Variation: Sources, Types and Evolutionary Significance | 变异：来源、类型与进化意义

📚 Variation: Sources, Types and Evolutionary Significance | 变异：来源、类型与进化意义

Variation refers to the differences between individuals within a species or population. These differences may be visible in morphology, physiology, behaviour or biochemistry. For A-Level Biology, it is essential to distinguish continuous from discontinuous variation, to explain how genetic and environmental factors interact, and to appreciate why variation is the raw material for natural selection. This article breaks the topic into key exam-ready sections with paired English and Chinese explanations.

变异是指同一物种或种群内个体之间的差异。这些差异可以表现在形态、生理、行为或生化等方面。在 A-Level 生物中，必须区分连续变异与不连续变异，解释遗传因素和环境因素如何相互作用，并理解变异为什么是自然选择的原材料。本文将主题拆分为适合考试的关键小节，每节均提供中英对照讲解。

1. Defining Variation | 变异的定义

Variation is the presence of differences in traits among individuals of the same species. These traits may be anatomical, physiological or behavioural. Without variation, every individual would be genetically and phenotypically identical, and evolution by natural selection could not occur. In a population, variation is usually described statistically using the mean, range and standard deviation, because individuals rarely share the exact same phenotype.

变异是指同一物种个体之间性状存在差异。这些性状可以是解剖结构、生理功能或行为方式。如果没有变异，所有个体在遗传和表型上都完全相同，自然选择驱动的进化就不可能发生。在种群中，变异通常用平均值、极差和标准差等统计量来描述，因为个体很少具有完全相同的表型。

• Intraspecific variation: differences within one species | 种内变异：同一物种内部的差异
• Interspecific variation: differences between species | 种间变异：不同物种之间的差异

2. Continuous and Discontinuous Variation | 连续变异与不连续变异

Continuous variation shows a complete range of phenotypes between two extremes, such as height, mass or leaf length. Traits are usually quantitative and controlled by many genes plus environmental influences. When plotted on a frequency histogram, continuous data often form a bell-shaped normal distribution. There are no clear-cut categories; intermediate phenotypes are common.

连续变异在两个极端之间表现出完整的表型范围，例如身高、体重或叶片长度。这些性状通常是数量性状，受多个基因和环境因素共同控制。用频率直方图绘制时，连续数据常呈钟形正态分布。连续变异没有明确的类别，中间表型很常见。

Discontinuous variation falls into distinct groups with no intermediates. Examples include ABO blood groups, tongue rolling, sex in mammals and seed shape in peas (round or wrinkled). These traits are usually controlled by one or a few genes and are less influenced by the environment. Data are best represented by bar charts rather than histograms.

不连续变异分为截然不同的组别，没有中间类型。例子包括 ABO 血型、卷舌、哺乳动物性别以及豌豆种子形状（圆粒或皱粒）。这些性状通常由一个或少数几个基因控制，受环境影响较小。此类数据最适合用条形图而不是直方图表示。

3. Genetic Causes of Variation | 变异的遗传原因

Genetic variation arises from differences in alleles, genes and chromosome combinations. It is inherited and can be passed to offspring. The main genetic sources are mutation, meiosis (crossing over and independent assortment) and random fertilisation. These processes create new alleles or new combinations of existing alleles without necessarily changing allele frequencies.

遗传变异来源于等位基因、基因和染色体组合的差异。这种变异可以遗传给后代。主要的遗传来源包括突变、减数分裂（交叉互换和独立分配）以及随机受精。这些过程产生新的等位基因或现有等位基因的新组合，但不一定改变等位基因频率。

• Mutation creates new alleles | 突变产生新等位基因
• Meiosis reshuffles alleles | 减数分裂重排等位基因
• Random fertilisation combines gametes | 随机受精组合配子

4. Environmental Causes of Variation | 变异的环境原因

Environmental variation results from differences in climate, diet, light intensity, water availability, pH and other external factors. These effects are not inherited; they alter the phenotype without changing the genotype. For example, soil pH can change hydrangea flower colour, and poor nutrition can reduce final height. Identical twins raised apart often show environmental differences in body mass or skin colour.

环境变异由气候、饮食、光照强度、水分供应、pH 值等外部因素差异引起。这些影响不会遗传；它们改变表型而不改变基因型。例如，土壤 pH 可改变绣球花的花色，营养不良会降低最终身高。分开抚养的同卵双胞胎常在体重或肤色上表现出环境差异。

5. Mutation as the Ultimate Source | 突变作为最终来源

Mutation is the only process that creates brand-new alleles. A gene mutation changes the DNA base sequence, which may alter the polypeptide produced. Chromosome mutations

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