生物专业英语(英文)课件

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,cyclic,Lesson 9,The origin of species,1.The biologists define a species,Modern biologists generally define a species as group of actually or potentially,interbreeding,populations that is reproductively isolated from other such groups . Members of species can interbreed with each other , but they cannot breed with organisms belonging to another species . One advantage of the standard of reproductive isolation is that it is very precise . Notice ,however , that it can only be applied to organisms that reproduce sexually . Asexual reproducers, including most prokaryotes, many plants, and some animals, must be classified into species on the basis of physical (biochemical or morphological) traits.,2. Preventing gene exchange,Two general types of mechanisms operate to block the exchange of genes between individuals of related groups . The first general type is made up of,prezygotic,isolating mechanisms - mechanisms that prevent the formation of zygotes . Prezygotic isolation falls into two categories: ecological and behavioral . In the first case, two related group may become adapted to slightly different environments _ perhaps varying soil types or food sources. Over time , these genetic differences become so great that successful cross-fertilization can no longer take place . In behavioral isolation , related groups evolve differing behaviors-such as species mating,rituals,-that restrict the exchange of genes to members of the same group.,-,Preventing gene exchange,Sometimes the differences that produce,prezygotic,isolation involve mechanical isolation . That is , mating is physically impossible between members of different species because,genitals,of males and females are structurally incompatible or because molecules on the surfaces of sperm and egg fail to bind .,A final type of prezygotic mechanism is temporal isolation , in which time-related environmental cues that trigger reproductive processes are different for related species.,In postzygotic isolating mechanisms mating occurs, but the resulting hybrid organism is,inviable,or,sterile,. In a special case of,hybrid sterility,termed,hybrid breakdown, the second and subsequent generations after a cross show reduced reproductive success .,In,postzygotic,isolating mechanisms mating occurs, but the resulting hybrid organism is,inviable,or,sterile,. In a special case of,hybrid sterility,termed hybrid breakdown , the second and subsequent generations after a cross show reduced reproductive success .,Contrast this face with the very different outcome of crossbreeding between two genetically distant members of the same species , where the result is often,heterozygote,advantage (,hybrid vigor,).,Populations of a species that are spread out over abroad geographical range are often arrayed in a cline-a gradual change in one or more characteristics as each population evolves adaptations to its own local environment. Along a cline , subspecies with distinct characteristics may arise . Often ,individuals at either end of a cline are reproductively isolated .,-,Preventing gene exchange,Becoming a species : how gene pools become isolated,Ernst,mayrs,model of,allopatric,speciation,proposes that species can originate in a two stage process . In the first stage , populations of existing species are separated by physical or geographical barrier . As a result , over time genetic differences leading to pre- or,postzygotic,isolation arise between the two groups . In the second stage , the diverged populations may again come into contact . If this happens, speciation becomes complete through the action of natural selection.,4. The genetic bases of speciation,the extent of differences between populations that are diverging into separates species or between species that have already diverged is represented by a statistic,called,genetic identity,- the relative proportion of the same,structural Genes,present in members of groups being compared . In general, biologists believe that the,genetic events leading to speciation take place gradually,. Once a new species has arisen, it tends to diverge genetically from related species at a more rapid pace . In some cases, such as primate order, major differences in body form are not reflected by corresponding divergences in structural genes .,This has led biologists to hypothesize that small changes in,regulatory genes,may account for many of the large-scale changes responsible for species and the origin of higher taxonomic groups.,one mechanisms that may rapidly split populations genetically is,polyploidization,- the sudden multiplication of an entire complement of chromosomes . This can result in,sympatric speciation, in which new species arise even though no geographical isolation has taken place . A phenomenon similar to polyploidization involving the rearrangement of chromosomes has been proposed to explain the evolutionary origin of giant pandas . Clearly, species can originate in variety ways.,-The genetic bases of speciation,5. Explaining macroevolution: higher-order changes,The changes that generate species are sometimes termed,microevolution,; those that produce the major,phenotypic,differences that separate genera , classes , orders, and son on are termed,macroevolution,. Some,lines of descent,can be traced by studying the fossil record . In other cases relationships must be inferred by comparison of related living organisms. When lines of descent over evolutionary time are constructed, the result is a,phylogeny,.,The,rationale,for building a phylogeny is simple : it assumes that similarities in body structure , biochemistry, reproductive strategies, and other features of organisms can be used to trace lines of common descent .The process is complex because evolution proceeds in different patterns. In cases of,parallel evolution,two or more lineages evolve along similar lines . In,convergent evolution,very distantly related lineages become more alike as similar adaptations take hold in response to demands of the environment . Thus similar structures in different organisms may reflect,homology,( derivation from a common ancestor) or,analogy,(independent origin of structures used for similar purposes).,One of the most common evolutionary patterns that can be constructed from the fossil record is,divergent evolution,or,radiation,. It is represented by the branching and rebranching of a single line. Another common feature of evolution is extinction the complete loss of a species. Mass extinctions have occurred at least five times in the earths history.,Gaps in the fossil record have led some,paleontologists,to propose the,punctuated equilibrium theory,of evolution. The theory holds that evolution proceeds by,spurts,- radical changes over short (in geological time) periods of timewith intervening periods of equilibrium. The theory is controversial and tends not to be supported when an abundant fossil record is available.,6. The role of microevolution,Biologists have no certain a number of questions about large-scale evolutionary changes. These questions range from whether novel higher taxa,result from,as yet undescribed radical genetic processes, to whether known processes such as genetic Drift and small gene changes can plausibly account for the evolution of new genera, families , and orders. Investigators are exploring these areas using traditional methods as well as the newer techniques of molecular biology,.,Lesson ten,number.,谢谢,