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Davies et  al. (1999a) used the same intron loci to distinguish between “alter-
native hypotheses concerning the source of medfly infestations in California.”
In their study, intron sequences from Villablanca et al. (1998) were used, as well
as newly obtained intron sequences from Medfly samples in California, Costa
Rica, Guatemala, Mexico, Brazil, Peru, Greece, Hawaii, and Africa. A total of 237
sequences were obtained for the four loci in 74 individuals. The data from all
Medflies in California were treated as a “single population for the purposes of
statistical analysis. Under the null hypothesis that there is a resident medfly popu-
lation in California, we assume that these flies, captured in the same geographic
area, represent a single biological population ( Carey 1991 ).” To assess whether
the recent outbreak was due to a new invasion, the authors focused on a single
fly (B-96) captured in southern California (Burbank) in 1996. AMOVA produced
indices of population subdivision analogous to standard F statistics. Another
program, TFPGA, was used to calculate the average theta. Yet another pro-
gram, IMMANC, was used to carry out an assignment test. Davies et  al. (1999a)
concluded that, “Because the B-96 genotype was included in the Californian
population (and not the potential source) when estimating the “resident” allele
frequencies, the test is conservative with respect to the null hypothesis that
B-96 is a resident--in this case of California.” Davies et  al. (1999a) concluded
that the single B-96 Medfly studied was “less likely ( α< 0.05) to be a resident of
California than an immigrant from no less than four potential sources: Costa Rica,
Guatemala, Mexico, or Peru.” Finally, Davies et al. (1999a) concluded,
“More work is clearly needed to explore the phylogenetic consequences of
invasions and a better understanding of invasion genetic patterns will provide
a deeper insight into the ecological and evolutionary processes that underlie
bioinvasions. It is important to consider that invasions often involve a
hierarchy of events, the totality of which might be termed a metainvasion.
The metainvasion begins with a primary invasion, when a species first
colonizes a new area from its ancestral source. Subsequently, secondary and
tertiary invasions arise as the newly established populations themselves seed
new areas. The genetic changes that result from these events are complex
and phylogenetic analyses may be informative at some levels but not others.
A primary invasion of the medfly occurred from Africa to the Mediterranean.
The invasion of Latin America may be another primary invasion, direct from
Africa, or a secondary invasion from the Mediterranean. Californian medfly
invasions thus represent secondary or tertiary events in the global medfly
metainvasion. Indeed, California may be subject to repeat invasions that could
superimpose on one another.”
Although by 1998, powerful genetic tools had been brought to bear on
the California Medfly colonization question and the statistical methods for
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