distributional boundaries to be set by biological
factors rather than an inability to reach mainland
areas.
The labelling of particular species or traits as
‘relictual’ and the relative importance of palaeo- and
neoendemism on islands has proven controversial
(Chapter 2; Bramwell 1979; Cronk 1992; Elisens
1992; Carlquist 1995). One example discussed in
Chapter 7 is the question of whether woodiness
of island forms of normally herbaceous taxa
could be considered relictual (basal) or a derived
characteristic. The application of modern genetic
techniques argues convincingly for the latter (Kim
et al. 1996; Panero et al. 1999). In practice, as more
modern genetically derived phylogenies become
available it seems likely that the dichotomy between
palaeo- and neoendemic forms will become
increasingly irrelevant. Instead, we will be able to
compare the complex, and often individualistic,
patterns of lineage development across and between
islands, with recent work even pointing to cases of
back-colonization of island forms to mainlands (e.g.
Carineet al. 2004; Nicholson et al. 2005).
8.3 Mechanisms of speciation
When examining the mechanisms of speciation
rather than focusing on the geographical circum-
stances, it becomes clear that there are different
processes at work. Rosenzweig (1995) summarizes
these under the three headings: geographicalor
allopatric speciation,competitive speciation, and
polyploidy(involving an increase in the chromo-
some number). Each form holds relevance to the
island context, but as insularity defines the island
condition, the allopatric form of speciation will be
dealt with first.Allopatric or geographical speciation
The essence of this model has been summarized by
Rosenzweig (1995, p. 87):
●A geographical barrier restricts gene flow within
a sexually reproducing population.
●The isolated subpopulations evolve separately
for a time.202 SPECIATION AND THE ISLAND CONDITION
St. HelenaTrochetiopsisDombeyaTrochetiaParadombeyaPentapetesMelhaniaHarmsiaCheirolaenaAstiria/RuiziaDendroleandria/ParamelhaniaFigure 8.1Distribution of genera in the subtribe Dombeyeae (Sterculiaceae). The closest relatives of the Trochetiopsisof St Helena are Trochetia
(Mauritius) and Dombeya. The centre of generic diversity is in Madagascar and the Indian Ocean islands. According to Cronk, present-day ocean
currents (arrows) differ from those predominant at the time the ancestral Dombeyeae stock colonized St Helena during the late Tertiary, when
there was a stronger flow from the Indian Ocean to the South Atlantic. (Redrawn from Cronk 1990, Fig. 3, with permission of the New
Phytologist Trust.)