Data from: Ancestral state reconstruction sheds new light on the loss of divarication hypothesis on New Zealand’s outlying islands

The New Zealand flora is remarkable in many respects, but one of its most
notable features is the convergent evolution of many plant lineages toward
a divaricate habit. The adaptive significance of divarication remains
controversial, but it has usually been considered a response to harsh
climates or a defence mechanism against the diverse array of Ratites
(large browsing birds) that once populated New Zealand. The loss of
divarication hypothesis posits that divaricate-related traits should be
lost on New Zealand’s outlying islands, as these islands were never
reached by Ratites. Like most evolutionary components of the island
syndrome, this hypothesis was tested through the pairwise comparison
method, i.e. by comparing traits of island endemics to those of their
closest relative(s) on the mainland, assuming that the most recent common
ancestor (MRCA) was divaricate. This assumption was challenged by recent
results but has never been tested. Here, we test this
assumption. We collated data for all but one genus comprising at
least one divaricate species and one island endemic in the New Zealand
region. We used ancestral range reconstruction to determine which endemics
colonized the islands from New Zealand. We then used ancestral state
reconstruction to test whether each MRCA was divaricate and to quantify
the degree of morphological evolution in leaf size and branching angle
(i.e. significant divaricate traits) on islands. 21 out of 29
MRCAs were estimated as most likely non-divaricate, meaning that in most
cases divarication evolved on the mainland and was never present on New
Zealand’s outlying islands. Island endemics also evolved larger leaves and
smaller branching angles regardless of whether the MRCA was divaricate or
not. Synthesis. These findings show how most island lineages were never
divaricate, and advance our understanding of the evolution of divarication
in the New Zealand region. In addition, they have broader implications for
the field of island biogeography, as they highlight the limits of the
pairwise comparison method and provide a way to overcome them.