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. 在过去的150年里,大马哈鱼栖息地的范围因人类的活动诸如采矿,家畜的放养,木材的砍伐,以及农业休闲和城镇开发用地而大幅度消失了。 The numerical effect is obvious: there are fewer salmon in degraded regions than in pristine ones; 数字上的影响十分的明显:受影响区域的大马哈鱼要比未受影响的区域少得多。 however, habitat loss also has the potential to reduce genetic diversity. 然而,栖息地的减少同样也潜在的较少了基因的多样性。 This is most evident in cases where it results in the extinction of entire salmon populations. 这在那些因此而导致的全部的大马哈鱼灭绝的地方是非常明显的。 Indeed, most analysts believe that some kind of environmental degradation underlies the demise of many extinct salmon populations. 确实,大部分分析家相信一些环境的恶化是导致大量灭绝的大马哈鱼死亡的基础。 (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,因为环境问题) 尽管一些河流被重新开拓,但最初的唯一的基因已经消失了。 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. 一个区域的大规模的骚乱同样可能会潜在的改变其相邻区域的成员的基因结构,即使这些区域是未受到恶化的。 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. 为什么呢?尽管,使得大马哈鱼回到他们出生地的自导引的本能很强,但是一小部分在回程中的鱼(大约为15%)会在其附近的水域中迷路或产卵。 Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be (40) repopulated should the fish there disappear. 低程度的迷路非常重要,因为该过程提供一种新的基因和机构使得那些鱼儿消失的区域可以被重新入住。 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. 然而,大量的鱼儿迷路将会带来很大的问题,因为迷路的鱼会与那些已经当地血统的鱼进行杂交到一个使得本地鱼存在的特征被冲淡的程度。 Straying rates remain relatively low when environmental conditions are (50) stable, but can increase dramatically when streams suffer severe disturbance. 当环境的情况比较稳定时,迷路的比例还是相对较低的,但是当河流遭受严重的扰动时,这个比例会非常明显的增加。 The 1980 volcanic eruption of Mount Saint Helens, for example, sent mud and debris (55) into several tributaries of the Columbia River. 例如,1980年的MSH的火山爆发将很多的泥浆和碎片送到哥伦比亚河的几条支流中。 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. 在接下来的二年里,从大海里返回产卵的st(大马哈鱼的一种)就需要找到其他代替的流域. As a consequence, their rates of straying, initially 16 percent, rose to more than 40 percent (65) overall.(另外后果及原因:干扰同样导致基因结构变化,因为洄游路线频率上升) 其中一个后果就是,他们迷路的比例从原有的16%升到了40%. Although no one has quantified changes in the rate of straying as a result of the disturbances caused by humans, there is no (70) reason to suspect that the effect would be qualitatively different than what was seen in the aftermath of the Mount Saint Helens eruption. 尽管没有人量化因为人类的骚扰而迷路的比例的变更,但没有理由怀疑这个效果会在本质上与MSH火山爆发那次所带来的后果有什么不同. Such a dra- (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.(最终结果:后代适应性变差) 这种明显的恶化的区域比未受恶化的区域的迷路的增加导致了实质上的基因的变动,而这种变动也相应的降低了后代的适应性. |