📚 Evolution | 进化
Evolution is the change in the heritable characteristics of biological populations over successive generations. It is the unifying theory of biology, explaining both the diversity of life and the shared ancestry of organisms.
进化是指生物种群在连续世代中遗传特征的变化。它是生物学的统一理论，解释了生命的多样性以及生物的共同祖先。
1. Introduction to Evolution | 进化简介
At its core, evolution is defined as a change in allele frequencies within a gene pool over time. This process occurs at the level of a population, not at the level of an individual organism. A gene pool comprises all the alleles for all genes in a reproducing population.
进化的核心定义为基因库中等位基因频率随时间的变化。这一过程发生在种群层面，而不是个体生物层面。基因库包含一个繁殖种群中所有基因的全部等位基因。
Evolution does not necessarily lead to increasing complexity; it simply results in adaptations that enhance survival and reproduction in a given environment. Microevolution refers to small-scale changes in allele frequencies, while macroevolution describes the large-scale patterns, such as the origin of new species.
进化不一定会导致复杂性的增加；它只是产生那些在特定环境中增强生存和繁殖的适应。微进化指等位基因频率的小规模变化，而宏进化描述大规模的模式，例如新物种的起源。
2. Darwin's Theory of Natural Selection | 达尔文的自然选择学说
Charles Darwin and Alfred Russel Wallace independently proposed evolution by natural selection. Darwin's observations included: overproduction of offspring, heritable variation within populations, competition for limited resources, and differential survival and reproduction of individuals best suited to the environment.
查尔斯·达尔文和阿尔弗雷德·拉塞尔·华莱士独立提出了自然选择进化论。达尔文的观察包括：后代过度生产、种群内的可遗传变异、对有限资源的竞争，以及最适应环境的个体差异化的生存和繁殖。
Natural selection acts on phenotypes that are at least partly heritable. Individuals possessing advantageous traits are more likely to survive to reproductive age and produce more offspring, passing on the alleles for those traits. Over many generations, the frequency of favourable alleles increases, leading to adaptation.
自然选择作用于至少部分可遗传的表型。拥有有利性状的个体更有可能存活到繁殖年龄并产生更多后代，从而传递这些性状的等位基因。经过许多代后，有利等位基因的频率增加，导致适应。
Darwin's theory does not propose that individuals change during their lifetimes; rather, the proportion of individuals with certain genotypes in a population shifts gradually across generations.
达尔文的理论并不主张个体在其一生中发生改变；而是种群中具有特定基因型的个体比例在世代间逐渐变化。
3. Sources of Genetic Variation | 遗传变异的来源
Genetic variation is the raw material for evolution. Mutation, the random change in DNA base sequences, is the ultimate source of all new alleles. Meiosis generates new combinations of existing alleles through crossing over and independent assortment. Random fertilisation further increases variation by uniting unique gametes.
遗传变异是进化的原材料。突变，即DNA碱基序列的随机变化，是所有新等位基因的根本来源。减数分裂通过交叉和独立分配产生现有等位基因的新组合。随机受精通过将独特的配子结合，进一步增加变异。
Gene flow, the movement of alleles between populations via migration, can also introduce new alleles or change existing frequencies. Although mutations are rare and often neutral or harmful, occasionally a mutation confers a selective advantage that can spread in a population.
基因流动，即等位基因通过迁移在种群间移动，也能引入新的等位基因或改变现有频率。尽管突变罕见且通常为中性或有害，但偶尔也会产生选择性优势，并在种群中传播。
4. Types of Natural Selection | 自然选择的类型
Three main patterns of natural selection can be distinguished based on how phenotypic distributions change over time: directional selection, stabilising selection, and disruptive selection.
根据表型分布随时间的变化方式，可以区分三种主要的自然选择模式：定向选择、稳定化选择和分裂选择。
| Selection Type | Description | Effect on Phenotype | 选择类型 | 描述 | 对表型的影响 |
|---|---|---|---|---|---|
| Directional | Favours one extreme phenotype, causing the mean to shift. | Population curve shifts towards the favoured extreme. | 定向选择 | 偏爱一个极端表型，导致平均值偏移。 | 种群曲线向有利的极端移动。 |
| Stabilising | Favours intermediate phenotypes; extremes are selected against. | Narrows the curve around the mean. | 稳定化选择 | 偏爱中间表型，极端表型被淘汰。 | 使曲线围绕平均值变窄。 |
| Disruptive | Favours both extreme phenotypes while intermediate forms are selected against. | Creates a bimodal distribution. | 分裂选择 | 同时偏爱两种极端表型，中间型被淘汰。 | 形成双峰分布。 |
Directional selection is exemplified by the evolution of antibiotic resistance in bacteria or by the increase in average beak size in Darwin's finches during a drought. Stabilising selection is seen in human birth weight, where very low or very high birth weights carry increased mortality. Disruptive selection can occur in environments with distinct resources, leading to a bimodal distribution, such as in African seedcrackers with different beak sizes adapted to hard and
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