0851996884.pdf

loss in quality occurring during the initial
stages of domestication. Such a pattern sug-
gests investment in improving the rearing
conditions so that the selection that results in
an excessive loss of quality is reduced.
In a captive environment, we can expect
the ‘domesticated’ stock (Fig. 6.1) to be rela-
tively stable. However, over time, we would
expect the accumulated effects of inbreeding
to result in a slow general decline that
reduces both quantity and quality (the lower
part of the curves shown in Fig. 6.1).

Genetic Change due to Captive Rearing

Most of the studies of genetic change occur-
ring in captive-reared populations have been
on the tephritid fruit flies used in SIT. These
studies are useful because they illustrate the
dramatic changes that occur when insects are
intensively cultured to produce very large
numbers. There is no reason to believe that
the data from fruit flies are unusual. Genetic
changes are inevitable whenever a genetically
variable population is reared in a novel envi-
ronment. Natural selection will act and adap-
tation to the new environment will occur.

Mating behaviour

One very common adaptation to captive
rearing is earlier mating and oviposition (e.g.
melon fly: Miyatake, 1998; medfly: Rössler,
1975; Wong and Nakahara, 1978; Vargas and
Carey, 1989; oriental fruit fly: Foote and
Carey, 1987; tobacco budworm: Raulston,
1975). The time scale of this adaptation is
quite rapid. Using a wild-caught population
of tobacco budworm, Raulston (1975) found
that, after seven generations of captive rear-
ing, the shift to the domesticated pattern of
early mating was almost complete.
More complex changes in mating behav-
iour may also occur. Haeger and O’Meara
(1970) showed that the captive rearing of the
mosquito Culex nigripalpusresulted from a
shift in female behaviour. The mating success
of colony females was about 70%, regardless
of whether the males were from the colony or
from the wild; however, under similar condi-

76 L. Nunney

log (quantity)

log (quantity)

log (quality)

log (quality)

Wild

population

Wild population

and optimum

strategy

Optimum

strategy

Domesticated

stock

Domesticated

stock

Fig. 6.1.The expected trade-off between the

numbers produced in mass rearing (quantity) and

field performance (quality). The solid curve defines

the trade-off. The natural (wild) population is

arbitrarily placed at the origin and the position of a

population adapted to the captive-rearing facility

over many generations (domesticated stock) is

shown. The region of the trade-off curve below the

point of domestication defines decreasing quality

and quantity, due to the effect of long-term

inbreeding depression. The dashed lines link

combinations of quality and quantity that are

equally effective (as defined by equation 1). The

upper graph shows a trade-off curve with an

optimum strategy of partial domestication; the lower

graph shows a curve with no such optimum – the

best strategy is to minimize domestication.