The process of speciation within insular populations has been discussed
by many authors. Hesse, Allee, and Schmidt (1937:517) list the motives
for differentiation as, "Special character of insular faunae rests on
the conditions common to all islands--isolation, freedom from
competition, space restriction, and special insular climates." This
combination of characteristics is seldom found elsewhere in nature, and
as Murphy (1938:357) points out, an island is the nearest approach to a
"man-controlled laboratory." Isolation of small populations is probably
the most influential factor in the process of speciation in insular
organisms. Lack (1947:134) writes that "in all organisms the isolation
of populations is an essential preliminary to the origin of new
species." Buxton (1938:265) also stresses this point with regard to the
formation of species of insects in Samoa and emphasizes that evolution
may occur more quickly in small populations. When mutations appear in
such small and isolated populations, they have a greater chance to
become fixed than do mutations in less restricted populations in a
larger land mass, where such a mutation might be lost by the swamping
effects of outbreedings. In addition, Wright (1931 and elsewhere)
suggests the possibility of change by accidental elimination and
recombination of hereditary characters in micropopulations. This
mechanism could well be a factor in Micronesian bird populations, many
of which possess no more than a few hundred individuals. Huxley
(1938:256) emphasizes that "accidental" mutations may be perpetuated in
small, isolated groups. It might be added that such changes might be
either advantageous or disadvantageous to the organism concerned. Huxley
(1938:263) states also that geographic isolation may promote nonadaptive
differentiation, which may be caused by "colonization by a random
sample" or by subsequent "preservation of nonadaptive mutations in
numerically small isolated groups." Mayr (1942b:237) cites the
importance of the "founder" principal for reduced variability in small
populations. He points out that if the "founders" of the population
carried with them only "a very small proportion of the variability of
the parent population," one would expect to see divergence from the
ancestral stock.
Freedom from competition, especially interspecific strife, is an
important factor in differentiation; this is especially true in the
early period of colonization. Lack (1947:113) points to the absence of
food competitors, especially in the initial period of colonization, as
an important influence in the evolution of Darwin's finches at the
Galapagos Islands. Once a population has become established and
"adjusted" to a given environment on a small island, intraspecific
competition might bring about adaptative selection. Subsequent colonists
might be eliminated by the competition brought about by these previously
adapted organisms, especially if both organisms were adapted for life in
the same ecologic niche. Space restriction may be important in such
Micronesian birds as _Rhipidura_ and _Myiagra_, which appear to possess
recognizable territories. A new colonist entering the territory of one
of these birds might be forced out. This competition might not play such
an important part among birds, which live in flocks and do not range in
closely guarded territories; birds in this group include some pigeons,
starlings, and white-eyes.
Freedom from the pressure of predation probably exerts a direct
influence on formation of species. Aside from a few migrant hawks and
two kinds of resident owls, most of the avifauna feeds on vegetable and
invertebrate foods. The large lizard _Varanus_ may be classed as the
only native predator on many of the islands. Man has been responsible
for the introduction of rats, house cats, and other mammals, which may
be destructive to birds. Thus, before the advent of man the factor of
predation may not have been of great consequence. As mentioned
previously, nonadaptive modifications may be perpetuated where the
"weeding-out" process by predation is not an influence. Flightless rails
have apparently developed in the absence of predation.
The absence of the pressure of predation should remove a certain amount
of control on the population turn-over. As Hesse, Allee, and Schmidt
(1937:521) write, a characteristic of the faunas of oceanic islands is
the fact that they are distinguished by the occurrence of
"disproportionately developed taxonomic groups in which one or a few
basic types have undergone adaptative radiation and come to fill unduly
large proportions of the population as compared with conditions that
obtain on neighboring continents." Lack (1947:114) writes, "that the
absence of predators may well have accelerated the adaptative radiation"
in the Galapagos finches. In Micronesia, the starling (_Aplonis opacus_)
dominates much of the available habitat on some of the Caroline atolls,
and even on "high" islands, where other land birds are present. There
appears to be no tendency towards selective adaptations occurring, or
towards ecologic isolation.
Available data indicate that the life spans of individual birds in
Micronesia may be short. For example, it was obvious on many of the
islands visited by the NAMRU2 party that starlings (_Aplonis opacus_) in
immature plumage outnumbered starlings in adult plumage, although it is
possible that immature plumages are retained longer in these island
birds than in others. Similar observations were made by Coultas, who
noted the ratio of birds in immature plumage to birds in adult plumage
at Kusaie to be 5 to 1. If the life span is shorter in these insular
forms as compared with that of the ancestral stocks, the higher annual
population turn-over would allow for the speed of genetic changes to be
accelerated.
The origin of species by hybridization between different kinds of
organisms has been a subject of frequent discussion. Lack (1947:100)
concludes that it is improbable that hybridization has played an
important part in the origin of new kinds of birds. Nevertheless, the
absence of sufficient mates in the confines of a small island probably
stimulates the crossbreeding between two species of birds. Fertile
offspring of such a cross might conceivably account for some of the
populations, the origins of which are puzzles to present day
taxonomists. Such Micronesian forms as _Metabolus_ and _Cleptornis_
could conceivably have been derived in such a manner. Yamashina (1948)
has described the origin of _Anas oustaleti_ as a result of
hybridization between _A. platyrhynchos_ and _A. poecilorhyncha_. It
might be difficult to explain every case of the formation of other
insular species on the basis of the effects of isolation and paucity
alone. However, Mayr (1942b:236) includes the development of
questionable and unusual kinds of insular forms in a general statement:
"The potentiality for rapid divergent evolution in small populations
explains also why we have on islands so many dwarf or giant races, or
races with peculiar color characters (albinism, melanism), or with
peculiar structure (long bills in birds), or other peculiar characters
(loss of male plumage in birds)."
Nutrition may be also a factor influencing speciation in bird life. The
types of food plants (coconut, papaya, breadfruit, pandanus, etc.) might
be similar on a Micronesian island and on a continental island in the
Philippine region; however, the value of these plants as foods might
vary and might reflect differences in mineral content of the soils. For
example, if the soils on an island lack, or by leaching out have lost,
sufficient amounts of potassium and other elements, plants may store
foods, not as proteins, but possibly as carbohydrates, simple sugars, or
alkaloids. Whether nutritional influences might have a selective effect
on the bird populations, has not been ascertained.
In summary, it may be said that genetic change altering the phenotypic
expression of avian characteristics is no more apt to happen in insular
populations than in continental populations but genetic change may have
a greater chance of being perpetuated in small insular populations where
isolation, limited competition, freedom from the selective influences of
predation, and other factors exert influences.