Biology Reference
In-Depth Information
classes, in all countries.” Had he expanded on this theme—perhaps in a sequel called
Onthe
Origins and Ubiquity of Rare Species
—the topic of rarity might have become more central
to the scientific orthodoxy much earlier.
Rarity is all around us, but an excellent starting point for understanding its generation, su-
perior to the Juan Fernández group in its illustrative qualities, is on a large, highly mountain-
ous tropical island. Even better is if such a landmass is far from a major continent and has
remained separated from it for eons. In this environment we could seek a rich assemblage
of plants and animals illustrating the first condition of rarity—a narrow range. Such an ideal
locale sits on the other side of the southern Pacific Ocean from the Juan Fernández Islands:
the huge island of New Guinea. Here we can clearly observe how geologic events and evol-
utionary mechanisms influence rarity; indeed, they help to create it. New Guinea's array of
fabled birds of paradise and tree kangaroos illustrates how such episodes and processes lead
to the separation of populations and the surfacing of new species, a number of which earn the
moniker “rare.”
Geologic events are harbingers of evolution. By this I mean that they often create the con-
ditions that divide previously continuous populations into subpopulations that are isolated
from one another, the condition that enables them to evolve separately. In New Guinea, for
example, the formation of rugged mountain ranges isolated populations of the same species
from one another by distances so great that individuals no longer dispersed between them.
Over many generations, through genetic mutation and adaptation to differences in the envir-
onments on the isolated ranges, the populations may diverge enough in important character-
istics that if members of the divergent groups are reunited, they can no longer interbreed. In
this way new species are born—a process called speciation—with such new species often
starting their existence as rare forms with a narrow range and low numbers. For this process
to occur for birds and mammals, the isolated populations must persist long enough for the re-
quired genetic changes to accumulate. Speciation in birds and mammals does not happen on
small islands because distances are not great enough to isolate populations for a long enough
time. In contrast, amphibians do speciate on small islands, owing to their reduced mobility.
But New Guinea is large enough and rugged enough for its lofty mountain ranges, surroun-
ded by tropical lowlands unsuitable for mountain animals, to have witnessed many bouts of
speciation. Although much of the seminal work on speciation used islands as the model, spe-
ciation is hardly restricted to islands and occurs on large landmasses as well. So, a critical
insight about rarity is that island life per se is not the key to frequent evolution of new species
and new rarities; rather, it is isolation, which can be provided by island archipelagoes or large
islands.
If all islands are physically isolated from a mainland, are all islands, by the very nature of
their limited range, repositories of rarities? Not necessarily, because several factors influence
evolutionary processes on islands. Among them are distance from a mainland, length of time
