1. GWD-1-Q9 to Q12环境退化导致鲑鱼数量下降
Over the last 150 years, large stretches of salmon habitat have been eliminated by human activity: Line mining, livestock grazing, timber(5) harvesting, and agriculture as well as recreational and urban devel-opment. The numerical effect is obvious: there are fewer salmon in degraded regions than in pris-(10) tine ones; however, habitat loss also has the potential to reduce genetic diversity. This is most evident in cases where it results in the extinction of entire salmon(15) populations. Indeed, most *ysts believe that some kind of environmental degradation underlies the demise of many extinct salmon populations. (20) Although some rivers have been recolonized, the unique genes of the original populations have been lost. 提出现象：数量减少，基因多样性下降，因为环境疑问Large-scale disturbances in (25) 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(30) 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(35) streams. Low levels of straying are crucial, since the process provides a source of novel genes and a mechani* by which a location can be (40) repopulated should the fish there disappear. Yet high rates of straying can be problematic because misdirected fish may interbreed with the existing stock(45) to such a degree that any local adaptations that are present become diluted. Straying rates remain relatively low when environmental conditions are(50) stable, but can increase dramati-cally when streams suffer severe disturbance. The 1980 volcanic eruption of Mount Saint Helens,for example, sent mud and debris(55) into several tributaries of the Columbia River. For the next couple of years, steelhead trout (a species included among the salmonids) returning from the(60) 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(65) overall.此外后果及原因:干扰同样导致基因结构变化，因为洄游路线频率上升Although no one has quantified changes in the rate of straying as a result of the disturbances caused by humans, there is no(75) matic 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.很终结果：后代适应性变差结论解释
1P:samlon habitat eliminated by… effect obvious: fewer; however, reduce genetic diversity
2P: also alter genetic structure. Low crucial; yet high, problematic.
*: although, no reason to suspect…. Increase result in, in turn lower the …