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Reference

How stabilized?

Invasiveness

Occurs in human-disturbed areas?

M. Blumler, personal communication

Selfing genotype

Spreading rapidly

Yes

Parker and Bartsch (1996)

Coalescent complex

Noxious weed

Yes

Gallagher et al. (1997); Vilà and D'Antonio (1998)

Clonal growth

Replacing one parent

Yes

Strefeler et al. (1996)

Clonal growth

Noxious weed

Yes

O'Hanlon et al. (1999)

Coalescent complex

Weed

Yes

Panetsos and Baker (1967)

Coalescent complex

Weed

Yes

Abbott and Milne (1995); Milne and Abbott (2000)

Coalescent complex

Noxious weed

Yes

Sun and Corke (1992)

Coalescent complex

Weed

Yes

Ayres et al. (1999); Daehler and Strong (1997)

Clonal growth

Replacing one parent

Yes

Neuffer et al. (1999)

Coalescent complex

Invading polluted forests

Yes

traits tend to be concentrated within a narrow subset of those traits possible. Almost all of our examples are herbaceous (24 of 28). However, the majority of the cases involve perennial species (19 of 28). Interestingly, these characteristics also are found to be frequent among cases of spontaneous hybridization. For example, Ellstrand et al. (1996) examined the 10 genera in the British flora with the highest number of different spontaneous hybrids. They found that most were perennial herbs.

These trends make sense. Perennial hybrids will persist longer than will annuals, giving more time for stabilization opportunities to occur, especially if clonal reproduction is available (as it is in about half of our examples). The predominance of herbaceous over woody examples in our Tables is consistent with Harper's (1977) prediction that colonizing plants allocate more resources to reproductive rather than to vegetative growth. Iteroparous perennial herbs appear to maximize fitness by investing in sexual structures and vegetative spread instead of investing in permanent structures (Crawley, 1986).



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