1、cyclic Lesson 9Lesson 9 The origin of speciesThe origin of species1.The biologists define a species1.The biologists define a species Modern biologists generally define a species as group Modern biologists generally define a species as group of actually or potentially of actually or potentially inter
2、breedinginterbreeding populations that populations that is reproductively isolated from other such groups.is reproductively isolated from other such groups.Members of species can interbreed with each other,but Members of species can interbreed with each other,but they cannot breed with organisms bel
3、onging to another they cannot breed with organisms belonging to another species.One advantage of the standard of reproductive species.One advantage of the standard of reproductive isolation is that it is very precise.Notice,however,that it isolation is that it is very precise.Notice,however,that it
4、can only be applied to organisms that reproduce sexually.can only be applied to organisms that reproduce sexually.Asexual reproducers,including most prokaryotes,many Asexual reproducers,including most prokaryotes,many plants,and some animals,must be classified into species plants,and some animals,mu
5、st be classified into species on the basis of physical (biochemical or morphological)on the basis of physical (biochemical or morphological)traits.traits.2.Preventing gene exchange2.Preventing gene exchange Two general types of mechanisms operate to block Two general types of mechanisms operate to b
6、lock the exchange of genes between individuals of related the exchange of genes between individuals of related groups.The first general type is made up of groups.The first general type is made up of prezygoticprezygotic isolating mechanisms-mechanisms that prevent the isolating mechanisms-mechanisms
7、 that prevent the formation of zygotes.Prezygotic isolation falls into formation of zygotes.Prezygotic isolation falls into two categories:ecological and behavioral.In the first two categories:ecological and behavioral.In the first case,two related group may become adapted to slightly case,two relat
8、ed group may become adapted to slightly different environments _ perhaps varying soil types or food different environments _ perhaps varying soil types or food sources.Over time,these genetic differences become so sources.Over time,these genetic differences become so great that successful cross-fert
9、ilization can no longer great that successful cross-fertilization can no longer take place.In behavioral isolation,related groups take place.In behavioral isolation,related groups evolve differing behaviors-such as species mating evolve differing behaviors-such as species mating ritualsrituals-that
10、restrict the exchange of genes to members-that restrict the exchange of genes to members of the same group.of the same group.-Preventing gene exchange Sometimes the differences that produce Sometimes the differences that produce prezygoticprezygotic isolation involve mechanical isolation.That is,mat
11、ing is isolation involve mechanical isolation.That is,mating is physically impossible between members of different physically impossible between members of different species because species because genitals genitals of males and females are of males and females are structurally incompatible or becau
12、se molecules on the structurally incompatible or because molecules on the surfaces of sperm and egg fail to bind.surfaces of sperm and egg fail to bind.A final type of A final type of prezygotic mechanism is temporal isolation,in which prezygotic mechanism is temporal isolation,in which time-related
13、 environmental cues that trigger reproductive time-related environmental cues that trigger reproductive processes are different for related species.processes are different for related species.In postzygotic isolating mechanisms mating occurs,but In postzygotic isolating mechanisms mating occurs,but
14、the resulting hybrid organism is the resulting hybrid organism is inviableinviable or or sterilesterile.In a.In a special case of special case of hybrid sterilityhybrid sterility termed termed hybrid hybrid breakdownbreakdown,the second and subsequent generations after ,the second and subsequent gen
15、erations after a cross show reduced reproductive success.a cross show reduced reproductive success.In In postzygoticpostzygotic isolating mechanisms mating occurs,but isolating mechanisms mating occurs,but the resulting hybrid organism is the resulting hybrid organism is inviableinviable or or steri
16、lesterile.In a.In a special case of special case of hybrid sterilityhybrid sterility termed hybrid termed hybrid breakdown,the second and subsequent generations after breakdown,the second and subsequent generations after a cross show reduced reproductive success.a cross show reduced reproductive suc
17、cess.Contrast this Contrast this face with the very different outcome of crossbreeding face with the very different outcome of crossbreeding between two genetically distant members of the same between two genetically distant members of the same species,where the result is often species,where the res
18、ult is often heterozygoteheterozygote advantage(advantage(hybrid vigorhybrid vigor).).Populations of a species that are spread out over Populations of a species that are spread out over abroad geographical range are often arrayed in a cline-a abroad geographical range are often arrayed in a cline-a
19、gradual change in one or more characteristics as each gradual change in one or more characteristics as each population evolves adaptations to its own local environment.population evolves adaptations to its own local environment.Along a cline,subspecies with distinct characteristics may Along a cline
20、,subspecies with distinct characteristics may arise.Often,individuals at either end of a cline are arise.Often,individuals at either end of a cline are reproductively isolated.reproductively isolated.-Preventing gene exchange Becoming a species:how gene pools Becoming a species:how gene pools become
21、 isolatedbecome isolated Ernst mayrs model of Ernst mayrs model of allopatric speciationallopatric speciation proposes that species can originate in a two stage proposes that species can originate in a two stage process.In the first stage,populations of existing process.In the first stage,population
22、s of existing species are separated by physical or geographical species are separated by physical or geographical barrier.As a result,over time genetic differences barrier.As a result,over time genetic differences leading to pre-or leading to pre-or postzygoticpostzygotic isolation arise isolation a
23、rise between the two groups.In the second stage,the between the two groups.In the second stage,the diverged populations may again come into contact.If diverged populations may again come into contact.If this happens,speciation becomes complete this happens,speciation becomes complete through the act
24、ion of natural selection.through the action of natural selection.4.The genetic bases of speciation 4.The genetic bases of speciation the extent of differences between populations the extent of differences between populations that are diverging into separates species or between that are diverging int
25、o separates species or between species that have already diverged is represented by a species that have already diverged is represented by a statisticstatistic called called genetic identitygenetic identity-the relative proportion -the relative proportion of the same of the same structural Genesstru
26、ctural Genes present in members of present in members of groups being compared.In general,biologists believe groups being compared.In general,biologists believe that the that the genetic events leading to speciation take genetic events leading to speciation take place graduallyplace gradually.Once a
27、 new species has arisen,it.Once a new species has arisen,it tends to diverge genetically from related species at a tends to diverge genetically from related species at a more rapid pace.In some cases,such as primate order,more rapid pace.In some cases,such as primate order,major differences in body
28、form are not reflected by major differences in body form are not reflected by corresponding divergences in structural genes.corresponding divergences in structural genes.This has This has led biologists to hypothesize that small changes in led biologists to hypothesize that small changes in regulato
29、ry genesregulatory genes may account for many of the large-may account for many of the large-scale changes responsible for species and the origin scale changes responsible for species and the origin of higher taxonomic groups.of higher taxonomic groups.one mechanisms that may rapidly split populatio
30、ns one mechanisms that may rapidly split populations genetically is genetically is polyploidizationpolyploidization-the sudden multiplication of-the sudden multiplication of an entire complement of chromosomes.This can result in an entire complement of chromosomes.This can result in sympatric specia
31、tionsympatric speciation,in which new species arise even ,in which new species arise even though no geographical isolation has taken place.A though no geographical isolation has taken place.A phenomenon similar to polyploidization involving the phenomenon similar to polyploidization involving the re
32、arrangement of chromosomes has been proposed to rearrangement of chromosomes has been proposed to explain the evolutionary origin of giant pandas.Clearly,explain the evolutionary origin of giant pandas.Clearly,species can originate in variety ways.species can originate in variety ways.-The genetic b
33、ases of speciation 5.Explaining macroevolution:higher-order changes5.Explaining macroevolution:higher-order changes The changes that generate species are sometimes termed The changes that generate species are sometimes termed microevolutionmicroevolution;those that produce the major;those that produ
34、ce the major phenotypicphenotypic differences differences that separate genera,classes,orders,and son on are termed that separate genera,classes,orders,and son on are termed macroevolution macroevolution.Some .Some lines of descentlines of descent can be traced by studying the can be traced by study
35、ing the fossil record.In other cases relationships must be inferred by fossil record.In other cases relationships must be inferred by comparison of related living organisms.When lines of descent over comparison of related living organisms.When lines of descent over evolutionary time are constructed,
36、the result is a evolutionary time are constructed,the result is a phylogeny phylogeny.The The rationalerationale for building a phylogeny is simple:it assumes that for building a phylogeny is simple:it assumes that similarities in body structure,biochemistry,reproductive strategies,similarities in b
37、ody structure,biochemistry,reproductive strategies,and other features of organisms can be used to trace lines of common and other features of organisms can be used to trace lines of common descent.The process is complex because evolution proceeds in different descent.The process is complex because e
38、volution proceeds in different patterns.In cases of patterns.In cases of parallel evolutionparallel evolution,two or more lineages evolve along ,two or more lineages evolve along similar lines.In similar lines.In convergent evolutionconvergent evolution,very distantly related lineages ,very distantl
39、y related lineages become more alike as similar adaptations take hold in response to become more alike as similar adaptations take hold in response to demands of the environment.Thus similar structures in different demands of the environment.Thus similar structures in different organisms may reflect
40、 organisms may reflect homology homology(derivation from a common ancestor)(derivation from a common ancestor)or or analogyanalogy(independent origin of structures used for similar purposes).(independent origin of structures used for similar purposes).One of the most common evolutionary patterns tha
41、t One of the most common evolutionary patterns that can be constructed from the fossil record is can be constructed from the fossil record is divergent divergent evolutionevolution or or radiationradiation.It is represented by the branching .It is represented by the branching and rebranching of a si
42、ngle line.Another common feature of and rebranching of a single line.Another common feature of evolution is extinction the complete loss of a species.evolution is extinction the complete loss of a species.Mass extinctions have occurred at least five times in the Mass extinctions have occurred at lea
43、st five times in the earths history.earths history.Gaps in the fossil record have led some Gaps in the fossil record have led some paleontologistspaleontologists to propose the to propose the punctuated equilibrium theorypunctuated equilibrium theory of evolution.of evolution.The theory holds that e
44、volution proceeds by The theory holds that evolution proceeds by spurtsspurts-radical -radical changes over short(in geological time)periods of timewith changes over short(in geological time)periods of timewith intervening periods of equilibrium.The theory is intervening periods of equilibrium.The t
45、heory is controversial and tends not to be supported when an controversial and tends not to be supported when an abundant fossil record is available.abundant fossil record is available.6.The role of microevolution 6.The role of microevolution Biologists have no certain a number of questions about Bi
46、ologists have no certain a number of questions about large-scale evolutionary changes.These questions range large-scale evolutionary changes.These questions range from whether novel higher taxa from whether novel higher taxa result fromresult from as yet as yet undescribed radical genetic processes,
47、to whether known undescribed radical genetic processes,to whether known processes such as genetic Drift and small gene changes can processes such as genetic Drift and small gene changes can plausibly account for the evolution of new genera,families,plausibly account for the evolution of new genera,f
48、amilies,and orders.Investigators are exploring these areas using and orders.Investigators are exploring these areas using traditional methods as well as the newer techniques of traditional methods as well as the newer techniques of molecular biologymolecular biology.Lesson tenLesson tennumber.number.
