Transcript Evolution of Invasiveness

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?
1. Evolution in the native range (pre-adaptation).
2. Evolution related to transport from native to invaded
habitats.
3. Evolution in the invaded habitat during introduction,
establishment and spread.
In part, this relates to the filter model you’ve 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.
Model to Predict Invasions
Species Pool
A B C D E F G
Transport
(Dispersal) Filter
you must
get there
Physiological
Filter (+/-)
you must
survive
conditions
Biotic Filter (+/-)
Natural Colonization
E
you must
tolerate
species
already
present
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
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.
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:
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.
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:
Positive Z scores in the discriminant analysis are an indication
the species is invasive; negative Z scores occur in noninvasive species.
Pinus contorta – an invasive species
Common name: lodgepole pine
Distribution:southeastern 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
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:
1. 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.
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.
Table 1 Rhagoletis completa populations used in the study
Location
Mean number of alleles Total number of alleles
Introduced
Wapato, Washington
4.83
29
Heterozygosity
0.57  0.33
Medford, Oregon
4.67
28
0.51  0.29
Lakeport, California
Lodi, California
Hollister, California
Tulare , California
4.50
4.33
4.17
3.17
27
26
25
19
0.50  0.29
0.49  0.29
0.46  0.27
0.49  0.29
Native
Columbia, Missouri
Blackjack, Missouri
Kerrville, Texas
Austin, Texas
3.83
5.00
5.17
4.83
23
30
31
29
0.48  0.28
0.53  0.31
0.54  0.31
0.53  0.31
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’.
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.
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.
What types of genetic change are frequently observed?
1. hybridization, particularly among individuals introduced
from different sources. Looking back to Chen’s 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.
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.
References
Barrett, S.C.H., R.I.Colautti and C.G.Eckert. 2008. Plant reproductive systems and
evolution during biological invasion. Molecular Ecology 17:373-383.
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 149:656-667.
Novak, S.J. 2007. The role of evolution in the invasion process. PNAS 104:3671-2.
Rejmanek, M. and D.M.Richardson. 1996. What attributes make some plant species
more invasive? Ecology 77:1655-1661.
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 17:351-360.