Literature review: Impact of Chilean needle grass ... - Weeds Australia

origins of their parents (Petit 2004). The normally strongly outcrossing S. alterniflora Loisel., introduced from the east coast of
North America to San Francisco, rapidly evolved high rates of self-fertilisation in its new evironment (Cox 2004). Stebbins
(1972) recorded the invasive nature of an artificial Ehrharta erecta Lam. autopolyploid he created by colchicine treatment and
released on the Berkeley campus of the Universityof Califorina. Stipeae as a whole apparently consists largely of species
resulting from frequent and widespread hybridisation of divergent elements (Johnson 1972, Tsvelev 1977).
Single genes or a few genes might effect invasiveness or weediness traits. Sorghum halepense, one of the world’s worst weeds
(Parsons and Cuthbertson 1992), has few genes affecting weediness that distinguish it from non-invasive grain Sorghum spp.
(Lee 2002). When S. halepense (tetraploid) pollinates the diploid Grain Sorghum, S. bicolor (L.) Moench, sterile triploids are
produced that are weedy in successive crops (Parsons and Cuthbertson 1992).
Selection pressure after naturalisation can produce phenotypes with altered morphology, physiology and phenology, or with
greater plasticity in response to environmental variables (Lee 2002) and these changes can occur within periods as short as a few
plant generations (Cox 2004). For example invasive populations of Echinochloa crus-galli (L.) P. Beauv. in Canada have
evolved greater catalytic efficiency of some enzymes, which compensates for the poor adaptation of their C 4 photosynthetic
system to the cold climate (Lee 2002, see her references). Selection can occur in response to environmental gradients, the
resident biota and control activities (Lee 2002). Weed mimicry of crop species e.g. by E. crus-galli (Lee 2002), is another
example of relatively rapid evolutionary adaptation, well known in grasses (Barrett 1983).
Release from predation and competition in the invaded environment removes some selection pressures on the invading plant and
may release characters associated with defense mechanisms from evolutionary canalisation (phenotype limitation due to the
constraints imposed by developmental pathways) and result in rapid evolution (Lee 2002). This has occurred with Silene latifolia
Poir. in North America which has apparently allocated resources, previously used in defence, to enhanced reproduction (Whitney
and Gabler 2008). Rapid evolution can also occur if plant predators and specialist herbivores expand or shift their host
preferences to consume invasive plants (Cox 2004).
Complex patterns of evolutionary change should be expected in each particular invasion, with particular traits favoured at
different stages of the invasion, and possible reversals of trait changes (Whitney and Gabler 2008).
Complementary evolutionary changes may also be expected in the invaded community – the more serious the invader, the
greater the selective pressure it imposes – and those that have been investigated can also occur rapidly, on timescales of