8.5 Summary
In order to understand evolutionary change it is
necessary to have a basic familiarity with the cur-
rency of evolution and knowledge of the problems
associated with it. This chapter therefore begins by
discussing some of the problems of the species
concept, the arguably arbitrary distinction between
species and varieties and the higher-order phyloge-
nies of relationships that can be constructed for -
particular lineages. Recent developments in
phylogeny, including the use of cladistic and
molecular biological techniques, have provided
many new insights into long-standing island
biogeographical issues, although such techniques
should be viewed with proper caution as merely
constructing realistic hypotheses of the unknown
real genealogies.
A distinction is drawn between distributional,
locational (and historical), mechanistic, and phylo-
genetic frameworks for describing island evolution.
It is shown that divergence can take place both
through isolation (allopatry) and in conditions of
distributional overlap (sympatry), and indeed that
these concepts can be applied on different scales:
between island and mainland populations, within
an archipelago, and within an island. Evolutionary
change within a lineage may be accomplished by a
variety of genetic mechanisms, including hybridi-
zation and polyploidy, and by more subtle alter-
ations following from founding events, genetic
drift, and natural and sexual selection in the novel
biotic conditions of oceanic islands. Sympatric
speciation may occur through more than one route.
Perhaps the classic island examples are those that
follow the competitive speciation model of niche
expansion and break-up in to daughter populations
exploiting different resource peaks. Polyploidy
does not seem to have been a major route of speci-
ation on islands, although it undoubtedly occurs.
The final framework considered in the chapter is
that of phylogenetic trees, which can be used to
denote whether lineages develop without branch-
ing (anagenesis), with branching and extinction of
the original form (cladogenesis), or by branching
and survival of the original line (anacladogenesis).SUMMARY 2075%71% 24%ANAGENESIS
(progenitor
becomes
extinct)CLADOGENESIS
(progenitor is
partitioned into
two lines and
becomes extinct)ANACLADOGENESIS
(progenitor survives
with little change)Figure 8.2General patterns of
phylogeny in the endemic vascular
flora of the Juan Fernández
islands. The figures give the
percentage of each form of
lineage development evident in
123 endemic plant species. (From
data in Stuessy et al. 1990.)