impacts extraterrestrial impacts are known to have battered the moon and earth repeatedly since 4 bya more than 100 large craters are known on earth: ten meteors, each one km in diameter, are estimated to have each produced 20-kmwide craters at a frequency of one every 400,000 years a 50-kmwide cra

60、ter is produced every 12.5 my a 150-kmwide crater is produced every 100 my extraterrestrial impacts frequently cause extinctions, but other than the k-t cretaceous event, are probably not the single major cause of mass extinctionsextraterrestrial impactscosmic forces & periodic galactic cycles supernova (explosio