#lineages KEITOU

#clue KAGI ascribe MOTODUKU ledger tallying albeit hedged


■Effects of exotic species on evolutionary diversification

Mark Vellend, , Luke J. Harmon, Julie L. Lockwood, Margaret M. Mayfield, A. Randall Hughes, John P. Wares and Dov F. Sax

Available online 7 March 2007.


  • Allopatric speciation 異所的種分化
  • Biodiversity:種内の遺伝的多様性
  • Directional selection 方向性選択
  • Disruptive selection 分断型選択
  • Exotic species外来種
  • Evolutionary diversification 進化的多様化
  • Genetic bottleneck ボトルネック
  • Genetic drift 遺伝的浮動
  • Hybridization 交雑
  • Introgression 浸透性交雑
  • Species invasion:外来種の最初の点から自然な範囲以上への拡大
  • Polyploidy倍数体

■Exotic species and biodiversity 


  • Most study: negative effects on biodiversity
  • Potentially importance: part of the equation
  • -> Promote evolution
  • -> offset the loss (at least partially)
  • Review: When and how exotic species might promote diversification?
    • Empirical evidence
    • Conceptual framework uniting the various mechanisms

■Exotic species and evolutionary diversification  


  • Definetion of Evolutionary diversification:
    • an increase in genetic variation among populations within a particular species or species complex, including adaptive divergence of lineages as well as the special case of the formation of new hybrid lineages.
    • the actual creation of new species (reproductively isolated lineages)
  • Focus onthe early stages of evolutionary diversification
    (because of the short time from the human mediated invasion)
    • by increases in genetic differentiation among populations within particular species for quantitative traits or molecular marker data.
  • Exotic species can contribute simultaneously
    • Decreases in biodiversity
    • Increases in biodiversity via evolutionary diversification.
  • -> Interest is evaluating ignored positive side of the effects of exotic species on biodiversity.
  • Ideal stage for evolutionary diversification:
    • new environment & geographic distance
    • e.g.) altered selection regime, ecological opportunities, or possibilities for hybridization
  • Evolutionary diversification...Fig.1
    • 1)following introduction to new regions
    • 2)of native species in response to exotic species
    • 3)via hybridization, (native * exotic or exotic1 * exotic2)
  • -> This review "Our review of the literature is not exhaustive, but draws on a range of case studies that provide compelling evidence that evolutionary diversification is a frequent result of exotic species invasions with potentially profound implications for the global biota. "

□Figure 1. 外来種の侵入が分化をもたらす本レビューの枠組みの概念図

■Diversification of exotic species: theory  



  • Classical view of early stage
    • Low genetic diversity:sampled from the native range of a species
    • == genetic bottle neck -> minimal evolutionary divergence (especially asexual)
  • Auther's view of early stage
    • Less bottle neck [7]:review of 29 animal (17% reduction of heterozygosity)
    • No reduction case [6]:Multiple introduction, source populations
    • Effect of intron diversity in evolution: molecular marker not reflect genetic variation for traits which affects natural selection [8]
  • Genetic variation <== local new selection + geographic isolation
  • == text bookish "allopatric speciation"

"Thus, evolutionary theory leads us to expect that invasions will often be followed by evolutionary diversification."


  • Bottleneck -> decreased genetic diversity -> constrain evolution
  • cf.) Repeated invsion inhibit local adaptation or facilitation of diversification

"The key points for our purposes here are that the available evidence suggests that genetic bottlenecks often do not constrain evolutionary change in exotic species, and that selection is often sufficiently strong to result in adaptive evolution in the face of gene flow [6] and [7]."

  • Evolutionary diversification of exotic species (Figure 1 upper left).
    • 1) bottlenecks and drift in the new allopatric population
      	(diverge from populations in the native range. )
    • 2) directional selection by new environment
      	(divergence from populations in the native range)
      	---Low genetic variation of intro.. than native..
      	---High genetic differentiation between intro.. and native..
    • 3) "disruptive selection" under heterogeneous environment within the introduced range.

■Diversification of exotic species: empirical evidence  


  • Since neutral molecular markers...
    • Between native and intro: founder effects and genetic drift causing divergence
    • Intra specific diversity: genetic variation will increase if differentiation among native and exotic exceed native population.★few research just suggestive
  • e.g.)
  • Common mynas Acridotheres tristis, from India[12]
    • New Zealand, South Africa, Hawaii, Australia and Fiji
    • Genetic different in enzyme loci
  • house finch Carpodacus mexicanus[13]
    • native range in western North America
    • differentiation at AFLP markers
    • morphological differentiation.
  • -> examples imply directional natural selection beyound differenced view between molecular marker loci and phenotype adaptivity
  • animals have more information about responce traits
  • "The evolution of increased competitive ability (EICA) hypothesis [19]"
    • No enemies -> less defence type and more competitive type will be appear
    • mixed evidence still just speculation
    • Changes of interspecific interactions affected in case of diverbence of intoro from native
    • example of divergence within an introduced range (Box 1).

Finally, exotic species might evolve in response to the widespread availability of suitable but unoccupied habitat. In cane toads Bufo marinus in Australia, populations at the leading edge of expansion appear to have evolved longer legs and increased locomotion speed because of the substantial fitness advantage of arriving at a site first and forming the founding population [25].

■Box 1. Evolution of geographical clines following invasion

Environmental gradients can impose spatially varying selection on exotic species, which, in turn, can lead to evolutionary diversification within the introduced range. A century after house sparrows Passer domesticus were introduced from England and Germany into North America, there was clear evidence for adaptive divergence under different climatic conditions [58] and [59]. Birds from cool and humid climates were darker in plumage than were birds in hot and dry climates. Similarly, birds living in boreal climates with severe winters were larger in body size compared with those living in milder climates. These patterns conform to plumage and body size clines found among native North American bird species and follow the well-documented Gloger's and Bergmann's rules, respectively, for native species [60]. To the extent that these patterns mirror those found along the latitudinal gradient in Europe, they might not represent an increase in genetic variation among populations of house sparrows, but instead a duplication of variation found in the native range.

However, similar studies of genetically based clines in introduced taxa of Drosophila subobscura [15] provide evidence to the contrary. After 20 years in the New World, separate D. subobscura populations in both North and South America had evolved a pattern of increased wing length with increasing latitude that mirrored the pattern found in the Old World (Figure Ia), but the developmental and genetic basis of the variation in wing length varied among the three continents (Figure Ib). Thus, similar phenotypic patterns were underlain by evolutionary diversification in D. subobscura.

■Diversification of native species in invaded regions: theory  


  • Exotic spp. =change=> communities[4] & environment(physical and chemical structure)[26]
  • -> new selective pressures to natives(e.g.reviewed in detail by Strauss and colleagues [5])
  • Normal view: Exotic reduce or extinct natives
  • == no evolutionary diversification of natives
  • Authour's first view: selection-via-invaders is not so strong but strong enough to induce phenotypic change [5]
    • but, it will be directional or uniform
  • -> no diversification in native
  • two scenario of diversification of native sp.(Figure 1)
    • 1)local disruptive selection by invaders
      • #e.g.) by providing an alternative host for herbivorous insects.
    • 2)part of population invaded case genetic differentiation between invaded and uninvaded populations cause dicersification
      • #ちょっと無理があるけど、浸透性交雑みたいな侵入のイメージ?

■Diversification of native species in invaded regions:empirical evidence 


  • Shift of host plant in native phytophagous insects(Box 2 and more).
  • Directional selection caused by invasion affect only in invaded patchs [3] and [5].
  • There are many way to change selection
    • competition mediation
    • resistance, tolerance, or avoidance of exotic predators
    • cope with exotic pray

■Box 2. Host shifts of native insects onto exotic plants

Some of the clearest examples of evolutionary diversification of native species in response to invaders come from host shifts in phytophagous insects. The two best-documented cases are that of native apple maggot fly Rhagoletis pomonella, which has evolved genetically differentiated ecotypes that feed on native hawthorns and introduced apples in North America [61] and [62] (Figure I) and the North American soap-berry bug Jadera haematoloma, which has evolved different beak lengths to feed on the fruit from exotic tree species [63]. In both cases, genetically differentiated populations feed on native and exotic host plants, with restricted gene flow between the new and old host races.

□Figure I. Host shifts onto exotic plants.

Apple maggot fly Rhagoletis pomonella (b): Hawthorn Crataegus spp. (a) (native host)v--> Malus spp. (c)(introduced)

■Hybridization: theory  


  • Hybridization:Unambiguous rapid evolutionaly diversification
    • Three cases: Exotic+Exotic or Exotic+Native, (Native+Native)in changed community
    • Introgression of genetic material into both sp. or makes hybrid
  • negative aspect to biodiversity
    • inhibit local adaptation and prevent speciation in distinctive selective environment [36]
    • interbreeding or competitive superiority of hybrids affect extinction of rare sp.
  • positive aspect to biodiversity
    • lead to speciation as shown in plant [38] and [39]
    • accelate adaptation in both plants and animals [40] and [41]
    • might have an important role in adaptive radiation [42] and [43].
  • Fixation of new species from hybrid
    • genetic criteria:there must be factors, such as polyploidy or other chromosomal changes, that prevent the expansion or collapse of the hybrid zone through gene flow and introgression [44].
  • ecological criteria: distinctness of the new species from any parental species with which it shares habitat will promote persistence of the hybrid [45].
  • how diversity can be increase from hybrid
    • stable hybrid taxon
    • not extinct parental form

■Hybridization: empirical evidence 

(Spartina sppの交雑種)

  • how ecological and genetic factors can interact to determine the impacts of hybridization on diversity

In Great Britain, native Spartina maritima hybridized with exotic Spartina alterniflora introduced from eastern North America. This hybridization was followed by chromosome doubling, which led to a new, reproductively isolated species, Spartina anglica [46]. Coexistence with native S. maritima was facilitated by the fact that the new species can occupy a habitat, bare tidal flats, which is not available to either parental species [47]. However, in San Francisco Bay, hybridization of native Spartina foliosa with the same exotic species, S. alterniflora, has resulted in hybrids that occupy the same habitats, and can interbreed with and outcompete the native species [48], suggesting that evolutionary diversification is unlikely.

Thus, although strikingly similar hybridization events can have different results, evolutionary diversification is one possible outcome. There are many additional examples of stable new plant species formed through invasion and hybridization (e.g. Circaea [49], Mentha [50], Viola [51], Rhododendeon [52], Tragopogon [53] and Plantanus [37]). Another particularly compelling study illustrating a complex combination of factors involved in exotic?native hybridizations involves ragworts in the genus Senecio (Box 3).

  • fruit flies of the genus Rhagoletis. Schwarz et al. [35] affected by honeysuckle(スイカズラ)invasion
    • The new species hybrid between two natives, specialized to different host plants by ecological separation

□Box 3. Geographical isolation and hybrid speciation

Ragworts, Senecio spp., provide a fascinating example of the factors influencing genetic stability in exotic hybrids [67]. Two species, Senecio aethnensis and Senecio chrysanthemifolis, form a natural hybrid zone on Mount Etna, Sicily. Where the native ranges of these two species overlap, there is evidence for extensive hybridization, and for the maintenance of the hybrid zone by a balance between dispersal and extrinsic selection [67].

Within the past 300 years, individuals from this hybrid zone were introduced to the British Isles, where they have spread as an exotic species, known as Senecio squalidus, which is morphologically and ecologically distinct from both parents and also from individuals in the hybrid zone in the native range. It is likely that spatial segregation that was lacking in the native range provided the impetus needed for hybrid speciation without a change in chromosome number [67]. Thus, the invasion facilitated a speciation event that has not occurred in the native range where these two species hybridize. As an additional complication, the new exotic species (Figure Ic; reproduced with permission from R. Abbott) has hybridized with a local native species, Senecio vulgaris (Figure Ia), leading to the origin of three new hybrid taxa, S. vulgaris var. hibernicus [68] Senecio eboracensis [69] and Senecio cambrensis [70] (Figure Ib).

■Lessons from the fossil record  


  • Accuracy of consequence of avove theory.
  • Possible to use large-scale biotic exchanges e.g.)Panama land bridge
  • General change of number of species in the fossil record(Vermeij [54])...
    	extinct > adaptive radiations and evolutionary diversification of invading species 
  • Difference of invasion between past and present
    • 1)Increase of transportation distances
    • 2)Multiple source regions simultaneously (many contemporary exotic species) "It's mekes difficult to predict, but processes of diversification was not different in today."

■Spatial and temporal scale  


  • consept of temporal scale
    • evolutionary diversification occurs over a wide range of scales.
    • there is no discrete point during the process of diversification
      • continuously degree of adaptive differentiation and reproductive isolation [55]
      • even most species are not 100% reproductively isolated from their closest relatives [55]
  • time scale of evolution in each scenario
    • a day: Hybridization can create species
    • a century: Adaptive diversification of native phytophagous insects(Box 2)
    • years to decades: non-hybrid exotic or native species exhibit adaptive genetic differentiation[15]
    • thousands or millions of years: in most cases of evolution we would predict
    • speculation... reproductive isolation will eventually evolve between native and introduced ranges
  • spatial scale
    • varies tremendously

■Concluding thoughts 


  • No information about proportion of diversification.
  • But exotic species invasions possibly generate evolution.
  • It will not overcome negative effect
    • 1.In Australia (spatial place) negative effect on biodiversity are probably greater than positive aspect[25] and [31]
    • 2.Not randomly sampled species successfully invade (already successful lineages, such as finches, mice and grasses)
      we will lose much broader lineages.
    • 3.Endemic, phylogenetically distinct lineages have the greatest conservation value Nonetheless, many exotic species provide important ecosystem services to humanity (e.g. many food plants and animals)
  • Question ..."whether species that originate via evolutionary diversification of an exotic species are considered native to the introduced region"
    • ... we feel non native, but must face such question.
  • Future research
    • net effects of particular exotic species on biodiversity.
    • taxonomic distribution of new lineages originated from exotic species invasions

"At present, it seems plausible to suggest that, in the long term, moving species to new regions with different environmental conditions and altering the biotic environment of native species will lead to speciation in many branches in the tree of life. We have already seen the formation of new reproductively isolated and ecological distinct taxa via hybridization stimulated by species invasions; speciation events that do not involve hybridization will simply take much longer to happen."


・多様性の指標とは? レアファクションのような時間軸を入れた計り方も比較したほうがいい。 ・侵入とは? 帰化定着後の話を主にしていて、帰化定着するまでの過程についてはほとんどしていない。侵入初期の研究ばかりが多いので、斬新さがあるのかもしれないけど、対象とする種がそこまで侵入成功する可能性を考えてみると意外と限定的。 ・具体性は? 事例は個別個別でどれも一般的でいまいち単調、理屈で押し切っている感じ。取捨選択するほど様々な事例がないから?グローバルにdiversificationするのかとかもう少しネットワークの強度の違いをメタ解析できたりすると面白いが・・・ ・この先にあるもの 野鳥や人間の行動様式レベルの話でアナロジーできる。(「銃・病原菌・鉄」がお勧め。)  そこではさらにinvasionして、定着後のGlobal化の段階にまで進んでいる。(特に島国)

  • > コスモポリタンの進化ばかり進んで系統樹全体の多様性は減る。   しかし、情報はスケールフリーにできても、交配は地理的に縛られる
  • > 石油危機などで人間活動がなくなると、地球中で新たな進化が起こるかも。地球規模での分業化の促進。
  • > 遺伝的距離が近いけど、全く違うニッチの種ができる?

・個人的教訓 研究の流れは大事だけど、流れの中で偏り無く物事をみることが大事。このシナリオは僕も1年生の時に保全生態学の本を読んで考えていたので、時代が追いついてきた感じ。もっと、ボトルネックによる表現型遺伝子の固定についてもう少し標本などを使ってやったらメカニズムの解明につながって面白そう。


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