200 SPECIATION AND THE ISLAND CONDITION
Table 8.2A simplified framework for speciation patterning
Form Pattern
Distributional
Sympatric Overlapping
Allopatric Separate
Parapatric Contiguous
Locational (and historical)
Neo-endemic Change on island
Palaeo-endemic Island form relict
Mechanistic
Allopatric Drift and selection
Polyploidy Changes in chromosome number
Competitive speciation Other sympatric mechanisms
Tree form (phylogeny)
Anagenesis Replacement of original
Anacladogenesis Alongside original
Cladogenesis Lines diverge and replace original
Box 8.2 On archipelago speciation, allopatry and sympatry—a clarification
We provide this simple summary as a quick look-
up and clarification of the ideas involved.
●Allopatric speciationis speciation occurring
between two (or more) populations that are geographically
isolated from each other. All island neo-endemics exemplify
allopatry in the sense that they are isolated from the main-
land source pool. However, within an archipelago, popula-
tions may also be allopatric at the inter-island scale, or
even within islands, for example, isolated within forest
islands surrounded by lava (kipukas, Hawaii).
●Sympatric speciationis a term applied to cases
where speciation occurs without geographical separation
of the populations involved, i.e. the opposite of allopatric
speciation. In practice, the past roles of allopatry and sym-
patry can be difficult to distinguish at the intra-island
scale, or even at the intra-archipelago scale where the
islands are close to one another and the organisms
involved have high dispersal abilities.
●Parapatric speciationrefers to situations interme-
diate between the extreme cases of allopatry and sympa-
try, where the geographical segregation is incomplete, and
two separating forms are contiguous in space with a
hybrid zone in the area of contact/overlap.●Archipelago speciationis a term sometimes used
where speciation of island lineages has occurred mostly
between islands rather than within them. This tends to imply
an allopatric model, especially where inter-island distances
are considerable, but may in fact involve both allopatric and
sympatric phases. The taxon cycle model, as applied to the
West Indian avifauna, is also an example of archipelago spe-
ciation. Sometimes, environmental differences across an
archipelago such as the Canaries are great enough that
adaptive differencesare evident, but sometimes very
little niche segregation is evident, in which case the archipel-
ago speciation might be considered non-adaptive.
●Are these terms operational?Given the huge
environmental changes involved in the life cycle of isolated
volcanic islands, and archipelagos, it can be very difficult to
be sure that current distributions indicate the circum-
stances in which speciation occurred. Indeed, taking the
Galápagos finches as a classic case study of island evolu-
tion, it appears likely that this adaptive radiation has
involved allopatric, sympatric and parapatric phases
(Chapter 9). Similarly, the distinction between adaptive and
non-adaptive changes may not always be clear cut: evolu-
tionary change commonly involves a mix of stochastic and
directional mechanisms.it may be termed parapatric speciation, and if it
arises in a separate geographical area it may
be termed allopatric speciation. In reality the dis-
tinction between these conditions may be blurred
as: (1) the spatial scale we choose to describe the
geography of the populations may not be concor-
dant with the scale at which the members of the
populations interact; and (2) we may be wholly
ignorant of the past history of the distributional
overlap of the populations such that, for instance, a
currently sympatric species pair may well have
speciated while geographically isolated from one
another.
As evident from the debate concerning vicari-
ance vs dispersalist explanations in biogeography
(Chapter 3), many authors have assumed that
allopatric speciation is by far the dominant mode
of speciation. The demonstrable occurrence of
sympatric forms of speciation exposes the weakness