The evolution of invasive species

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The evolution of invasive species Invasion
success is determined both by the evolution of
the invader and the species in the invaded
community. Here we are interested only in
evolution of characteristics that promote the
success of NIS. The usual conditions of invasion
(i.e. small initial population size of
non-indigenous species, etc.) make possible rapid
evolution of these species. However, that is
putting the cart before the horse. Invasion is
a multi-stage process. Each stage acts as a
selective filter, and we need to consider
evolution as it relates to each stage. What are
those stages?
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• Evolution in the native range (pre-adaptation).
• Evolution related to transport from native to
  invaded habitats.
• Evolution in the invaded habitat during
  introduction, establishment and spread.
• In part, this relates to the filter model youve
  already seen. That model does not consider
  pre-adaptation it assumes the evolved
  characteristics of the species is suitable for
  invasion. In addition, it subdivides stage 3,
  separating the filters related to evolved
  physiology from the relationship the invader has
  to the invaded community.

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Model to Predict Invasions
A B C D E F G
Species Pool
Transport (Dispersal) Filter
you must get there you must survive
conditions you must tolerate species already
present
Physiological Filter (/-)
Biotic Filter (/-)
Natural Colonization
E
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Pre-adaptation Invasive species are not a random
group from within native biodiversity. Their
evolved traits predispose them to transport
(particularly human-mediated transport) from
native to invaded habitats. What are those
traits? high fecundity small body size
vegetative or asexual reproduction high
genetic diversity high phenotypic plasticity
physiological tolerance
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High fecundity (and regular reproduction)
produces high propagule pressure. Small body size
(and usually small propagule size or seed mass)
means that maturity can be reached rapidly, and
makes it more likely that high fecundity can be
achieved. The regularity of reproduction (and/or
short reproductive interval) means that
additional propagules are likely dispersed to an
invaded habitat after first arrivals. That
reduces possibly harmful Allee effects, increases
genetic diversity in the novel habitat, and
permits adaptive evolution in that habitat. High
genetic diversity in the native range means that
even a small initial population in the invaded
habitat can produce a broad range of genotypes
and phenotypes in the invaded area.
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Reproductive strategies of invasive plant species
frequently involve extensive vegetative
reproduction and/or asexual reproduction or
self-fertility. Any of these approaches minimizes
the requirement for mates within a local
area. Broad physiological tolerance in the native
habitat means that a potential invader is less
likely to be filtered out by environmental
conditions in the invaded habitat. The same end
point can be achieved by phenotypic
plasticity. Either of the above likely means we
would identify the species as being a habitat
generalist in its native range. Examples
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Pines (Pinus) Rejmanek and Richardson (1996)
separated species of pines into invasive and
non-invasive groups. They performed a
discriminant analysis on data that included
various biological characteristics of the
species. The discriminant function that best
separated the groups included seed mass,
interval between large seed crops and minimum
juvenile period.
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What about other plant species? Rejmanek and
Richardson found they could use the same basic
characteristics and add the potential for
vertebrate seed dispersal to produce a more
general pattern for what species are likely to be
invasive
Here the Z is their discriminant function score
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Positive Z scores in the discriminant analysis
are an indication the species is invasive
negative Z scores occur in non-invasive species.
Pinus contorta an invasive species Common name
lodgepole pine Distributionsoutheastern Alaska
to northern California, Sierra Nevada,
Rocky Mts, Black Hills Needles 3-4 cm Seeds 1-2
mm, wings to 12 mm
Pinus engelmanii non-invasive Common name
Aztec pine Distribution northern Mexico,
Arizona, New Mexico Seed size 8-9 mm
circumference, wings to 20 mm Needles
8-14 inches
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• Insects in particular a walnut husk fly,
  Rhagoletis completa
• Here we have an example that points to the value
  of both genetic diversity and a general purpose
  genotype (which might be either a species with
  broad physiological tolerance or phenotypic
  plasticity).
• We might expect a new invader to suffer a genetic
  bottleneck on invasion. Limited genetic diversity
  could inhibit success. However, there are two
  ways an invasive population could be successful
• An invader could very rapidly be selected for
  local adaptations. This would depend on
  sufficient local genetic diversity (e.g. by
  multiple invasions) or
• 2. If invasive species show general-purpose
  genotypes, or
• display sufficient phenotypic plasticity to
  thrive under a
• wide range of environmental conditions.

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R. completa apparently took advantage of both
approaches Colonization of California walnut
groves apparently occurred repeatedly and
sequentially from midwestern sources. Checks of
microsatellite genetic diversity in California do
not indicate a significant bottleneck. and
Even though there is a significant difference
in climate between California and midwestern
source areas, there was no significant difference
in diapause characteristics, normally induced by
climatic conditions. The flies carry either a
very large phenptypic plasticity and/or a very
general purpose genotype.
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Table 1 Rhagoletis completa populations used in
the study Location Mean number of
alleles Total number of alleles
Heterozygosity Introduced Wapato, Washington
4.83 29
0.57 ? 0.33 Medford,
Oregon 4.67
28 0.51 ?
0.29 Lakeport, California 4.50
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0.50 ? 0.29 Lodi, California
4.33 26
0.49 ? 0.29 Hollister,
California 4.17
25 0.46 ?
0.27 Tulare , California 3.17
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0.49 ? 0.29 Native Columbia, Missouri
3.83 23
0.48 ? 0.28 Blackjack,
Missouri 5.00
30 0.53 ? 0.31
Kerrville, Texas 5.17
31
0.54 ? 0.31 Austin, Texas
4.83 29
0.53 ? 0.31
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Diapause length was shorter in introduced
populations in both California and the midwest,
but climate difference did not significantly
affect the response. Instead, finding the same
response suggests there may be rapid local
adaptation, but certainly a general purpose
genotype.
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Evolution related to transport Any change that
increases propagule pressure would enhance the
probability of successful invasion. Evolutionary
changes related to this phase seem less well
studied. The number of propagules per
reproductive episode could be increased. The
interval between reproductive episodes could be
shortened or more regular. The nature or quality
of dispersal accessory structures or
attractiveness to vectors could be enhanced.
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Evolution in the introduced range There is
typically a more-or-less extended lag phase in
population size after introduction. That could be
just a typical growth response, but is suggested
more likely a result of adaptive evolution
following introduction. The usually accepted
hypothesis of founder effects and reduced genetic
diversity at introduction seem not to be
supported by data. Wares et al. (2005) found
invading animal species (29 species reported)
retain 80 of the genetic diversity of their
native source populations. Bottlenecks, where
they occur, seem to be short-lived. After
invasion we expect genetic changes in the
invading populations.
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• What types of genetic change are frequently
  observed?
• hybridization, particularly among individuals
  introduced
• from different sources. Looking back to
  Chens study of
• Rhagoletis, some introduced populations had
  greater
• genetic diversity than any of the native
  ones. This, she
• believes, was due to hybridizations among
  multiple
• invasions.
• Interspecific hybridization might also occur
  between
• introduced and similar native species.
• Combinations can also lead to new phenotypes
  (novel multi-locus genotypes Novak 2007) and
  novel epistatic interactions.
• 2. Another way to produce new genetic variants is
  through
• chromosomal or gene duplication. Multiple
  copies make
• possible novel adaptive traits.

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3. One way of achieving rapid growth (and thus
avoiding inbreeding depression or
bottlenecks during early phases of invasion)
available to plants is uniparental reproduction
selfing, asexual reproduction by clonal
propagation and apomixis). These may be
pre-adaptations or - Selection on invading
plant species seems to lead to loss of mating
types in species with polymorphic systems. Some
species shift to obligate asexuality in various
ways clonal propagation and selfing have
been reported in evolution of invading
populations. Barrett and his collaborators
have discovered loss of morphs in tristylous
Eichornia crassipes. Recessive modifiers that
appear in founder populations have been shown
responsible for evolution of selfing.
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References Barrett, S.C.H., R.I.Colautti and
C.G.Eckert. 2008. Plant reproductive systems and
evolution during biological invasion. Molecular
Ecology 17373-383. Chen, Y.H., S.B.Opp,
S.H.Berlocher and G.K.Roderick. 2006. Are
bottlenecks associated with colonization? Genetic
diversity and diapause variation of native and
introduced populations Rhagoletis completa
populations. Oecologia 149656-667. Novak, S.J.
2007. The role of evolution in the invasion
process. PNAS 1043671-2. Rejmanek, M. and
D.M.Richardson. 1996. What attributes make some
plant species more invasive? Ecology
771655-1661. Sax, D.F., J.J. Stachowicz and S.D.
Gaines (eds). 2005. Species Invasions Insights
into Ecology, Evolution and Biogeography.
Sinauer, Sunderland, MA. 495p. Suarez, A.V. and
N.D.Tsutsui. 2008. The evolutionary consequences
of biological invasions. Molecular Ecology
17351-360.