生物专业英语（英文）.ppt

1、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 can

2、not 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 ani

3、mals, 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 m

4、echanisms - 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 geneti

5、c 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 di

6、fferences 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 t

7、ype 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

8、 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 , th

9、e 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 sp

10、ecies 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

11、 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 ph

12、ysical 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

13、 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

14、 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

15、 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 rap

16、idly 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 t

17、he 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 microevolu

18、tion; 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

19、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

20、 .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 envir

21、onment . 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 evolutio

22、n 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 proc