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276
C.R. Largiadèr
Number of Publications
600
500
400
300
200
100
0
1956-60
1961-65
1966-70
1971-75
1976-80
1981-85
1986-90
1991-95
1996-00
2001-05
Fig. 16.1 Development of
natural and anthropogenic
hybridization
and introgression as
research topics. The literature
search was carried
out in the Web of
Science (Thomson Scientific)
citation database,
using the search
string ‘hybridization and
introgression’
Years
and species are defined also strongly influences the perception of the problem.
Recently, Mallet (2005) reviewed studies of natural interspecific hybridization
in plants and a variety of animals. He estimated that at least 25 % of plant
species, and 10 % of animal species are involved in hybridization and potential
introgression with other species. Plants seem more prone to hybridization
than are animals. However, this difference may at least partly be due to the different
historical attitudes of botanists and zoologists toward hybridization,
which resulted possibly in greater attention being paid to this phenomenon
by botanists (Dowling and Secor 1997). While botanists viewed hybridization
and introgression as important processes in adaptive evolution (Rieseberg
1997), zoologists have tended to see them rather as problems being the converse
of reproductive isolation, and thus challenging the ‘reality’ of biological
species. However, there is increasing evidence that hybridization and introgression
have also played an important role in the evolution of animals (Grant
and Grant 1996; Dowling and Secor 1997). Taking into account the potential
evolutionary importance of these processes changes considerably how we
may perceive them in the context of biological invasions, e.g., that they may
act as a stimulus for the evolution of invasiveness in transplanted species (Ellstrand
and Schierenbeck 2000).
In this chapter, I intend to give a non-exhaustive overview on the empirical
evidence of hybridization and introgression in the context of biological invasions.
When any two taxa hybridize, the outcome of this event is difficult to
generalize, and is modulated by many interacting external (e.g., habitat modifications)
and evolutionary factors (e.g., mate preferences, heterosis etc.): the
two taxa can merge completely, forming a ‘hybrid swarm’ leading to the
extinction of the native taxon, which is replaced by a single mongrel species.
Hybridization can be asymmetrical, i.e., genes introgress directionally from
one taxon into the gene pool of the other, and hence lead to an acceleration of