8.2 The geographical context of speciation events
Distributional context
In order to appreciate the full gamut of possibilities
involved in speciation it is helpful, if slightly
artificial, to distinguish the geography of specia-
tion from the mechanism (Table 8.2; Box 8.2). The
geographical context can be viewed either in
distributional or locational terms, i.e. distinguish-
ing the degree of overlap of populations involved
in speciation events on the one hand, from the
issue of where the evolutionary change is taking
place.
The terms sympatry and allopatry denote,
respectively, two populations (or species) which
overlap in their distributions and two populations
(or species) which have geographically separate
distributions. In geographical terms, a new species
(or subspecies) may, in theory, arise in one of three
circumstances. If the new form arises within the
same geographical area as the original, then it may
be termed sympatric speciation, if it arises in a
zone of contact (hybrid zone) between two speciesTHE GEOGRAPHICAL CONTEXT OF SPECIATION EVENTS 199Box 8.1 Molecular clocks
Dating events in evolutionary biogeography
presents considerable challenges because of the
extended time periods involved, extending way
beyond many conventional dating techniques
such as radiocarbon isotopes. Traditionally, fossils
are dated by means of the geological record and
their place in the stratigraphic column, but this
can be a very imprecise approach and doesn’t
provide a means of dating colonization and
speciation events in extant taxa. Molecular
clocksprovide a solution to this problem based
on the idea that proteins and DNA evolve at a
more or less constant rate. The extent of
molecular divergence is then used as a metric for
the timing of events within the development of
the lineage.
Geneticists have developed an impressive array
of rather complicated analytical procedures to
ensure that such molecular clocks are reasonable.
Nonetheless, they are inevitably based on limited
amounts of information and involve a number of
important assumptions. They should be regarded
as providing plausible hypotheses for the
developmental sequence and timing of particular
monophyletic lineages. Greater confidence can be
placed in such molecular clocks if they can be
independently calibrated.
In illustration, Carranza et al. (2006) have
made use of a molecular clock in their study ofthe phylogeography of the lacertid lizard
Psammodromos algirusin the Iberian peninsular
and across the Strait of Gibraltar in North Africa.
They sampled genomic DNA for three genes
from 101 specimens of the subfamily
Gallotiinae, mostly of the target species, but
including specimens of Gallotiaspecies from the
Canaries. These specimens provide a means of
‘rooting’ the phylogenetic tree, and also a
means of calibrating it. This is because it is
reasonable to assume that G. caesaris caesaris,
endemic to the island of El Hierro, commenced
divergence from its nearest relative, G. c.
gomerae, endemic to the island of La Gomera,
shortly after the formation of El Hierro
(approximately 1 Ma). The clock used by
Carranzaet al. indicated that diversification in
the genus Gallotiabegan on the Canaries about
13 Ma, with a rather earlier data of 25 Ma for
the first speciation events within the
Iberian/North African genus Psammodromus.
This case study is illustrative of a growing
number of studies that place the development of
a monophyletic island lineage into context
alongside related mainland lineages. This body of
work has produced some interesting surprises,
for instance showing evidence of successful
back-colonization of island forms into mainland
regions (e.g. Nicholson et al. 2005).