Evolution of Hawaiian Animals — Gosline 
pened with a garden spider ( Argiope appensa ) 
and the giant African snail ( Achatina fulica ), 
among the more conspicuous unintentionally 
introduced forms. If it can be assumed that this 
cycle happened in the past with our "native” 
biota, then certain postulates concerning selec- 
tion pressures would seem to follow. 
First, during the period of initial buildup of 
an introduced form, selection pressure must be 
very low. (Apparently the other members of the 
biota are not initially able to cope with or defend 
themselves from the new introduction.) How- 
ever, at some point in the buildup, the popula- 
tion becomes excessive, after which it falls 
drastically to a new fluctuating equilibrium well 
below the previous maximum. The nature of the 
factor that sooner or later kills back the initial 
overshoot is unknown in any particular instance. 
There is no reason to believe it is the same in all 
cases, or that it may not be a combination of 
factors. What is important to the present argu- 
ment is that after a period of relaxed selection 
during the population buildup a very severe 
selection pressure of some sort appears. Some of 
the various possibilities are as follows. 
First, the animal may eat out the available 
food supply and then die of starvation. This 
apparently happened to the rabbits introduced 
to Lisianski Island (Bryan, 1942:192, 193), 
and almost but, perhaps significantly, not quite 
with the rabbits on Laysan (Warner, 1963:6, 
7; cf. also Tomich, et al., in press). 
On a larger island with a more varied biota 
a second possible situation might occur after 
the immigrant population had overeaten its 
original food supply. Assume that an immi- 
grant adapts itself to an insular food supply as 
close as possible to that of its parental stock. 
Assume that, having adapted itself to this in- 
sular food source, the immigrant builds up a 
tremendous population under greatly relaxed 
selection pressure. At some point it will over- 
shoot its new food supply and a severe compe- 
tition for food will take place. This selection 
may preserve the best adapted individuals of 
the original immigrant type, if enough of the 
food supply is left. It may also preserve those 
individuals that have differentiated farthest in 
the direction of adapting to a new food source 
(Fig. 2). This theoretical possibility has been 
set up with the evolution of the Hawaiian 
271 
Fig. 2. On a background ( dotted contours') rep- 
resenting Sewall Wright’s adaptive peak concept are 
shown ( hatched areas): above , a theoretical popula- 
tion just before reaching the initial maximum (C in 
Fig. 1), and below, the same population after an 
equilibrium size had been reached ( D in Fig. 1). 
For discussion, see text. 
drepaniid finches, with their various beak types, 
in mind. 
Another possibility is that, following the 
initial population explosion, some factor other 
than food supply develops to keep subsequent 
numbers low. This could be disease or para- 
sitism, some change in other environmental 
features, or some other factor which would 
lower the reproductive rate. That the reproduc- 
tive rate may be diminished has been stressed 
by Lack (1954). Lack deals especially with 
changes in egg number in birds. But there is 
