Adler and Levins’ (1994) model is based on studies
of rodents, but there is no particular reason why
it could not be extended to and tested on other
systems.
7.7 Summary
This chapter has been concerned with what we
might think of as micro-evolutionary change; the
often small, incremental alterations in genotype,
and in niche, which provide the building blocks of
the more spectacular emergent patterns. We have
considered essentially random processes associated
with founding events and genetic drift, and we
have discussed at greater length an array of island
species traits or syndromes that appear to point to
general selective forces operating on islands. We
have also set out some of the many hypotheses put
forward in explanation of these changes.
The founder principle is that small bridgehead
populations bring with them a biased subset of the
genetic variation of the source population. Such
bottlenecks appear to be important in the rapid
evolutionary divergence of mainland and island
lineages, although the scale and significance of
founder effects remains a contested issue. Analyses
of reproductive behaviour and the sexual systems
of colonists provide further insights into evolution-
ary change on islands. Hybridization appears to
have been more important in island evolution than
once realized.
Island forms have undergone a wide variety of
forms of niche alteration, including shifts and
expansions in feeding niche, alterations in nesting
sites in birds, loss of defensive attributes, and loss
of dispersal powers. One of the most striking pat-
terns is the so-called island rule, of size increase in
small vertebrates, and size reduction in large verte-
brates. Each of these trends requires careful evalua-
tion before a significant island effect is accepted. In
some cases niche shifts can be due to character dis-
placement caused by competitive interactions, or to
ecological release in the absence of the usual array
of competing species. Particular syndromes of traits
can be identified which can be understood in rela-
tion to the geographical context of the island sys-
tems under examination. For example, island
pollination networks seem to be characterized by
the emergence of endemic super-generalists, which
can be linked to the disharmonic nature of remote
island ecosystems.
Although much of the discussion has been incon-
clusive, raising as many questions as answers, we
hope that we have set out some of the more impor-
tant evolutionary–ecological mechanisms, the tool
kit from which island evolution has fashioned the
peculiar features of island endemics. In the final
section, we have begun to see how these mecha-
nisms can be put together into more complex theo-
ries that might explain emergent patterns of island
faunas and floras, including the most spectacular
radiations of species and genera from single ances-
tral colonists. To understand these in more detail
we have now to consider the process of speciation,
what it entails, and how we can organize ideas
about island speciation into alternative explanatory
and descriptive frameworks. This we do in the next
chapter.194 ARRIVAL AND CHANGE