allozyme loci of 0.186, which is more than twice as
high as the mean reported for Pacific island
endemics (0.064) by DeJoode and Wendel (1992).
They suggest that this discrepancy might be
explained through two non-exclusive arguments.
The first argument is that the greater age of the
Canarian archipelago (e.g. 20 million years for
Fuerteventura; Anguita et al. 2002), means that
there has been a longer period of time for genetic
variation to accrue via mutation. However, phylo-
genetic studies of several endemic groups do not
support great ages for the majority of endemic line-
ages, hardly exceeding 4–5 million years (Marrero
and Francisco-Ortega 2001a,b). Moreover, there is
also evidence of a considerable degree of interfertil-
ity among several endemic species belonging to
radiating genera, and in cases even for interfertility
between members of distinct monophyletic genera,
which supports the supposition that they represent
relatively recent derivatives from common ances-
tors (Francisco-Ortega and Santos-Guerra 2001).
Second, the close proximity of the islands to Africa,
96 km today but only 60 km during glacial period
sea-level minima (García-Talavera 1999), would
have facilitated multiple introductions into the
Canaries and thus genetic bottlenecks may have
been less extreme than for some of the more remote
Pacific islands. Although a monophyletic origin has
been demonstrated for a large majority of Canarian
endemics, this does not mean that there could not
been several dispersal events prior to the radiation
processes (Francisco-Ortega et al. 2000). These ideas
are interesting but speculative: further work is
clearly needed to establish how genetic diversity
levels vary between and among archipelagos, and
then to test what factors may be responsible.
It may be relevant to this discussion that the
bulk of the genetic diversity displayed by the
Canarian endemic plants appears to reflect inter-
population, rather than intrapopulation variation.
This is consistent with (1) repeated founder
events within the archipelago, and even within
islands, (2) the apparent existence of dispersal
limitations of pollen and seed among popula-
tions, and (3) the prevalence of autogamic (self-
fertilization) reproduction in many of these
species. Moreover, when a species has been found
to hold a larger component of intrapopulation
than interpopulation genetic variation, as was
found for the Gomeran endemic shrub Echium
acanthocarpum, the population disjunction has
been interpreted in historical terms as constitut-
ing just the surviving remnants of a formerly
continuous island population (Sosa 2001). If
intraarchipelago events are crucial to explaining
patterns of genetic variation within lineages, com-
parisons between archipelagos or different ocean
basins may in some respects be confounded.
Finally, in this discussion we have assumed that
bottlenecks are essentially stochastic, but conceptu-
ally, at least, we might recognize that they could
sometimes be regarded as deterministic. Whereas
stochastic bottlenecks are survived by a random
subset of the population, deterministic bottlenecks
are survived by a specific genetically ‘pre-adapted’
subset. A deterministic bottleneck will give rise to a
greater subsequent degree of resistance to a new
episode of the same disturbance, whilst a stochastic
one won’t. We can illustrate this idea with two
hypothetical examples. First, in the event of the
introduction of the myxoma virus to an insular rab-
bit population, a few individuals may survive to
perpetuate the population, and these will tend to be
those least affected by the disease: subsequently
genetic resistance will spread through the popula-
tion. Second, imagine that a volcanic eruption pro-
duces a lava flow that destroys entirely a plant
community, except for those plants that happen to
have germinated on small hillocks that protrude
over the level of the lava. The survivors will domi-
nate the seed rain in the next generation, but have
not been selected for their resistance to being cov-
ered with lava.
In conclusion, what have we established in this
section? First, the restriction of populations to an
extremely small size, either on colonizing an island
(the founding event), or subsequently in the his-
tory of a lineage (other bottlenecks) may, in theory,
produce a variety of effects:●loss of genetic diversity (heterozygosity)
●addition of novel genetic combinations
●the introgression of genotypic variation from
related species (or varieties).FOUNDER EFFECTS, GENETIC DRIFT, AND BOTTLENECKS 171