📚 Species Concepts and Speciation | 物种概念与物种形成
In A-Level Biology, few topics bridge genetics, ecology, and evolution as elegantly as the study of species concepts and speciation. Understanding what defines a species and how new species arise is fundamental to interpreting the diversity of life on Earth and the mechanisms that drive evolutionary change.
在A-Level生物学中，很少有主题能像物种概念与物种形成的研究那样，将遗传学、生态学和进化论如此优雅地联系在一起。理解什么定义了物种以及新物种是如何产生的，对于解读地球上生命的多样性以及驱动进化变化的机制至关重要。
1. What Is a Species? | 什么是物种？
The concept of a “species” is one of the most fundamental yet debated ideas in biology. A species is commonly described as the smallest distinct group of organisms that share a common ancestry and can interbreed to produce fertile offspring. However, this seemingly simple definition becomes complex when applied to organisms that reproduce asexually, hybridise readily, or exist only in the fossil record.
“物种”的概念是生物学中最基本但也最具争议的观点之一。物种通常被描述为共享共同祖先并能交配产生可育后代的最小独立生物群。然而，当这个看似简单的定义应用于无性生殖、容易杂交或只存在于化石记录中的生物时，就变得复杂起来。
The word “species” originates from Latin, meaning “kind” or “appearance.” Early naturalists classified organisms primarily by visible morphology, but modern biology recognises that species are dynamic entities shaped by evolutionary forces, genetic isolation, and ecological interactions.
“物种”一词源自拉丁语，意为“种类”或“外观”。早期的博物学家主要根据可见的形态对生物进行分类，但现代生物学认识到，物种是由进化力量、遗传隔离和生态相互作用塑造的动态实体。
2. The Biological Species Concept | 生物学物种概念
The Biological Species Concept (BSC), proposed by Ernst Mayr in 1942, defines a species as a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. This concept emphasises reproductive compatibility as the key criterion for species membership.
生物学物种概念（BSC）由恩斯特·迈尔于1942年提出，将物种定义为实际或潜在能够相互交配的自然种群群体，这些群体与其他类似群体之间存在生殖隔离。该概念强调生殖相容性是判断物种归属的关键标准。
According to the BSC, members of the same species share a common gene pool because they can exchange genetic material through interbreeding. The crucial feature is that gene flow occurs within a species but is prevented between different species.
根据生物学物种概念，同一物种的成员共享一个共同的基因库，因为它们可以通过交配交换遗传物质。关键特征在于：基因流动在物种内部发生，但在不同物种之间被阻止。
The BSC works well for most animals and many plants that reproduce sexually and have distinct, observable mating behaviours. For example, the domestic dog (Canis lupus familiaris) and the grey wolf (Canis lupus) can interbreed, producing fertile offspring, so they are considered the same species under this concept.
生物学物种概念适用于大多数有性生殖且具有明显可观察交配行为的动物和许多植物。例如，家犬（Canis lupus familiaris）和灰狼（Canis lupus）可以交配并产生可育后代，因此在该概念下它们被认为是同一物种。
However, the BSC has limitations. It cannot be applied to: (a) asexual organisms such as bacteria and some fungi; (b) extinct organisms known only from fossils; and (c) organisms that are geographically separated and never encounter each other naturally, even if they could interbreed if brought together.
然而，生物学物种概念存在局限性。它不能应用于：（a）无性生殖生物，如细菌和某些真菌；（b）仅从化石中得知的已灭绝生物；（c）在地理上分离、自然环境中永远不会相遇的生物——即使它们被带到一起时能够交配。
3. Alternative Species Concepts | 其他物种概念
Because the Biological Species Concept cannot be universally applied, biologists have developed alternative definitions. Three major alternatives are the Morphological Species Concept, the Phylogenetic Species Concept, and the Ecological Species Concept.
由于生物学物种概念无法普遍适用，生物学家提出了替代性定义。三个主要的替代概念是形态学物种概念、系统发育物种概念和生态学物种概念。
The Morphological Species Concept (MSC) defines a species based on structural features such as size, shape, and colour. This concept is practical for field identification and can be applied to fossils. However, it faces the problem of cryptic species—organisms that are morphologically identical but reproductively isolated—and polymorphic species, where individuals look very different but belong to the same species. For instance, the African elephant was once considered a single species, but genetic analysis later revealed two distinct species: the savanna elephant (Loxodonta africana) and the forest elephant (Loxodonta cyclotis), which differ mainly in genetics rather than obvious morphology.
形态学物种概念（MSC）根据结构特征（如大小、形状和颜色）来定义物种。该概念便于野外鉴定，也可应用于化石。然而，它面临隐存种（形态完全相同但生殖隔离的生物）和多态种（个体外观差异很大但属于同一物种）的问题。例如，非洲象曾被认为是一个物种，但遗传分析后来揭示了两个不同的物种：草原象（Loxodonta africana）和森林象（Loxodonta cyclotis），它们主要在遗传学上而非明显的形态上有所不同。
The Phylogenetic Species Concept (PSC) defines a species as the smallest monophyletic group distinguishable by unique genetic or morphological characters. It is based on evolutionary relationships reconstructed from DNA sequences, and it can be applied to asexual organisms and fossils. The main disadvantage is that phylogenetic data are not always available, and different genes may yield different phylogenetic trees.
系统发育物种概念（PSC）将物种定义为可通过独特遗传或形态特征区分的最小单系群。它基于从DNA序列重建的进化关系，可应用于无性生殖生物和化石。主要缺点在于系统发育数据并非总是可获得，且不同基因可能产生不同的系统发育树。
The Ecological Species Concept defines a species as a lineage that occupies a distinct ecological niche or adaptive zone. This concept acknowledges that natural selection, rather than gene flow alone, maintains species boundaries. It is particularly useful for explaining how similar species can coexist in the same habitat by occupying different roles, a phenomenon known as resource partitioning.
生态学物种概念将物种定义为占据独特生态位或适应性区域的一个谱系。该概念认识到自然选择（而非仅仅是基因流动）维持着物种边界。它特别有助于解释相似物种如何通过占据不同角色在同一栖息地共存，这一现象被称为资源分配。
4. Reproductive Isolation Mechanisms | 生殖隔离机制
Regardless of which species concept one adopts, reproductive isolation is central to maintaining species boundaries. Reproductive isolating mechanisms are classified into two broad categories: prezygotic and postzygotic barriers. Prezygotic barriers prevent fertilisation from occurring, while postzygotic barriers operate after fertilisation has taken place.
无论采用哪种物种概念，生殖隔离对于维持物种边界都是核心要素。生殖隔离机制分为两大类：合子前屏障和合子后屏障。合子前屏障阻止受精发生，而合子后屏障在受精完成后起作用。
Prezygotic barriers include habitat isolation, temporal isolation, behavioural isolation, mechanical isolation, and gametic isolation. Habitat isolation occurs when species live in different habitats and rarely encounter one another. Temporal isolation occurs when species breed at different times of day, season, or year. Behavioural isolation involves differences in mating rituals or courtship signals; for example, female fireflies respond only to the specific flash pattern of their own species’ males.
合子前屏障包括栖息地隔离、时间隔离、行为隔离、机械隔离和配子隔离。栖息地隔离发生在物种生活在不同栖息地而极少相遇时。时间隔离发生在物种在一天中的不同时间、不同季节或不同年份繁殖时。行为隔离涉及交配仪式或求偶信号的差异；例如，雌性萤火虫只对自己物种雄性的特定闪光模式作出响应。
Mechanical isolation arises from anatomical incompatibilities in reproductive organs. Gametic isolation occurs when gametes cannot fuse successfully, even if mating has occurred; this is common in marine organisms that release sperm and eggs into the water, where biochemical incompatibilities prevent cross-species fertilisation.
机械隔离源于生殖器官的解剖学不相容。配子隔离发生在即使交配已经发生，配子仍无法成功融合时；这在将精子和卵子释放到水中的海洋生物中很常见，生化不相容性阻止了跨物种受精。
Postzygotic barriers include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown. Reduced hybrid viability means that hybrid zygotes fail to develop or hybrids fail to reach sexual maturity. Reduced hybrid fertility refers to hybrids that are vigorous but sterile—the classic example is the mule, a hybrid between a horse and a donkey, which is healthy and strong but cannot reproduce. Hybrid breakdown occurs when first-generation hybrids are viable and fertile, but their offspring are feeble or infertile.
合子后屏障包括杂种活力降低、杂种可育性降低和杂种衰退。杂种活力降低意味着杂合子无法发育或杂种无法达到性成熟。杂种可育性降低是指杂种健壮但不可育——典型例子是骡子，它是马和驴的杂交后代，健康强壮但无法繁殖。杂种衰退发生在第一代杂种存活且有繁殖力，但它们的后代虚弱或不育时。
5. Modes of Speciation | 物种形成的方式
Speciation is the evolutionary process by which new species arise. It requires the accumulation of genetic differences between populations, ultimately leading to reproductive isolation. Three primary modes of speciation are recognised: allopatric, sympatric, and parapatric speciation.
物种形成是新物种产生的进化过程。它要求种群之间遗传差异的积累，最终导致生殖隔离。三种主要的物种形成模式被确认：异域性物种形成、同域性物种形成和边域性物种形成。
Allopatric speciation, meaning “different homeland,” occurs when a physical barrier such as a mountain range, river, or ocean separates a population. Geographic isolation prevents gene flow, allowing populations to diverge independently through mutation, natural selection, and genetic drift. Over time, accumulated genetic differences may result in reproductive isolation. The classic example is Darwin’s finches in the Galápagos Islands: different island populations evolved different beak shapes adapted to different food sources, eventually becoming separate species. Another striking example is the formation of new species of cichlid fish in the isolated crater lakes of East Africa, where each lake harbours endemic species found nowhere else.
异域性物种形成（意为“不同家园”）发生在物理屏障（如山脉、河流或海洋）将种群分离时。地理隔离阻止了基因流动，使种群通过突变、自然选择和遗传漂变独立分化。随着时间的推移，累积的遗传差异可能导致生殖隔离。经典例子是加拉帕戈斯群岛的达尔文雀：不同岛屿的种群进化出适应不同食物来源的不同喙形，最终成为独立物种。另一个引人注目的例子是东非隔离火山湖中丽鱼科鱼新物种的形成，每个湖泊都栖息着其他地方找不到的特有物种。
Sympatric speciation, meaning “same homeland,” occurs when new species arise within the same geographic area without physical separation. This mode is more controversial and less common, but it is well-documented in plants through polyploidy—the duplication of entire chromosome sets. If a tetraploid plant arises within a diploid population, it can self-fertilise or mate with other tetraploids, but it cannot produce fertile offspring with diploids because the triploid offspring would be infertile due to uneven chromosome pairing during meiosis.
同域性物种形成（意为“同一家园”）发生在没有物理隔离的情况下，同一地理区域内产生新物种时。这种模式更具争议性且较少见，但通过多倍化——整个染色体组的复制——在植物中已得到充分证实。如果一个四倍体植物出现在二倍体种群中，它可以自花授粉或与其他四倍体交配，但不能与二倍体产生可育后代，因为三倍体后代在减数分裂期间染色体配对不均而无法生育。
Another mechanism of sympatric speciation is disruptive selection combined with habitat or host-plant preference. For example, the hawthorn fly (Rhagoletis pomonella) originally laid its eggs on hawthorn fruits. When apple trees were introduced to North America, some flies began laying eggs on apples. The two populations now exhibit genetic differences and temporal isolation because they emerge at different times of the year to match their respective fruit availability.
同域性物种形成的另一种机制是分裂选择与栖息地或寄主植物偏好相结合。例如，山楂蝇（Rhagoletis pomonella）最初在山楂果实上产卵。当苹果树被引入北美后，一些果蝇开始在苹果上产卵。这两个种群现在表现出遗传差异和时间隔离，因为它们在一年中的不同时间出现，以匹配各自果实的可用时间。
Parapatric speciation occurs when populations are adjacent but not completely separated, with a narrow zone of contact where some gene flow still occurs. Selection pressures differ across the geographic gradient, and reproductive isolation develops despite limited interbreeding. This mode is less common but has been observed in certain plant and grass species that grow across environmental gradients, such as heavy-metal-contaminated soils near abandoned mines.
边域性物种形成发生在种群相邻但未完全分开的情况下，存在一个狭窄的接触带，其中仍有部分基因流动。选择压力在地理梯度上有所不同，尽管有限的杂交仍在进行，但生殖隔离仍然发展起来。这种模式较少见，但已在某些植物和禾本科物种中被观察到，它们生长在环境梯度上，如废弃矿山附近受重金属污染的土壤中。
6. Genetic Drift and the Founder Effect | 遗传漂变与建立者效应
Genetic drift is the random change in allele frequencies within a population due to chance events, particularly significant in small populations. Unlike natural selection, which systematically favours advantageous alleles, genetic drift is non-adaptive and can lead to the random fixation or loss of alleles. The founder effect is a specific type of genetic drift that occurs when a small group of individuals colonises a new area, carrying only a fraction of the genetic diversity of the original population.
遗传漂变是由于随机事件导致种群内等位基因频率的变化，在小种群中尤为显著。与系统性地有利于有利等位基因的自然选择不同，遗传漂变是非适应性的，可能导致等位基因的随机固定或丢失。建立者效应是一种特定类型的遗传漂变，发生在一小群个体拓殖新区域时，只携带原始种群遗传多样性的一小部分。
The founder effect has profound implications for speciation. Isolated founder populations are particularly susceptible to genetic drift because their small size amplifies random fluctuations in allele frequencies. This can lead to rapid divergence from the parent population, potentially accelerating reproductive isolation. For example, the Amish community in Pennsylvania exhibits an unusually high frequency of certain genetic disorders, such as Ellis-van Creveld syndrome, because the original founders carried these alleles at higher frequencies than the general European population.
建立者效应对物种形成具有深远影响。孤立的建立者种群特别容易受到遗传漂变的影响，因为它们的小规模放大了等位基因频率的随机波动。这可能导致与亲本种群的快速分化，可能加速生殖隔离。例如，宾夕法尼亚州的阿米什社区表现出某些遗传疾病（如埃利斯-范克雷维尔德综合征）异常高的频率，因为最初的建立者携带这些等位基因的频率高于一般欧洲人群。
The bottleneck effect is another form of genetic drift. It occurs when a large population is drastically reduced in size—for example, by a natural disaster or overhunting—and the surviving individuals carry only a random subset of the original gene pool. The northern elephant seal experienced a severe bottleneck in the 1890s when hunting reduced its population to approximately 20 individuals. Today, the entire population of over 100,000 seals descends from those few survivors, exhibiting extremely low genetic diversity.
瓶颈效应是遗传漂变的另一种形式。当一个大种群因自然灾害或过度捕猎等原因急剧缩小，幸存个体只携带原始基因库的一个随机子集时，就会发生这种情况。北象海豹在19世纪90年代经历了严重的瓶颈，当时捕猎使其种群减少到约20个个体。如今，超过10万只海豹的整个种群都来源于那几个幸存者，表现出极低的遗传多样性。
7. Natural Selection and Adaptive Radiation | 自然选择与适应辐射
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. In the context of speciation, natural selection acts on populations in different environments, favouring different adaptations. When a single ancestor species gives rise to many new species, each adapted to a distinct ecological niche, the phenomenon is called adaptive radiation.
自然选择是个体因表型差异而产生的差异性生存与繁殖。在物种形成背景下，自然选择作用于不同环境中的种群，偏爱不同的适应性特征。当一个祖先物种产生许多新物种，每个物种适应于独特的生态位时，这一现象称为适应辐射。
Adaptive radiation is most dramatic when an ancestral species colonises an area with abundant unexploited resources, such as newly formed volcanic islands, post-extinction recovery zones, or newly formed lakes. The Hawaiian honeycreepers are a spectacular example: from a single ancestral finch species, more than 50 species evolved, each with a distinct bill shape suited to different feeding strategies—from nectar-feeding curved bills to seed-crushing heavy bills.
当一个祖先物种拓殖一个具有大量未开发资源的地区时，适应辐射最为剧烈，如新形成的火山岛、灭绝后的恢复区或新形成的湖泊。夏威夷蜜鸟是一个壮观例子：从一个祖先雀类物种进化出50多个物种，每个物种都有适合不同取食策略的独特喙形——从吸食花蜜的弯曲喙到压碎种子的重型喙。
The process of adaptive radiation involves repeated allopatric or sympatric speciation events driven by divergent natural selection. Each new species occupies a unique ecological role, reducing competition through niche partitioning. This pattern is so characteristic that the presence of many closely related but ecologically distinct species in one region is strong evidence that adaptive radiation has occurred.
适应辐射的过程涉及由分歧性自然选择驱动的重复异域性或同域性物种形成事件。每个新物种占据独特的生态角色，通过生态位分化减少竞争。这种模式非常典型，以至于一个地区存在许多亲缘关系密切但生态上不同的物种，是适应辐射发生的有力证据。
8. Rates of Speciation: Gradualism vs Punctuated Equilibrium | 物种形成的速率：渐变论与间断平衡论
Evolutionary biologists debate the pace at which speciation occurs. Two competing models have been proposed to describe the tempo of evolutionary change: gradualism and punctuated equilibrium.
进化生物学家一直在争论物种形成发生的速率。两个竞争模型被提出来描述进化变化的节奏：渐变论和间断平衡论。
Gradualism, the traditional Darwinian view, proposes that evolutionary change occurs slowly and steadily over long periods. Under this model, new species arise through the gradual accumulation of small genetic changes, and intermediate forms should be abundant in the fossil record. The evolution of the modern horse from Eohippus, a dog-sized forest dweller, to Equus, the large grazing horse of today, over a period of 50 million years, was long considered a classic gradual sequence.
渐变论是传统达尔文主义观点，提出进化变化在长期内缓慢而稳定地发生。在该模型下，新物种通过小遗传变化的逐渐积累而产生，中间形态应该在化石记录中丰富存在。现代马从狗大小的森林居住者始马进化到如今的大型食草马马属，历时5000万年，长期被视为经典的渐进序列。
Punctuated equilibrium, proposed by Eldredge and Gould in 1972, argues that speciation occurs in rapid bursts separated by long periods of stasis. In this model, species remain relatively unchanged for millions of years, then undergo rapid evolutionary change—usually associated with speciation events—before returning to a new period of stability. The fossil record often shows sudden appearances of new species without gradual transitional forms, which punctuated equilibrium explains by pointing to the rapid, geologically ‘instantaneous’ nature of speciation in small, isolated populations.
间断平衡论由埃尔德里奇和古尔德于1972年提出，认为物种形成以快速爆发的形式发生，中间被长时期的停滞所分隔。在该模型中，物种在数百万年内保持相对不变，然后经历快速的进化变化——通常与物种形成事件相关——随后进入新的稳定期。化石记录常常显示新物种突然出现而没有渐进的过渡形态，间断平衡论将其解释为小规模隔离种群中物种形成的快速、在地质学上“瞬间”的性质。
In reality, both models likely contribute to evolutionary history. A growing body of evidence suggests that different lineages may exhibit different tempos of evolution depending on environmental stability, population size, and the strength of selection pressures. The stickleback fish in post-glacial lakes in British Columbia provide contemporary evidence for rapid speciation: isolated lake populations have diverged in body shape, armour plating, and feeding behaviour within just a few thousand years since the lakes formed.
事实上，两种模型都可能对进化历史有所贡献。越来越多的证据表明，不同谱系可能根据环境稳定性、种群大小和选择压力的强度表现出不同的进化节奏。不列颠哥伦比亚省冰后湖泊中的三刺鱼为快速物种形成提供了当代证据：自湖泊形成以来的短短几千年内，隔离的湖泊种群在体型、甲板和摄食行为上已经发生分化。
9. Case Study: Darwin’s Finches | 案例分析：达尔文雀
Darwin’s finches, also known as Galápagos finches, are arguably the most famous example of speciation in action. They comprise approximately 18 species that evolved from a single ancestral species that colonised the Galápagos Islands from mainland South America an estimated 2–3 million years ago.
达尔文雀，又称加拉帕戈斯雀，可以说是物种形成研究中最著名的实例。它们包括大约18个物种，由一个约200-300万年前从南美大陆拓殖加拉帕戈斯群岛的单一祖先物种进化而来。
The key to their diversification lies in the availability of varied food sources across different islands. Ground finches (Geospiza spp.) have strong, thick beaks adapted for cracking seeds; tree finches (Camarhynchus spp.) have more pointed beaks for capturing insects; and the sharp-beaked ground finch (Geospiza difficilis) even uses cactus spines to extract insects from crevices. This morphological diversity directly corresponds to ecological niche partitioning.
它们多样化的关键在于不同岛屿上各种食物来源的可获得性。地雀（地雀属）有强壮厚实的喙，适合压碎种子；树雀（树雀属）有更尖锐的喙，用于捕获昆虫；而尖嘴地雀（难地雀）甚至使用仙人掌刺从缝隙中提取昆虫。这种形态多样性直接对应于生态位分化。
Research by Peter and Rosemary Grant, who studied Darwin’s finches for over four decades, demonstrated that beak size varies with environmental conditions. During droughts, when small seeds become scarce, finches with larger beaks survive better because they can crack larger, harder seeds. This directional selection is observable over just a few generations. Furthermore, the Grants documented a case where a small population of finches on a tiny island, Daphne Major, became reproductively isolated and formed a new species—a rare direct observation of speciation in real time.
彼得和罗斯玛丽·格兰特的研究持续研究达尔文雀超过四十年，证明喙的大小随环境条件变化。在干旱期间，当小种子变得稀缺时，喙较大的雀类生存得更好，因为它们能压开更大更硬的种子。这种定向选择在短短几代内就可观察到。此外，格兰特夫妇记录了一个案例：达芙妮主岛上的一小群雀类形成了生殖隔离并形成了新物种——这是对物种形成的难得直接实时观察。
10. Polyploidy as a Rapid Speciation Mechanism | 多倍化作为快速物种形成机制
Polyploidy, the possession of more than two complete sets of chromosomes, is a major mechanism of sympatric speciation, particularly in plants. It is estimated that up to 70% of flowering plant species have undergone polyploidisation at some point in their evolutionary history.
多倍化——拥有两套以上完整染色体组——是同域性物种形成的重要机制，在植物中尤为突出。据估计，多达70%的开花植物物种在其进化史上的某个时期经历过多倍化。
Polyploidy can arise in two ways: autopolyploidy, where an individual has multiple chromosome sets from the same species, usually due to failure of meiosis or mitosis; and allopolyploidy, where an individual has chromosome sets from two different species, usually following hybridisation followed by chromosome doubling. Both mechanisms create immediate reproductive isolation, because the polyploid individual cannot produce fertile offspring with the original diploid population.
多倍化通过两种方式产生：同源多倍化，即个体拥有来自同一物种的多套染色体组，通常由减数分裂或有丝分裂失败引起；异源多倍化，即个体拥有来自两个不同物种的染色体组，通常是在杂交后染色体加倍产生。两种机制都立即产生生殖隔离，因为多倍体个体无法与原来的二倍体种群产生可育后代。
The formation of the bread wheat Triticum aestivum is a classic allopolyploidy example. It arose from the natural hybridisation of three different grass species: Triticum urartu (AA), Aegilops speltoides (BB), and Aegilops tauschii (DD). The resulting hexaploid wheat (AABBDD, 2n = 42) is fertile with itself but cannot interbreed with its diploid ancestors, making it a distinct species that arose through a series of hybridisation and chromosome-doubling events.
面包小麦普通小麦的形成是异源多倍化的经典例子。它产生于三种不同禾本科植物的自然杂交：乌拉尔图小麦（AA）、拟斯卑尔脱山羊草（BB）和节节麦（DD）。由此产生的六倍体小麦（AABBDD，2n = 42）自身可育，但无法与其二倍体祖先杂交，使其成为一个通过一系列杂交和染色体加倍事件产生的独特物种。
11. Speciation in Bacteria: The Challenge of Asexuality | 细菌的物种形成：无性生殖的挑战
Bacteria reproduce asexually through binary fission, so the Biological Species Concept cannot be applied directly. Yet bacteria clearly exist as distinct lineages with identifiable characteristics. Speciation in bacteria occurs through mechanisms such as mutation, horizontal gene transfer, and ecological specialisation.
细菌通过二分裂进行无性生殖，因此生物学物种概念不能直接应用。然而细菌显然以具有可识别特征的独立谱系存在。细菌的物种形成通过突变、水平基因转移和生态特化等机制发生。
Horizontal gene transfer complicates the concept of species boundaries in bacteria, as genes can be exchanged between distantly related lineages through transformation, transduction, and conjugation. Nevertheless, ecologically distinct populations—such as different pathogenic strains adapted to different host species—can accumulate sufficient genetic differences to be considered distinct species under a phylogenetic or ecological species concept.
水平基因转移使细菌的物种边界概念变得复杂，因为基因可以通过转化、转导和接合在远缘谱系之间交换。然而，生态上不同的种群——如适应不同宿主物种的不同致病菌株——可以积累足够的遗传差异，在系统发育或生态学物种概念下被视为不同的物种。
The emergence of antibiotic resistance provides a contemporary example of bacterial evolution that can be viewed through a speciation lens. Strains that acquire resistance genes through mutation or horizontal transfer occupy a new adaptive zone—environments containing antibiotics—and may diverge significantly from their ancestral populations over time.
抗生素耐药性的出现提供了一个当代的细菌进化例子，可以从物种形成的角度加以解读。通过突变或水平转移获得耐药基因的菌株占据了新的适应区——含有抗生素的环境——并可能随着时间的推移从其祖先种群显著分化。
12. Isolating Mechanisms in Practice: Hybrid Zones | 隔离机制的实践：杂交带
When two closely related species come back into contact after a period of isolation, they may form a hybrid zone—a geographical area where interbreeding occurs and hybrid offspring are produced. The outcome depends on the strength of reproductive isolation mechanisms and the fitness of hybrids.
当两个近缘物种在隔离一段时间后重新接触到一起时，它们可能形成杂交带——一个发生种间交配并产生杂交后代的地理区域。结果取决于生殖隔离机制的强度和杂种的适合度。
If hybrids have reduced fitness, reinforcement may occur: natural selection favours individuals that choose mates from their own species, strengthening prezygotic isolation and completing the speciation process. This has been observed in several bird and insect systems, where hybrid zones remain narrow and stable, with character displacement in mating signals.
如果杂种适合度降低，强化作用可能发生：自然选择偏爱选择本物种配偶的个体，加强合子前隔离并完成物种形成过程。这在几种鸟类和昆虫系统中已被观察到，杂交带保持狭窄和稳定，交配信号出现性状替换。
Alternatively, if hybrids are viable and fertile, they may serve as a bridge for gene flow between species, potentially leading to hybrid speciation—the formation of a new species through hybridisation. Hybrid speciation has been documented in sunflowers, where three hybrid species of Helianthus have arisen from crosses between parent species and have colonised extreme habitats—salt marshes, sand dunes, and desert floors—that neither parent can tolerate.
或者，如果杂种可存活且可育，它们可能充当物种间基因流动的桥梁，可能导致杂交物种形成——通过杂交形成新物种。杂交物种形成已在向日葵中得到证实，其中三个杂交物种的向日葵属源于亲本物种之间的杂交，并拓殖了两个亲本都无法忍受的极端栖息地——盐沼、沙丘和沙漠地面。
Hybrid zones provide natural laboratories for studying the dynamics of speciation. By comparing genetics, behaviour, and ecology across hybrid zones, researchers can investigate which genes contribute to reproductive isolation and how selection acts on hybrids—offering insight into the evolutionary forces that create and maintain biodiversity.
杂交带为研究物种形成动态提供了天然实验室。通过比较杂交带中个体的遗传、行为和生态，研究人员可以调查哪些基因有助于生殖隔离以及选择如何作用于杂种——为理解创造和维持生物多样性的进化力量提供了洞见。
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