ME408, Alien species, Spring 2010
Group 4: Heidi Kallioniemi, Petri Sola & Sari Suhonen
Evolutionary aspects of species introductions
Exotic species introductions offer us an extreme number of natural experiments on evolution
The most dramatic examples of evolutionary change often occur when a lineage is faced with a new
set of biotic conditions. E.g. A species arriving in a new geographic place. Species introductions
occur on a large spatial and temporal scale, many times into environments that are new to those
species. The main advantages of studying introduced species are that introductions are replicated
across space (the same species is often introduced into multiple places) and across taxa (multiple
species are often introduced into the same place) and all in natural conditions. Very often we also
know how long a population has been isolated from its historical range. Species invasion allows us
studies in spatial and temporal scales that could never be accomplished in laboratory or simplified
field experiments.
The development of different laboratory techniques has helped a lot in analyzing the effects of
introductions. Molecular markers have allowed enormous advances in studying exotic species
invasions. E.g. In locating the source(s) of exotic introductions and providing tests for genetic
bottlenecks.
The study of species introduction and invasion can help us answer the following questions:
1. Does genetic isolation lead to rapid novel speciation? Will we experience the birth of new species
due to introducing species into new areas separated geographically from the original population?
2. Is the lag phase just a result of the time it takes for adaptive evolution to produce invasive
phenotypes? An invasive species does not usually become invasive immediately after introduction.
Usually there is a time lag, before the invasion begins.
3. How often is the failure of exotic species to become invasive due to a lack of appropriate genetic
variation and therefore evolutionary potential? Are those species that do not become invasive just
not capable of evolving to the new biotic and abiotic conditions they meet in their new
environment?
4. How does invasion affect on the evolution of the invasive species itself and/or the native species?
Invasion may affect not only introduced species, but also native species, so that they too have to
evolve in order to be able to co-exist with the invasive species.
Mechanisms that drive evolutionary change
Genetic variation is considered to be the most important factor in a species ability to evolve. Many
succesful invaders often pass through short-lived bottlenecks followed by rapid population
expansion. The length of the bottleneck period may be more crucial than the bottleneck itself.
Endangered species often remain at low effective population sizes for extended periods, which may
lead to significant losses in variability, and therefore weakens the species survival. Introduced
populations often retain 80% or more of the genetic variation present in their source(s). Studies
have shown that eventhough only little allelic diversity may be lost due to a bottleneck, rare alleles
may change in frequency or be lost (Brookes et al. 1997), and the phenotypic effect of one allele or
gene could effectively increase or decrease due to reduced variation at one or more epistatically
associated loci (Brodie 2000).
Most introduced species (those that have become established) are capable of responding to selection
and genetically diverging from their source populations. Introduced species often exhibit dramatic
phenotypic changes that are much larger than the changes observed in the same populations at
neutral locations. Thus, selection is likely to be an important force in most examples of evolutionary
change in small populations. (Wares et al. 2003)
Hybridization and introgression can help a species to become invasive. Hybrids may absorb the
parental stocks through introgression (e.g. GM plants) and introgressive hybridization may lead to
genetic extinction of native species. (Ellstrand and Schierenbeck 2000)
Additive genetic variance and phenotypic responses to selection
It is important when measuring changes in the phenotypic composition of newly founded
populations to separate the contributions of environmental, additive and non-additive genetic effects
(Lõpez-Fanjul et al. 2003). Selection in natural populations may be quite effective at generating
rapid and heritable phenotypic evolution (Reznick and Ghalambor 2001). The magnitude and
duration of bottlenecks that accompany succesful introductions are often not severe enough to
prevent substantial phenotypic evolution of invading species entering new environments and
communities (Grant et al. 2001).
Evolution of the niche
How often is the evolution of species' niches directly responsible for invasion success? The lag
observed in many invasions can reflect the fine-tuning of adaptation to new environments. Ability
to evolve quickly may determine whether or not the invasion is ultimately succesful. If an exotic
species establishes foothold in one habitat, it can then repeatedly send out propagules into other
habitats leading to repeated evolutionary trials in the new environment. If niche evolution is
required for an invading species to adapt and spread in a heterogenous landscape, this process may
be more likely to be observed for species with female-biased dispersal. (Holt et al. 2003).
Evolution of invasive species
Requirements for a successful invasion
To make a successful invasion, Lee (2002) argues that a species must respond to new conditions. It
must be able to respond to requirements of environmental gradients and other species. Reaction can
be due to a genetic and plastic response. For example the European wild rabbit (Oryctolagus
cuniculus) has a leaner body and longer ears in warm Australia, to where it was introduced in 1859,
than in Europe. Morphological changes were due to a genetic and plastic response, and the traits
were heritable.
When invading, species meet new species and interact with them. To make invasion successful,
species have to respond to resident species. For example the velvetleaf Abutilon tehophrasti
(Malvaceae) is invading cultivated fields in the USA. It competes for light with cultivated plants
and has evolved different life-history strategies depending on the competitive species.
Invasive species have also a possibility, that the release from familiar predation can potentially
release defensive traits. This can be an opportunity to invasive species and it can lead to rapid
evolution. Some researchers have challenged this theory, but on the other hand it hasn’t been
studied so much yet (Lee 2002).
Silent nights in Kauai, Hawaii
Hawaii has many introduced species, so species are forced to meet many species they haven’t met
ever before. For example field cricket (Teleogryllus oceanicus), which is native to Australia and
Oceania and parasitoid fly (Ormia ochracea), which is native to North America, are both introduced
to Hawaii and are interacting with each other (Zuk et al. 1993). Male crickets call and play a
courtship song to female crickets by using their wings. Males have a stridulatory apparatus in their
wings to produce the voice (Zuk et al. 2006). The Parasitoid fly is specialized in locating calling
crickets. When a female fly finds a male cricket, it deposits its larvae near the male. Larvae burrows
inside the cricket, consumes its tissues and when emerges, kills the male cricket. Natural selection
against calling males is therefore strong. Nevertheless sexual selection is affecting in different
direction, so there is a conflict between sexual and natural selection (Tinghitella 2008).
Against their normal sexual behaviour, male crickets in one of the islands, Kauai, are silent. It took
less than 20 generations that over 90 % of males had become silent and instead of normal wings,
they have flat wings (Zuk et al. 2006). Researchers found a mutation from a sex-linked locus. Sexlinked inheritance is thought to facilitate rapid evolution. The usually maladapted type has become
general really fast, although mutation has eliminated sexual signals. Female crickets are forced to be
less picky and accept silent males. In this case selection against normal type and adaptation of
females were both needed to make mutation successful (Tinghitella 2008).
The Argentine ants of Southern Europe
The Argentine ants (Linepithema humile) were introduced to Southern Europe from Southern
America. They live in small, mutually aggressive colonies in their native areas. They fight, if they
meet ants from a foreign nest. The introduction of the Argentine ant in Europe caused a loss of
inter-nest aggression leading to the formation of two huge supercolonies (Giraud et al. 2002). It is
unusual, that a genetic bottleneck improves fitness. The argentine ants got benefit from it, because
they don´t fight inside the supercolony. Loss of intraspesific competition enabled the transformation
into a nonaggressive multiqueen supercolony with a high population density. That successfully
outcompetes native species. Also in Northern America, the invasion of the Argentine ant, has
resulted in reduced intraspesific aggression, leading to the formation of supercolonies (Tsutsui
2000).
Evolution of native species
Snake and toad in Mallorca
Endemic midwife toad (Alytes muletensis) have morphological changes caused by introduced
viperine snake (Natrix maura) in Mallorca (Moore at al. 2004). Snakes are eating tad-poles and it
causes size-selective predation. In some areas the snake has eaten the toads to extiction, but in this
area they can live together. Snakes eat only mid-size tad-poles, and small and big tad-poles survive.
Also tad-poles plasticity causes variation in toads morphology, for example their tails size
metamorphosed.
Snake and toad in Australia
Toxic cane toad (Bufo marinus) is a very bad invasive species in Australia. It was introduced to
Australia from South and Central America as a method of agricultural pest control. Toads have
poison glands and tad-poles are highly toxic. Only part of the native snakes can eat them. The
ability of the snake to get poisoned by consuming a toad depends on it's head size relative to it's
body mass (Phillips & Shine 2004). According to this research, snakes who are toad-vulnerable,
have morphological changes. Their body size increased after the arrival of toxic toads. Also the
relative head size of the snakes decreased. This lead to the snakes being more able to tolerate
poison.
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