Biology Reference
In-Depth Information
Figure 5.1.
Possible evolutionary consequences of contact between a native
species and an introduced species of pathogen. Similar evolutionary interactions
also can occur between higher plants and animals. (Reprinted with permission from
C. M. Brasier. 2001. Rapid evolution of introduced plant pathogens via interspecific
hybridization.
BioScience
51:123-133. © 2001 American Institute of Biological
Sciences.)
behave normally in meiosis, hybrid forms may be fertile. If hybridization
is frequent and widespread, the parental forms may essentially merge to
form one reproductive complex. This process, known as introgressive
hybridization, may enhance genetic variability, and the progeny may show
hybrid vigor. If substantial chromosomal or genetic differences exist, the
hybrid offspring may show reduced fertility or complete sterility. In
plants, however, doubling of the chromosomes of sterile hybrids can
occur, restoring fertility, since chromosomes are able to pair and behave
normally in meiosis.This, in fact, occurred with the cordgrasses discussed
at the chapter outset.
The potential for hybridizations involving alien species is great. For
example, the plant genus
Centaurea
, to which the knapweeds and starthis-
tles belong, contains about 1,350 species, among which at least 232
hybrids have been recorded (Roché and Roché 1991). In the British Isles
alone, 95 putative hybrid plants involving one or more aliens have been
recorded (Stace 1991). In Ontario, Canada, 34 hybrids involving one or
more alien plants have been reported (Vilà et al. 2000).The potential for
