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.
注：适应是针对环境的；如果环境以特定的方式发生变化，一个有利的特性就会变成不利的。
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。
Stabilising selection 稳定化选择
Initially there is a wide range of fur length about the mean of 1.5cm.
最初，毛皮长度的范围很广，平均为1.5厘米。
Due to rapid breeding in either very cold or very warm weather, animals with extreme fur lengths survive.
由于在非常寒冷或非常温暖的天气中快速繁殖，具有极端毛发长度的动物得以生存。
When the temperature remains constant with little variation, the individuals with very short or very long hair become less numerous and are eventually eliminated from the population.
当温度保持不变，变化不大时，毛发很短或很长的个体数量就会减少，并最终从种群中被淘汰。
Directional selection 方向性选择
If the temperature falls, the individuals with longer fur length are at an advantage as they have better insulation against the cold.
如果气温下降，毛发较长的个体处于优势地位，因为它们有更好的绝缘性能来抵御寒冷。
There is a selection pressure favouring the animals with longer fur so these animals are more likely to survive and thus reproduce.
有一种有利于长毛的动物的选择压力，所以这些动物更有可能生存下来，从而进行繁殖。
Over several generations, the average fur length increases as more young have inherited the genes for long fur. When the mean fur length has reached the most advantageous length, the selection pressure ceases.
经过几代人的努力，平均毛皮长度增加，因为更多的年轻人继承了长毛皮的基因。当平均毛皮长度达到最有利的长度时，选择压力就停止了。
Disruptive selection 破坏性选择
If the temperature difference between summer and winter increases, long hair for animals being active during the winter or short hair for animals being active during the summer is advantageous.
如果夏季和冬季的温差增大，在冬季活动的动物的长毛或在夏季活动的动物的短毛是有利的。
Intermediate fur length is disadvantageous. Therefore, two sub populations are formed over time.
中间的毛发长度是不利的。因此，随着时间的推移，会形成两个亚种群。