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Taxonomy

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Sedum was first formally described by Carl Linnaeus in 1753, with 15 species.[1] Of the genera encompassed by the Crassulaceae family, Sedum is the most species rich, the most morphologically diverse and most complex taxonomically. Historically, it was placed in the subfamily Sedoideae, of which it was the type genus. Of the three modern subfamilies of the Crassulaceae, based on molecular phylogenetics, Sedum is placed in the subfamily Sempervivoideae. Although the genus has been greatly reduced, from about 600[2] to 420–470 species,[3] by forming up to 32 segregate genera,[4] it still constitutes a third of the family and is polyphyletic.[5]

Sedum species are found in four of six major crown clades within subfamily Sempervivoideae of Crassulaceae and are allocated to tribes, as follows:[6]

Clades and tribes within Sempervivoideae
Clade Tribe
Hylotelephium Telephieae
Rhodiola Umbiliceae
Sempervivum Semperviveae
Aeonium Aeonieae
Acre Sedeae
Leucosedum
Note

Clades containing Sedum, shown in blue

In addition, at least nine other distinct genera appear to be nested within Sedum. However, the number of species found outside of the first two clades (Tribe Sedeae) are only a small fraction of the whole genus. Therefore the current circumscription, which is somewhat artificial and catch-all must be considered unstable.[5] The relationships between the tribes of Sempervivoideae is shown in the cladogram.

Cladogram of Sempervivoideae tribes[6]
Sempervivoideae
 

Telephieae

 
 
 

Umbilicieae

 
 
 

Semperviveae

 
 
 

Aeonieae

 
 

Sedeae (Leucosedum+Acre)

 
 
 
 
 

There are now thought to be approximately 55 European species in the genus. Sedum demonstrates a wide variation in chromosome numbers, and polyploidy is common. Chromosome number is considered an important taxonomic feature.[7]

Earlier authors placed a number of Sedum species outside of these clades, such as S. spurium, S. stellatum and S. kamtschaticum (Telephium clade),[8] that has been segregated into Phedimus (tribe Umbiliceae).[6][9][10][11] Given the substantial taxonomic challenges presented by this highly polyphyletic genus, a number of radical solutions have been proposed for what is described as the "Sedum problem", all of which would require a substantial number of new combinations within Sempervivoideae. Nikulin and colleagues (2016) have recommended that, given the monophyly of Aeonieae and Semperviveae, species of Sedum outside of the tribe Sedeae (all in subgenus Gormania) be removed from the genus and reallocated. However, this does not resolve the problem of other genera embedded within Sedum, in Sedeae.[5] In the largest published phylogenetic study (2020), the authors propose placing all taxa within Sedeae in genus Sedum, and transferring all other Sedum species in the remaining Sempervivoideae clades to other genera. This expanded Sedum s.l. would comprise about 755 species.[12]

Subdivision

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Linnaeus originally described 15 species, characterised by pentamerous flowers, dividing them into two groups; Planifolia and Teretifolia, based on leaf morphology, with 15 species, and hence bears his name as the botanical authority (L.).[13] By 1828, de Candolle recognized 88 species in six informal groups.[14] Various attempts have been made to subdivide this large genus, in addition to segregating separate genera, including creation of informal groups, sections, series and subgenera. For an extensive history of subfamily Sedoideae, see Ohba 1978.

Gray (1821) divided the 13 species known in Britain at that time into five sections; Rhodiola, Telephium, Sedum, (unnamed) and Aizoon.[15] In 1921, Praeger established ten sections; Rhodiola, Pseudorhodiola, Giraldiina, Telephium, Aizoon, Mexicana, Seda Genuina, Sempervivoides, Epeteium and Telmissa.[16] This was later revised in what is the best known system, that of Berger (1930), who defined 22 subdivisions, which he called Reihe (sections or series).[17] Berger's sections were:

  • Rhodiola
  • Pseudorhodiola
  • Telephium
  • Sedastrum
  • Hasseanthus
  • Lenophyllopsis
  • Populisedum
  • Graptopetalum
  • Monanthella
  • Perrierosedum
  • Pachysedum
  • Dendrosedum
  • Fruticisedum
  • Leptosedum
  • Afrosedum
  • Aizoon
  • Seda genuina
  • Prometheum
  • Cyprosedum
  • Epeteium
  • Sedella
  • Telmissa

A number of these, he further subdivided.[17] In contrast, Fröderströmm (1935) adopted a much broader circumscription of the genus, accepting only Sedum and Pseudosedum within the Sedoideae, dividing the former into 9 sections.[18] Although this was followed by numerous other systems, the most widely accepted infrageneric classification following Berger, was by Ohba (1978).[19] Prior to this, most species in Sedoideae were placed in genus Sedum.[9] Of these systems, it was observed "No really satisfactory basis for the division of the family into genera has yet been proposed".[20]

Some other authors have added other series, and combined some of the series into groups, such as sections.[21] In particular, Sedum section Sedum is divided into series (see Clades) [5][22] More recently, two subgenera have been recognised, Gormania and Sedum.[5]

  • Gormania: (Britton) Clausen. 110 species from Sempervivum, Aeonium and Leucosedum clades. Europe and North America.
  • Sedum: 320 species from Acre clade. Temperate and subtropical zones of Northern hemisphere (Asia and the Americas).[23]

Subgenus Sedum has been considered as three geographically distinct, but equal sized sections:[23]

  • S. sect. Sedum ca. 120 spp. native to Europe, Asia Minor and N. Africa, ranging from N. Africa to central Scandinavia and from Iceland to the Ural Mountains, the Caucasus and Iran.
  • S. sect. Americana Frod.
  • S. sect. Asiatica Frod.

S. sect. Sedum includes 54 species native to Europe, which Berger classified into 27 series.[23]

Clades

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Species and series include[24][25][26][8][6][5][4][27]

Subgenus Gormania
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Semperviveae
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Of about 80 Eurasian species, series Rupestria forms a distinct monophyletic group of about ten taxa, which some authors have considered a separate genus, Petrosedum.[29][30][31] It was series 20 in Berger's classification. Native to Europe it has escaped cultivation and become naturalized in North America.[32]

Aeonieae (N Africa)
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Embedded within series Monanthoidea are three Macaronesian segregate genera, Aichryson, Monanthes and Aeonium.[6]

Sedeae - Leucosedum (Europe/Mediterranean/Near East/Central Asia)
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  • S. series Aithales (Med)
  • S. series Alba (Med)
  • S. series Alsinefolia All. (Med)
  • S. series Atrata (Med)
  • S. series Brevifolia (Med)
  • S. series Cepaea (Med)
  • S. commixtum Moran & Hutchison
  • S. series Convertifolia (Med)
  • S. series Dasyphylla (Med)
  • S. series Glauco-rubens (Med)
  • S. series Gracile (Med)
  • S. series Hirsuta (Med)

In the Levant, one species of this succulent (S. microcarpum) covers the stony ground like a carpet where the soil is shallow, growing no higher than 5–10 cm. At first, the fleshy leaves are a light green, but as the season progresses, the fleshy leaves turn red.

Europe/Mediterranean/Near East/Central Asia
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  • Sedum series Inconspicua (Med)
  • S. ince 't Hart & Alpinar
  • S. lydium Boiss.
  • S. microcarpum (Sm.) Schönland
  • S. series Monregalense (Med)
  • S. moranii R.T.Clausen
  • S. series Nana (Med)
  • S. series Pedicellata (Med)
  • S. sedoides (Jacquem. ex Decne.) Pau
  • S. series Steico (Med)
  • S. series Subrosea (Med)
  • S. series Subulata (Med)
  • S. series Telmissa (Med)
  • S. series Tenella (Med)
  • Med = Mediterranean distribution

Embedded within the Leucosedum clade are the following genera: Rosularia, Prometheum, Sedella and Dudleya.[6] Rosularia is paraphyletic, and some Sedum species, such as S. sempervivoides Fischer ex M. Bieberstein are assigned by some authors to Rosularia, as R. sempervivoides (Fischer ex M. Bieberstein) Boriss.[34]

Subgenus Sedum
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Sedeae - Acre (Asia/Europe/Macaronesia/N. America)
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Embedded within the Acre clade are the following genera: Villadia, Lenophyllum, Graptopetalum, Thompsonella, Echeveria and Pachyphytum.[6] The species within Acre, can be broadly grouped into two subclades, American/European and Asian.[35][8]

List of selected species

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主条目：List of Sedum species

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Previous phylogenetic studies found that Nomocharis is nested within Lilium and is a sister taxon to L. nepalense. These studies contained N. saluenensis (I˙kinci et al. 2006; Nishikawa et al.1999, 2001), N. aperta, and sect. Eunomocharis species (Gao et al. 2012a). In our analysis, we obtained similar results (Fig. 1).

2013年Du等人和2019年Kim等人先后用不同类型的DNA数据，以不同的采样规模分析了百合属物种的演化历史，并将结果与Comber早在1949年发表的传统分类系统相对比。Du等人以中国分布的百合属为采样重点，使用了98种的细胞核糖体轉錄間隔區（英语：Internal transcribed spacer）数据进行分析，其结果大体支持Comber对喇叭花组基于鳞茎颜色的分类，并认为南川百合和湖北百合应从卷瓣组归入喇叭花组。

Kim等人的分析采用了28种百合属的质体基因组及片段数据，

支持、南川百合与亲缘较近，

发现本属主要分化为两大支系，一支为东亚及欧洲分布，另一支为横断山脉及北美洲分布。在组的层面，根茎组和喇叭花组均为多系群，而轮叶组的祖先则可能演化自卷瓣组[36]。该文章并未进行任何分类学处理和修订，但不排除未来出现采样更全面的系统演化研究以及分类修订的可能性。

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演化生物学
约翰·古尔德繪製的達爾文雀圖像
過程與結果
自然史
演化论發展史
學科與應用
社會反應

群体遗传学（population genetics）又稱族群遺傳學、種群遺傳學，是研究在自然选择、性選擇、遺傳漂變、突变以及基因流動这五种演化动力的影响下，等位基因的分布和改变。通俗而言，群体遗传学就是在种群水平上进行研究的遗传学分支，探究适应、物种形成、种群结构（英语：Population structure (genetics)）等自然现象的格局、规律和成因。

群体遗传学理论在现代进化综论中至关重要。本学科在传统上是高度数学化的学科，例如自1980年代起便成为核心计算方法的溯祖理论（英语：Coalescent theory），而现代的群体遗传学包括理论的、实验室的和实地的工作。得出的群体遗传模型既可用于特定概念的证明或证伪，也可对DNA序列进行统计推断。

该学科的主要创始者包括休厄尔·赖特、约翰·伯顿·桑德森·霍尔丹和羅納德·費雪等，也是定量遗传学领域的理论奠基者。

演化動力

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突變

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更多信息：突變

是多樣性的來源。在群體遺傳學主要區分為中性突變、有益突變和有害突變。

可逆的突變可以作如下表示：[37]

其中p和q代表等位基因的的頻率，μ和ν是突變率，t是時間。

平衡狀態是：

自然選擇

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更多信息：自然選擇

自然選擇發生於不同的基因型有不同適應度時：[38]

其中f(x)為x的頻率，是x的相對適應度。即是整個種群的平均適應度。適應度也可用選擇係數（selection coefficient）表示為。值得注意的是適應度不一定是一個常數，而可能是基因頻率的函數，這種情況稱為頻率依賴選擇（frequency-dependent selection）。負為頻率依賴選擇，也就是頻率低的基因較適應的情況，是維持基因多樣性的一個重要機制。另一個可以維持基因多樣性的情況是超顯性，即異型合子的適應度最高。

基因流

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更多信息：基因流

當個體在不同種群間移動時，稱為遷徙（migration）或基因流（geneflow）。

在兩個面積類似的棲地之間（兩島嶼模型）的基因流可用如下式子表示：

其中和分別代表某個等位基因兩個棲地中的頻率，m是遷徙率。

當一個棲地遠大於另一個時（陸地—島嶼模型），則用如下式子：

C和I分別代表陸地和島嶼的基因頻率。如果沒有別的演化動力，最終島嶼的基因頻率會和陸地相同。

性選擇

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更多信息：性選擇

各種非隨機交配會造成性選擇。

x是基因型頻率，是雌i對雄j的偏好，1代表AA，2代表Aa。

遺傳漂變

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更多信息：遺傳漂變

當族群大小有限時，會因為單純的機率造成基因頻率的改變。可以用二項分布描述基因頻率從一個值變為另一個值的機率。當族群大小是N時，等位基因有2N個，經一個世代後，族群中會有j個A基因的機率是：[39]

因為0<q<1，族群大小（N）越小時，這個機率越高。

除了用二項分布配合馬可夫鏈來計算，漂變也可以用一維布朗運動的擴散方程來描述。[40]

因為模型都經過一定的簡化，方程式中的N並不完全對應到真實世界的族群大小，而被稱為有效族群大小。[41]


应用

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推斷群體結構

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对于雙倍體、有性繁殖的物种，最简单的群体结构检测方法是计算其种群中基因型的频率是否符合哈代-溫伯格平衡律（简称哈温平衡）的预测，即该物种种群中单个位点的基因型是否符合不受上述五种演化动力影响时，理论上会呈现的频率（沒有天擇、沒有性擇、沒有突變、種群無限大、沒有遺傳漂變、沒有基因流）。

将该位点的两个等位基因A和a的频率分别标记为 p 和 q，哈温平衡下三种基因型的频率应分别为：[42]


當演化動力存在時，會造成基因型頻率偏離哈溫平衡，包括等位基因頻率的改變和連鎖不平衡。

描述多樣性

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遗传学
重要概念
历史及分支
研究领域
专业方向

Neutral theory predicts that the level of nucleotide diversity in a population will be proportional to the product of the population size and the neutral mutation rate. The fact that levels of genetic diversity vary much less than population sizes do is known as the "paradox of variation".[66] While high levels of genetic diversity were one of the original arguments in favor of neutral theory, the paradox of variation has been one of the strongest arguments against neutral theory.

中性演化理論認為種群內的遺傳多樣性應與有效種群大小成正比，而在實際調查中，種群遺傳多樣性的差異程度遠小於種群規模之差異的現象被稱為"遺傳差異的悖論"（paradox of variation）。

Neutral theory predicts that the level of nucleotide diversity in a population will be proportional to the product of the population size and the neutral mutation rate. The fact that levels of genetic diversity vary much less than population sizes do is known as the "paradox of variation".[66] While high levels of genetic diversity were one of the original arguments in favor of neutral theory, the paradox of variation has been one of the strongest arguments against neutral theory.

性狀由遺傳和環境的交互作用決定。多樣性的來源包括基因、環境和和兩者的交互作用。照定義，只有可遺傳的部份會影響生物演化，所以群体遗传學主要用等位基因的頻率來表示生物多樣性，並追蹤其變化來了解一個性狀的演化。一個種群中，某個性狀的多樣性源自遺傳差異的比例，稱為可遺傳性（heritability）。


連鎖不平衡

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主条目：連鎖不平衡

當基因座之間彼此獨立時，一個個體共時帶有A基因和B基因的機率f(AB)應該要等於f(A)×f(B)。但是有些演化動力或分子機制會造成基因座之間不獨立。連鎖不平衡描述的就是不同基因座之間，某些基因非隨機共同出現的機率。[43]

f(AB) = f(A)f(B) + D
f(Ab) = f(A)f(b) - D
f(aB) = f(a)f(B) - D
f(ab) = f(a)f(b) + D

D即是A和B之間的連鎖不平衡。也可以用如下公式計算：

D = f(AB)f(ab) - f(Ab)f(aB)

检测选择压力

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遗传系统演化

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參見

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參考資料

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  1. ^ Linnaeus 1753.
  2. ^ Ohba 1977.
  3. ^ Fu & Ohba 2001，第221頁.
  4. ^ 4.0 4.1 Mort et al 2001.
  5. ^ 5.0 5.1 5.2 5.3 5.4 5.5 Nikulin et al 2016.
  6. ^ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 Thiede & Eggli 2007.
  7. ^ Hart 1985.
  8. ^ 8.0 8.1 8.2 van Ham & Hart 1998.
  9. ^ 9.0 9.1 Ohba et al 2000.
  10. ^ Fu & Ohba 2001，第220頁.
  11. ^ ICN 2019.
  12. ^ 12.0 12.1 Messerschmid et al 2020.
  13. ^ Hart & Jarvis 1993.
  14. ^ de Candolle 1828.
  15. ^ Gray 1821.
  16. ^ Praeger 1921.
  17. ^ 17.0 17.1 Berger 1930.
  18. ^ Fröderströmm 1935.
  19. ^ Ohba 1978.
  20. ^ Tutin et al 1993.
  21. ^ Uhl 1978.
  22. ^ Ito et al 2017.
  23. ^ 23.0 23.1 23.2 Hart & Alpinar 1991.
  24. ^ Hart 1995.
  25. ^ Hart 1995a.
  26. ^ Hart 1997.
  27. ^ Ding et al 2019.
  28. ^ TPL 2013.
  29. ^ van Ham et al 1994.
  30. ^ Gallo 2017.
  31. ^ Gallo 2017a.
  32. ^ Gallo & Zika 2014.
  33. ^ Hart 2003，p. 41.
  34. ^ WFO 2019.
  35. ^ Mort et al 2009.
  36. ^ Kim, Hyoung Tae; Lim, Ki-Byung; Kim, Jung Sung. New Insights on Lilium Phylogeny Based on a Comparative Phylogenomic Study Using Complete Plastome Sequences. Plants. 2019-11-27, 8 (12): 547. doi:10.3390/plants8120547. 
  37. ^ Hartl & Clark 2007，第156頁.
  38. ^ Hartl & Clark 2007，第208-209頁.
  39. ^ Hartl & Clark 2007，第95-97頁.
  40. ^ Hartl & Clark 2007，第105-111頁.
  41. ^ Hartl & Clark 2007，第121頁.
  42. ^ Hartl & Clark 2007，第48-49頁.
  43. ^ Hartl & Clark 2007，第77-78頁.

参考書目

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