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Neobromus reached North America and eventually South America by the end of the
Pliocene, with diploid and tetraploid Ceratochloa and Neobromus species eventu-
ally becoming extinct and only New World species remaining extant. Grasses in
section Bromopsis also spread to Africa and the Americas during the Pliocene. The
species in sections Bromus and Genea (subgenera Bromus and Stenobromus ) prob-
ably developed from different early Central Asian Bromopsis species during the
Pleistocene in conjunction with human activities. They also spread into Europe
during the Pleistocene and are considered archaeophytes (introduced before about
1500 AD) (Stebbins 1981 ; Preston et al. 2004 ).
Annual species in sections Genea and Bromus are species that have long been
associated with human habitation and have adapted to grow and reproduce effectively
in disturbed areas and in association with crops (Stebbins 1981 ). They are native to the
Eastern Hemisphere, but many have been introduced into the Americas. The evolu-
tionary association of the species in these sections with grazing and agriculture sug-
gests that they have developed adaptations that are likely to make them highly
competitive weeds in areas heavily used by humans. The Genea species, especially,
have seeds with long awns and other features that facilitate distribution by grazing
animals. This section has only eight species, but six are widely naturalized and are
destructive crop weeds, ruderal weeds, and environmental weeds (Andersson et al.
2002 ; Kleeman and Gill 2006 ; Fortune et al. 2008 ; Williams et al. 2011 ). Section
Genea includes the seriously invasive species B. tectorum and B. rubens.
Many species in section Bromus are widely introduced and weedy, with some
species falling into all three weed categories. Out of 32 species in section Bromus ,
Fig. 6.1 Number of species in each taxonomic section of the Bromus genus for all introduction
and impact categories combined ( left panel ) or separated ( right panel ). Species may be both widely
introduced and weedy and may be included in multiple weed categories. “A” and “P” are used to
show sections that are (almost) all annual or perennial. Note the different y -axis scales for the left
(total) and right (by category) panels
S.Y. Atkinson and C.S. Brown