270 
PACIFIC SCIENCE, Vol. XXII, April 1968 
Insular selection pressures (except, perhaps, 
for species introduced by and dependent on 
man) are similarly at an angle to those exerted 
on the mainland parental form. Insofar as the 
island biota is different from that whence the 
immigrants came, it is inevitable that the bio- 
logical selection pressures on islands will differ. 
Any immigrant to an island will leave behind 
at least some of the predators, competitors, 
diseases, and parasites that the parental main- 
land stock had to cope with. On the other hand 
the initial immigrants may well have to adapt 
to new forms of food, cover, etc. (This will be 
less true only in degree if a species arrives by 
a series of island hops.) 
There is also evidence that selection pressures 
on small islands are likely to be more radically 
different than they are on large islands. Thus, 
on Manana Island, a small outlier of Oahu 
without domestic cats, the cat flea ( Ctenoce - 
p halides felts felts') has developed an ecto- 
parasitic existence on rabbits ( Oryctolagus 
cunt cuius) (Tomich, et ah, in press). Again, 
in the Balearic Islands off Spain, Eisentraut 
(1949) showed that on the smaller outliers the 
food of lizards ( Lacerta ) differed considerably 
from that on the main islands. As the normal 
insect food became more restricted, these lizards 
added the normally avoided ants to their diets, 
and on very small islands ate even flower petals 
and young plant shoots. 
Eisentraut believed that this change in diet 
had a direct metabolic effect resulting in the 
melanism frequently found in the small-island 
populations. To me (cf. Dowdeswell and Ford, 
1953) it seems more likely that the morpho- 
logical changes so frequently found in small 
poulations are in part the indirect effect of 
altered selection pressures working with time 
on the, in part randomly, depleted gene pool of 
small populations. A gradual reintegration of 
such a gene pool in response to altered selec- 
tion pressures would likely involve a change in 
phenotypic characters that are not themselves 
selected. Such an interpretation (cf. Mayr, 
1954; Dobzhansky and Pavlovsky, 1957) seems 
to me to provide the best available explanation 
for the often rather heterogeneous differenti- 
ating characteristics of insular endemics, e.g., 
Hawaiian inshore fishes. 
The main reason why the peculiarities of 
Hawaiian endemic fishes cannot, apparently, be 
attributed to small-population losses alone is 
that in many instances the Hawaiian endemics 
are not characterized by a simple increase in 
variability (as in Dobzhansky and Pavlovsky’s 
flies) but rather by new and fairly constant 
characters entirely outside the range of vari- 
ability of the ancestral populations (as in 
King’s results). Presumably such characters 
must have arisen through a reintegration and/ 
or evolution from the ancestral genetic system 
via direct or indirect selection. The same rea- 
soning would seem to apply to at least some of 
Eisentraut’s melanic lizard populations. 
Judging by personal observation and com- 
mon knowledge concerning recent successful ter- 
restrial introductions of animals to the Hawaiian 
islands (see also Mead, 1961:180-182 ; Tomich, 
et ah, in press), there is often (presumably 
following a longer or shorter period of small 
numbers) a tremendous initial build-up and 
"overshoot” in population number (Fig. 1). 
During my 18 years in Hawaii this has hap- 
. 1 \ 
variety \ 
B ■ — 
A 
time ► 
Fig. 1 . Theoretical population size {above) and 
allele variety {below) plotted against time in a ter- 
restrial animal that successfully immigrates into the 
Hawaiian Islands for the first time. A, Date of 
arrival; B, time when population becomes sufficiently 
large that small-population genetic factors will per se 
cause no further loss of alleles; C, initial peak of 
abundance; and D, subsequent equilibrium. For dis- 
cussion, see text. 
