Over the last 150 years, large stretches of salmon habitat have been eliminated by human activity:mining, livestock grazing, timber harvesting, and agriculture as well as recreational and urban development. The numerical effect is obvious: there are fewer salmon in degraded regions than in pristine 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 populations. Indeed, most analysts believe that some kind of environmental degradation underlies the demise of many extinct salmon populations. Although some rivers have been recolonized, the unique genes of the original populations have been lost.
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 of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated 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 salmonids) 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 dra- 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.
The author mentions the "aftermath of the Mount Saint Helens eruption" most likely in order to
在过去的150年里，大量鲑鱼栖息地的分枝已经因为人类活动而消亡：采矿、家畜放牧、木材收割、农业以及消闲娱乐和城市的发展。数量效应十分显著：相比原始栖息地，退化区域的鲑鱼更少；然而，栖息地的减少也有可能降低基因多样性。最明显的例子是基因多样性降低导致整个鲑鱼种群灭绝。的确，大多数分析人士相信一些环境的退化是许多灭绝鲑鱼种群走向灭亡的基础。尽管一些流域得到重新开拓，但是原始种群的独特基因已经流失了。
一个地区大规模的扰动有可能改变邻近区域种群的基因结构，即使那些区域有原始栖息地。为什么？尽管鲑鱼洄游本能（返回它们出生的溪流）很强，从海洋中洄游的鲑鱼中的一部分（很少超过15%）迷失在附近的溪流，并在其中产卵。低水平的迷失十分重要，因为这个过程为新基因提供了来源，并且提供了一种机制，通过这种机制当一个地区的鱼消失后，新种群可以重新入住。然而，高迷失率会带来问题，因为迷失方向的鲑鱼可能会与既有的当地鱼类杂交，杂交程度之高使得现存的当地适应性被稀释。迷失率在环境条件稳定时保持相对较低的水平，但是当溪流遭到严重扰动时会显著上升。比如说，1980年圣海伦斯火山的喷发将许多泥土和碎片送到了哥伦比亚河的一些支流。之后的一些年，从海洋洄游的steelhead trout（鲑鱼中的一种）被迫寻找代替的溪流。结果，他们的迷失率从最初的16%上升到超过40%。
尽管不曾有人量化有人类引起的扰动带来的迷失率，没有理由怀疑人类扰动带来的影响会和圣海伦斯火山喷发后带来的影响有什么本质不同。从被损害地区到更原始的支流的迷失率的显著上升导致了实质性的基因漂流，而基因漂流反过来降低了后代的整体适应性。
该题为手段目的题：正确答案为(C)。
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