... it is the circumstance, that several of the islands pos-
sess their own species of the tortoise, mocking-thrush,
finches, and numerous plants, these species having the
same general habitats, occupying analogous situations,
and obviously filling the same place in the natural econ-
omy of this archipelago, that strikes me with wonder. It
may be suspected that some of these representative
species, at least in the case of the tortoise and some of the
birds, may here-after prove to be only well-marked races;
but this would be of equally great interest to the philo-
sophical naturalist...
(Charles Darwin in 1845, writing about the Galápagos
archipelago. From Ridley 1994, p. 79)Having covered the more important micro-
evolutionary processes and outcomes in the previous
chapter, the aim of this chapter is to provide the
basic frameworks by which we understand macro-
evolutionary patterns of change on islands. We
thus start with the nature of the species unit, and
then we outline a set of alternative frameworks
within which we can organize ideas about
island evolution: specifically these frameworks
are distributional, locational, mechanistic, and
phylogenetic. We reserve consideration of the
most striking emergent patterns of island evolution
to Chapter 9.
Evolutionary change does not necessarily imply
speciation, and in the previous chapter we focused
on the changes in niche that characterize island
forms without paying much attention to whether
they constitute ‘good’ species or merely varieties. In
fact, the determination of the species unit, although
of central importance in ecology and biogeography,
is not always clear cut. As highlighted in the above
quotation, Darwin was uncertain as to the taxo-
nomic status of the members of that most emblem-
atic group of island evolution, the Galápagos
finches. When he collected them he viewed the dif-
ferent kinds of finches as merely variants of a single
species (Browne and Neve 1989). Moreover, the full
significance of his Galápagos collection did not
immediately strike Darwin. Rather, his key insights
into species transmutation came some time after
the voyage, as he was attempting to make sense of
his observations and of the taxonomic judgements
made of his collections. Although he was right to
state that the Galápagos biota would prove of
abiding interest whatever the taxonomic status of
the members of the radiating lineages, it is impor-
tant for the ‘philosophical naturalist’ to appreciate
the nature of the species unit.8.1 The species concept and its place in phylogeny
No one definition has as yet satisfied all naturalists; yet
every naturalist knows vaguely what he means when he
speaks of a species.
(Darwin 1859, p. 101)... it will be seen that I look at the term species, as one
arbitrarily given for the sake of convenience to a set of
individuals closely resembling each other, and that it does
not essentially differ from the term variety, which is given
to less distinct and more fluctuating forms.
(Darwin 1859, p. 108)
If we are to study distributions of organisms, let
alone speciation, it is a prerequisite that we have a
currency, i.e. units that are comparable. Thus
the most fundamental units of biogeography are
the traditional taxonomic hierarchies by which the
plant and animal kingdoms are rendered down
to species level (and beyond). So, for example,195CHAPTER 8