Taxacom: Ancestral-area algorithms are unreliable

[John Grehan]

This is the title of a new paper that any practitioner of ancestral areas
analysis should be informed about. This article provides a multitude of
examples of why ancestral areas analysis really just does not work, at
least when it comes to the origin of paraphyletic grades within an area, or
the star pattern of distribution (a pattern that is not all that rare, but
invisible to most biogeographers, and especially those working with
ancestral areas analysis). To my way of thinking, ancestral areas analysis
is just an analysis of a made up world. The world that is not made up is
the world of actual distributions. Anyway, you be the judge of your own
preferences, keeping in mind that no ancestral area proponent has yet been
able to refute this kind of challenge that has been floating around in the
literature for a couple of decades now.

Heads, M, Nicol, D. & Saldivia, P. 2025. Ancestral-area algorithms are
unreliable. Biological Reviews. doi: 10.1111/brv.70092
ABSTRACT
We examine two standard phylogenetic–biogeographic patterns and how these
are interpreted by current ancestral-area
algorithms. In the first pattern, a basal, paraphyletic grade is restricted
to one part of its clade’s overall range. In the second
pattern, the clades of a group overlap in one central area but are
allopatric elsewhere, resulting in a ‘star pattern’.
Ancestral-area algorithms will calculate a localised centre of origin for
both patterns in the area of overlap. Yet both patterns
can also be derived by vicariance (causing the allopatry) and subsequent
range expansion by normal dispersal (causing
the overlap). In this model, ancestors of allopatric clades were already
widespread and polymorphic before the
modern clades began to diverge. The overlap region is not a centre of
origin, it is a region of secondary range expansion.
There is no reason to assume that basal grades occupy ancestral areas or
habitats. There is also no need to use a priori
areas in biogeographic analysis. Instead, the distributional relationships
among the clade localities can be examined
directly. Distribution has emerged as a critical factor in molecular
systematics, as many clades are defined more easily
by their geographic distribution than by traditional morphological
characters. Thus, it makes sense to review the precise
geometry of the distributions in a study group, including details of
allopatry, disjunction, overlap and others. A distinction
is drawn here between secondary clade overlap that has developed following
the origin of the clades, and primary clade
overlap (true sympatry) that developed with the origin of the clades. In
current practice, distribution maps have been
replaced in biogeographic analysis by outputs from ancestral-area analyses.
But if the main clades recovered in a study
are mapped, simple, distinctive features, such as allopatry, marginal
overlap and disjunction often become evident, and
we encourage authors to map their clades. The patterns discussed here, for
example, the star pattern centred on
New Zealand, are repeated in unrelated groups. Thus, the regions
interpreted in ancestral-area analysis as centres of origin
can be re-interpreted as phylogenetic-biogeographic breaks (nodes) in
widespread ancestors. The method proposed
here – ‘allopatry indicates vicariance; overlap indicates dispersal’ – is
simpler to apply than the current algorithms, and
the results are much simpler, with a single cause explaining many
distribution patterns, rather than each component
clade of a biogeographic pattern having a separate history.

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