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For a species to survive, it must reproduce. However, the population is limited by environmental factors and so remains more or less constant over time.
一个物种要想生存，就必须进行繁殖。然而，种群受到环境因素的限制，因此随着时间的推移或多或少地保持不变。
There is competition between individuals of the same species (intraspecific competition) or between members of different species (interspecific competition) for resources.
同一物种的个体之间（种内竞争）或不同物种的成员之间（种间竞争）存在着资源的竞争。
Since there is variation within a population, some individuals are less well adapted to a particular environment. The less well adapted are 'weeded out' as the selection acts on the phenotype of the individual.
由于种群内部存在差异，一些个体对特定环境的适应性较差。由于选择作用于个体的表型，适应性较差的个体被 "淘汰 "了。
These individuals fail to reproduce or die, the more successful ones reproduce and pass on their genes to the next generation.
这些个体不能繁殖或死亡，较成功的个体则繁殖并将其基因传给下一代。
Note: Adaptations are environment-specific; an advantageous characteristic can become disadvantageous if the environment changes in a particular way.
The change in adaptation that occurs is called evolution.
所发生的适应性变化被称为进化。
There are three types of selection that occur in nature
在自然界中有三种类型的选择:
Stabilizing selection. 稳定化选择
Directional selection. 方向性选择
Disruptive selection. 破坏性选择
In each case we will use the illustration of a population of mammals and the characteristic being selected for or against is fur length.
在每一种情况下，我们都将使用哺乳动物种群的例子，被选择的特征是毛发长度。
In each situation, the population is normally distributed - there are a few individuals that have very short or very long fur length but most have an intermediate fur length.
在每种情况下，种群都是正态分布的--有几个个体的毛发长度非常短或非常长，但大多数个体的毛发长度处于中间水平。
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