26 an introduction to genetic analysisWord文件下载.docx

1、Key ConceptsEvolution consists of continuous heritable change of organisms within a single line of descent （phyletic evolution） and the differentiation between different lines of descent to form different species （diversification）.The Darwinian mechanism of evolution rests on three principles: （1） o

2、rganisms within a species vary from one another, （2） the variation is heritable, and （3） different types leave different numbers of offspring in future generations.Both phyletic change and diversification are the result of the interaction between the directional force of natural selection and random

3、 events.Natural selection is the differential reproduction of different genotypes that is a consequence of their different physiological, morphological, and behavioral traits.Random effects include the sampling of gametes each generation in finite populations and the random occurrence of mutations.A

4、 consequence of the random factors in evolution is that the same forces of natural selection do not lead to the same evolutionary result in independent lines of descent.Species are reproductively isolated populations of organisms that can exchange genes within the group but not with other species, b

5、ecause the groups are physiologically, behaviorally, or developmentally incompatible.Evolutionary novelties are possible because new DNA is acquired either by duplication and subsequent differentiation of DNA already present in the species or by the introduction of novel DNA from other species.Intro

6、ductionThe modern theory of evolution is so completely identified with the name of Charles Darwin （18091882） that many people think that the concept of organic evolution was first proposed by Darwin, but that is certainly not the case. Most scholars had abandoned the notion of fixed species, unchang

7、ed since their origin in a grand creation of life, long before publication of Darwins The Origin of Species in 1859. By that time, most biologists agreed that new species arise through some process of evolution from older species; the problem was to explain how this evolution could occur.Darwins the

8、ory of the mechanism of evolution begins with the variation that exists among organisms within a species. Individuals of one generation are qualitatively different from one another. Evolution of the species as a whole results from the differential rates of survival and reproduction of the various ty

9、pes, so the relative frequencies of the types change over time. Evolution, in this view, is a sorting process.For Darwin, evolution of the group resulted from the differential survival and reproduction of individual variants already existing in the groupvariants arising in a way unrelated to the env

10、ironment but whose survival and reproduction do depend on the environment.MESSAGEDarwin proposed a new explanation to account for the accepted phenomenon of evolution. He argued that the population of a given species at a given time includes individuals of varying characteristics. The population of

11、the next generation will contain a higher frequency of those types that most successfully survive and reproduce under the existing environmental conditions. Thus, the frequencies of various types within the species will change over time.There is an obvious similarity between the process of evolution

12、 as Darwin described it and the process by which the plant or animal breeder improves a domestic stock. The plant breeder selects the highest-yielding plants from the current population and （as far as possible） uses them as the parents of the next generation. If the characteristics causing the highe

13、r yield are heritable, then the next generation should produce a higher yield. It was no accident that Darwin chose the term natural selectionto describe his model of evolution through differential rates of reproduction of different variants in the population. As a model for this evolutionary proces

14、s, he had in mind the selection that breeders exercise on successive generations of domestic plants and animals.We can summarize Darwins theory of evolution through natural selection in three principles:1. Principle of variation.Among individuals within any population, there is variation in morpholo

15、gy, physiology, and behavior.2. Principle of heredity.Offspring resemble their parents more than they resemble unrelated individuals.3. Principle of selection.Some forms are more successful at surviving and reproducing than other forms in a given environment.Clearly, a selective process can produce

16、change in the population composition only if there are some variations among which to select. If all individuals are identical, no amount of differential reproduction of individuals can affect the composition of the population. Furthermore, the variation must be in some part heritable if differentia

17、l reproduction is to alter the populations genetic composition. If large animals within a population have more offspring than do small ones but their offspring are no larger on average than those of small animals, then no change in population composition can occur from one generation to another. Fin

18、ally, if all variant types leave, on average, the same number of offspring, then we can expect the population to remain unchanged.s principles of variation, heredity, and selection must hold true if there is to be evolution by a variational mechanism.The Darwinian explanation of evolution must apply

19、 to two different aspects of the history of life. One is the successive change of form and function that occurs in a single continuous line of descent time,phyletic evolution.Figure 26-1 shows such a continuous change over a period of 40 million years in the size and curvature of the left shell of t

20、he oyster,Gryphea.The other is thediversificationthat occurs among species: in the history of life on earth, there are many different contemporaneous species having quite different forms and living in different ways. Figure 26-2 shows some of the variety of bivalve mollusc forms that existed at vari

21、ous times in the past 130 million years. Every species eventually becomes extinct and more than 99.9 percent of all the species that have ever existed are already extinct, yet the number of species and the diversity of their forms and functions have increased in the past billion years. Thus species

22、not only must be changing, but must give rise to new and different species in the course of evolution. Both of these processes are the consequences of heritable variation within populations. Heritable variation provides the raw material for successive changes within a species and for the multiplicat

23、ion of new species. The basic mechanisms of those changes （as discussed in Chapter 24） are the origin of new variation by various kinds of mutational mechanisms, the change in frequency of alleles by selective and random processes, the possibility of divergence of isolated local populations because

24、the selective forces are different or because of random drift, and the reduction of variation between populations by migration. From those basic mechanisms, population genetics, as discussed in Chapter 24, derives a set of principles governing changes in the genetic composition of populations. The a

25、pplication of these principles of population genetics provides an articulated theory of evolution.Evolution, under the Darwinian scheme, is the conversion of heritable variation between individuals within populations into heritable differences between populations in time and in space, by population

26、genetic mechanisms.A synthesis of forces: variation and divergence of populationsIn evolution, the various forces of breeding structure, mutation, migration, and selection are all acting simultaneously in populations. We need to consider how these forces, operating together, mold the genetic composi

27、tion of populations to produce both variation within local populations and differences between them.The genetic variation within and between populations is a result of the interplay of the various evolutionary forces （Figure 26-3）. Generally, as Table 26-1 shows, forces that increase or maintain var

28、iation within populations prevent the differentiation of populations from each other, whereas the divergence of populations is a result of forces that make each population homozygous. Thus, random drift （or inbreeding） produces homozygosity while causing different populations to diverge. This diverg

29、ence and homozygosity are counteracted by the constant flux of mutation and the migration between localities, which introduce variation into the populations again and tend to make them more like each other.When Darwin arrived in the Galapagos Islands in 1835 he found a remarkable group of finchlike

30、birds that provided a very suggestive case for the development of his theory of evolution. The Galapagos archipelago is a cluster of 29 islands and islets of different sizes lying on the equator about 600 miles off the coast of Ecuador. Figure 26-4 shows the 13 Galapagos finch species. Finches are g

31、enerally ground-feeding seed eaters with stout bills for cracking the tough outer coats of the seeds. The Galapagos species, though clearly finches, have an immense variation in how they make a living and in their bill shapes and their behaviors, which underly these ecological differences. For examp

32、le, the vegetarian tree finch eats fruits and leaves, the insectivorous finch has a bill with a biting tip for eating large insects, and, most remarkable of all, the woodpecker finch grasps a twig in its beak and uses it to obtain insect prey by probing holes in trees. This diversity of species arose from an original population of a seed-eating finch that arrived in the Galapagos from the mainland of South America and populated the islands. The descendants of the original colonizers spread to the different islands and to