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etween natural selection and hybridization may significantly alter the population dynamics
of the introgressed taxa, and therein alter patterns of invasion and establishment (Lenormand
2002).
In general, crop-wild hybrid populations are expected to be more diverse than the nonhybridized
pa rental t axa. S ubsequently, h ybridized popul ations w ould d ifferent f rom t heir
parental ta xa o r mo re s imilar to t hem unde r na tural s election a cts, a nd t his pr ocess a lso
depends o n w hether p henotypes cr eated v ia their g enetic v ariation ar e b eneficial o r
deleterious to fitness. If the phenotype is advantageous, hybridization is expected to result in
long-term introgression. However, studies on whether genotypes of hybridized weeds tend to
converge on or di verge f rom t hose of t heir w eedy ancestors, a nd at w hat r ate, a re s cant
(Hegde et al., 2006; Whitney et al., 2006). Gene flow and introgression plays a ke y role in
maintaining cohesion a mong geographically s eparated populations in some species, such as
wild radish (Kercher and Conner 1996), maize (Ross-Ibarra et al. 2009), and rice (Zheng and
Ge 2010) . D istance di vergence c ould oc cur w hen na tural s election i s s ufficient s trong t o
overcome gene f low or w hen ge ne f low i s l imited ( Ross-Ibarra e t a l. 2009; Z heng a nd G e
2010). A lternatively, ge ne f low a nd i ntrogression f rom c rops m ight r esult i n no c orrelation
between genetic differences and geographic distance of crops (Tokunaga and Ohnishi 1992;
Kercher and Conner 1996).
Haygood e t a l. ( 2003) f ound t hat g enetic a ssimilation i nvolving t hresholds a nd
hysteresis through performing mathematical models of a wild population recurrently receiving
pollen from an introgressed crop, and that a small increase in immigration can lead to fixation
of a di sfavoured c rop g ene a nd e ven dr astic s hrinkage of t he w ild popul ation be cause
demographic swamping could give rise to ‘migrational meltdown’. Several studies have found
that h ybrids mi ght replace t heir p arents after crop gene i ntrogression i nto w ild r elatives
(Linder e t a l. 1998; C ompell e t a l. 2006) , a nd e ven r esulted i n t he extinction of t heir
progenitors (Hegde et al. 2006). Campbell et al. (2006) showed that hybrid radish population
after crop gene introgression had greater fecundity and survival than wild populations. Hedge
et al. (2006) found that hybrid derived descendants replaced the two local Raphanus species
using m orphological a nd a llozyme evidences i n C alifornia, and t hat C alifornia w ild r adish
separated f rom bot h of i ts pa rents a s a n e volutionary e ntity, s uggesting t he a ggressive
colonizing be havior of the h ybrids pr obably no t r esulting f rom genetic va riability of t he
population per se but from the combination of parental traits.
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