Over the last 150 years, large stretches of salmon habitat have been eliminated by human activity: mining, livestock grazing, timber
Large-scale disturbances in one locale also have the potential to alter the genetic structure of populations in neighboring areas, even if those areas have pristine habitats. Why? Although the homing instinct of salmon to their natal stream is strong, a fraction of the fish returning from the sea (rarely more than 15 percent) stray and spawn in nearby streams. Low levels ofstraying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulates should the fish there disappear. Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock to such a degree that any local adaptations that are present become diluted. Straying rates remain relatively low when environmental conditions are stable, but can increase dramatically when streams suffer severe disturbance. The 1980 volcanic eruption of Mount Saint Helens, for example, sent mud and debris into several tributaries of the Columbia River. For the next couple of years, steelhead trout (a species included among the salmon ) returning from the sea to spawn were forced to find alternative streams. As a consequence, their rates of straying, initially 16 percent, rose to more than 40 percent overall.
Although no one has quantified changes in the rate of straying as a result of the disturbances caused by humans, there is no reason to suspect that the effect would be qualitatively different than what was seen in the aftermath of the Mount Saint Helens eruption. Such a dramatic increase in straying from damaged areas to more pristine streams results in substantial gene flow, which can in turn lower the overall fitness of subsequent generations.
The author's argument that increased straying can “lower the overall
A. A disturbance of salmonid spawning streams caused by human activity could increase the straying rate of affected salmonid populations as much as the aftermath of the Mount Saint Helens eruption did.
B. In the streams in which the straying salmonid spawn, these straying salmonid would amount to no more than 40 percent of the total spawning population.
C. Salmonid in some streams benefit from particular local adaptations.
D. Nonenvironmental factors have no effect on salmonid straying rates.
E. At least some of the streams in which straying salmonid would spawn are pristine, affected by neither natural nor artificial disturbances.
- 【OG18-P680-668题】In a review of 2,000 studies of human behavior that date back to the 1940s, two Swiss psychologists, declaring that since most of the studies had failed to control for such variables as social class and family size, none could be taken seriously.
- Following several years of declining advertising sales, the Greenville Times reorganized its advertising sales force two years ago. Before the reorganization, the sales force was organized geographically, with some sales representatives concentrating on city-center businesses and others concentrating on different outlying regions. The reorganization attempted to increase the sales representatives’ knowledge of clients’ businesses by having each sales representative deal with only one type of industry or of retailing. After the reorganization, advertising sales increased.In assessing whether the improvement in advertising sales can properly be attributed to the reorganization, it would be helpful to find out each of the following EXCEPT:
- The author of the passage would most likely agree with which of the following statements about the link between increased solar activity and certain seasonal weather changes on the Earth?
- The passage provides information to support which of the following statement about La Nina?
- X: In order to reduce the amount of plastic in landfills, legislatures should impose a ban on the use of plastics for packaging goods.Y: Impossible! Plastic packaging is necessary for public safety. Consumers will lose all of the safety features that plastic offers, chiefly tamper-resistant closures and shatterproof bottles.Which of the following best describes the weak point in Y's response to X's proposal?