Chapter 8 Population Genetics and Natural Selection.docx

1、Chapter 8 Population Genetics and Natural SelectionChapter 8 Population Genetics and Natural SelectionIn mid October of 1835 under a bright equatorial sun, a small boat moved slowly from the shore of a volcanic island to a waiting ship.The boat carried a young naturalist who had just completed a mon

2、th of exploring the group of islands known as the Galapagos, which lie on the equator approximately 1,000 km west of the South American mainland (fig. 8. 1 ). As the seamen rowed into the oncoming waves, the naturalist, Charles Darwin, mused over what he had found on the island. His observations had

3、 confirmed expectations built on information gathered earlier on the other islands he had visited in the archipelago. Later Darwin recorded his thoughts in his journal which he later published (Darwin 1839), The distribution of the tenants of this archipelago would not be nearly so wonderful, if, fo

4、r instance, one island had a mocking-thrush, and a second island some other quite distinct genus-if one island had its genus of lizard and a second island another distinct genus, or none whatever. But it is the circumstance, that several of the islands possess their own species of the tortoise, mock

5、ing-thrush, finches, and numerous plants, these species having the same general habits, occupying analogous situations, and obviously filling the same place in the natural economy of this archipelago, that strikes me with wonder emphasis added.FIGURE 8.1 On the Galapagos Islands Charles Darwin encou

6、ntered many examples of readily observed plants and animal species that differed physically from one island to another island. Here a Gaapagos hawk lands on a giant tortoise for which the islands are named. Darwin wondered at the sources of the differences among clearly related populations and attem

7、pted to explain the origin of these differences. He would later conclude that these populations were descended from common ancestors whose descendants had changed after reaching each of the islands. The ship to which the seamen rowed was the H.M.S. Beagle. Halfway through a voyage around the world.

8、The main objective of the Beagles mission, charting the coasts of southern South America would he largely forgotten, while the thoughts of the young Charles Darwin would eventually develop into one of the most significant theories in the history of science. Darwins wonderlng, carefully organized and

9、 supported by a lifetime of observation, would become the theory of evolution by natural selection, a theory that would transform the prevailing scientific view of life on earth and rebuild the foundations of biology. Darwin left the Galapagos Islands convinced that the various populations on the is

10、lands were gradually modified from their ancestral forms. In other words, Darwin concluded that the island populations had undergone a process of evolution, a process that changes populations of organisms over time. Though Darwin left the Galapagos convinced that the island populations had evolved,

11、he had no mechanism to explain the evolutionary changes that he was convinced they had undergone. However, a plausible mechanism to produce evolutionary change in populations came to Darwin almost exactly 3 years after his taking leave of the Galapagos Islands. In October of 1838 while reading the e

12、ssay on populations by Thomas Malthus, Darwin was convinced that during competition for limited resourees, such as food or space, among individuals within populations, some individuals would have a competitive advantage. He proposed that the characteristics producing that advantage would be preserve

13、d and the unfavorable characteristics of other individuals would be destroyed. As a consequence of this process of selection by the environment, populations would change over time. With this mechanism for change in hand, Darwin sketched out the first draft of his theory of natural selection in 1842.

14、 However, it would take him many years and many drafts before he honed the theory to its final form and amassed sufficient supporting information. Darwins theory of natural selection can be summarized ecs follows:1. Organisms beget like organisms. (Offspring appear, behave, function, and so forth li

15、ke their parents.)2. There are chance variations between individuals in a species. Some variations (differences among parents) are heritable (are passed on to offspring).3. More offspring are produced each generation than can be supported by the environment.4. Some individuals, because of their phys

16、ical or behavioral traits, have a higher chance of surviving and reproducing than other individuals in the same population. Darwin (1859) proposed that differential survival and reproduction of individuals would produce changes in species populations over time. That is, the environment acting on var

17、iation among individuals in populations would result in adaptation of the population to the environment. He now had a mechanism to explain the differences among populations that he had observed on the Galapagos Islands. Still, Darwin was keenly aware of a major insufficiency in his theory. The theor

18、y of natural selection depended upon the passage of advantageous characteristics from one generation to the next. The problem was that the mechanisms of inheritance were unknown in Darwins time. In addition, the prevailing idea at the time, blending inheritance, suggested that rare traits, no matter

19、 bow favorable, would be blended out of a population, preventing change as a consequence.Darwin worked for nearly half a century to uncover the laws of inheritance. However, he did not. To do so required a facility with mathematics that Darwin had not developed. In a short autobiography, Darwin hims

20、elf (1859) remarked, I attempted mathematics, and even went during the summer of 1828 with a private tutor., but I got on very slowly. The work was repugnant to me, chiefly from my not being able to see any meaning in the early steps in algebra. This impatience was very foolish, and in after years I

21、 have deeply regretted that I did not proceed far enough at least to understand something of the great leading principles of mathematics, for men thus endowed seem to have an extra sense emphasis addedl. As Darwin explored the Galapagos Islands, halfway around the world in central Europe a school bo

22、y named Johann Mendel was studying under difficult conditions and developing the facility with mathematics necessary to complete Darwins theory of natural selection. At thirteen, Johann was half Darwins age, yet he had already set a course for a life of study which he followed as resolutely as the c

23、rew of the Beagle on their voyage around the world. At the end of his scientific voyage, Mendel would uncover the basic mechanisms of inheritance. Mendel was the oldest child of a family that farmed a small landholding near Brno, a town in what is now the Czech Republic. He would have had little sch

24、ooling if it were not for the philanthropy of the countess Walpurga Truchsess-Zeil who ruled the district in which Mendels family lived. The countess had a standing order to her advisors that they should identify all of the promising boys and girls living within her domain and send them to school, w

25、here she paid their room and board. Mendel had been one of those children. The countess was more than a philanthropist, however. She also paid attention to details, including the curriculum of her school, which she specified should include the natural sciences. Thus from the outset, Mendels studies

26、included a firm grounding in the sciences. A countess with foresight, intelligence, and heart and her perceptive advisors had discovered an intellectual treasure and provided for the blossoming of one of biologys great geniuses. Johann would be renamed Gregor Mendel when he joined the Augustinian or

27、der of monks that maintained a monastery near his birthplace. In a garden within the walls of the abbey, Mendel would discover what Darwins around-the-world voyage would not reveal. The two keys to Mendels discoveries would be excellent training in mathematics and physics from which he derived a sen

28、se of quantitative relationships and the power of experimental approaches to the study of the natural world. What did Mendel discover? Briefly he discovered what we now call Mendelian genetics, including the very fundamental concept of particulate inheritance. That is the concept that characteristic

29、s pass from parent to offspring in the form of discrete packets of information that we now call genes. Mendel also determined that genes come in alternative forms, which we term alleles. For instance, Mendel worked with alleles such as round versus wrinkled seeds and tall versus short plants. In add

30、ition, he found that some alleles prevent the expression of other alleles. We call such alleles dominant and the alleles that they suppress recessive. Mendels work also revealed the distinction between genotype and phenotype and the difference between homozygous and heterozygous genotypes. Mendels w

31、ork, which revealed still other aspects of the laws of inheritance, laid a solid foundation for the science of genetics. How did Mendel succeed, while so many others had failed? The sources of his success can be traced to his education and his own special genius. Mendels education at the University

32、of Vienna exposed him to some of the best minds working in the physical sciences and to an approach to science that emphasized experimentation. His introduction to the physical sciences included a solid foundation in mathematics, including probability and statistics. As a consequence.Mendel could qu

33、antify the results of his experimental research. Mendel chose to work with plants which could be maintained in the abbey garden. His most famous and influential work was done on the garden pea, Pisum sativum, that has many desirable traits (fig. 8.2). Many domestic varieties of peas, which showed a great deal of physical