[Taxacom] Plant Dispersal across the Atlantic (was: Biogeography of Meliaceae)

[Kenneth Kinman]

Hi All,
       To put this into a much broader context, I would recommend reading a more comprehensive study (by Renner, 2004; "PLANT DISPERSAL ACROSS THE TROPICAL ATLANTIC BY WIND AND SEA CURRENTS").  It includes up to 110 genera of plants which are thought to exhibit Trans-Atlantic dispersals (during the Cenozoic) by water currents or wind currents (rather than by older vicariance events).   A weblink to Renner's paper is given below.
                          -----------------------Ken Kinman

http://citeseerx.ist.psu.edu/viewdoc/download?doi.1.1.516.6358&rep=rep1&type=pdf



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From: Taxacom <taxacom-bounces at mailman.nhm.ku.edu> on behalf of John Grehan via Taxacom <taxacom at mailman.nhm.ku.edu>
Sent: Tuesday, October 15, 2019 10:50 AM
To: taxacom <taxacom at mailman.nhm.ku.edu>
Subject: [Taxacom] Biogeography of Meliaceae

Heads, M 2019. Biogeography and ecology in a pantropical family, the
Meliaceae. Gardens’ Bulletin Singapore 71(Suppl. 2):335-461.

A copy of this paper can be obtained via the author. It provides a
comprehensive approach to the biogeography of a widespread taxon. Also
included is a critique of fossil based molecular dating (excerpts below).
Any counter points posted to Taxacom would be of interest, but please read
the paper first. Note Bromham's (2019) characterization of molecular
dating. It seems that more researchers are starting to catch on to the
absurdity of magically transmogrifying minimums into maximums.

John Grehan

ABSTRACT. This paper reviews the biogeography and ecology of the family
Meliaceae and
maps many of the clades. Recently published molecular phylogenies are used
as a framework
to interpret distributional and ecological data. The sections on
distribution concentrate
on allopatry, on areas of overlap among clades, and on centres of
diversity. The sections
on ecology focus on populations of the family that are not in typical,
dry-ground, lowland
rain forest, for example, in and around mangrove forest, in peat swamp and
other kinds of
freshwater swamp forest, on limestone, and in open vegetation such as
savanna woodland.
Information on the altitudinal range of the genera is presented, and brief
notes on architecture
are also given. The paper considers the relationship between the
distribution and ecology of
the taxa, and the interpretation of the fossil record of the family, along
with its significance for
biogeographic studies. Finally, the paper discusses whether the evolution
of Meliaceae can be
attributed to ‘radiations’ from restricted centres of origin into new
morphological, geographical
and ecological space, or whether it is better explained by phases of
vicariance in widespread
ancestors, alternating with phases of range expansion.

Excerpts

Bromham (2019) suggested that molecular clock dating “requires us to
believe six impossible things before breakfast”. Many of the assumptions
“are based
on statistical tractability rather than being informed by improved
understanding of
molecular evolution, yet changing the assumptions can influence molecular
dates.
… If our assumptions are wrong, then our inference could be misleading,
however
much data we have…. All molecular dating analyses require us to make
statements
of belief, irrespective of whether we call them models, assumptions, or
priors. We
need to recognise that while some disagreements between molecular dates are
due
to differences in data or method, many are due to different beliefs
codified in the
analyses…”.
To summarise, a fossil gives a minimum age for its clade, and we do not know
how much older than this date the actual clade age is. Biogeographic
calibrations
suggest that many clades are much older than their oldest fossil (Heads,
2014).


Koenen et al. (2015) ran three analyses with different maximum age priors,
as
follows: 1. Using a fossil-calibrated crown age for Sapindales (c. 71 Ma)
as a hard
maximum bound; 2. Using a fossil-calibrated stem age of Sapindales (98 Ma)
as a
hard maximum bound; 3. Using the age of the first tricolpate pollen (127
Ma) as a hard
maximum bound for the age of eudicots.

The authors regarded the third method as most likely. Nevertheless, the
authors
found that in scenarios 2 and 3, the crown age of the Sapindales becomes
‘unrealistically
old’, especially when the tricolpate pollen age is used. (Using tricolpate
pollen, the
authors calculated that Sapindales originated at 120 Ma and Meliaceae at 80
Ma, but
biogeography indicates that this last date is still much too young).
By stipulating such narrow priors and thus calculating such young clade
ages,
the authors were able to support traditional chance dispersal over
vicariance: “…
Old World/New World disjunctions are indeed substantially too young
(Oligocene
or younger) for vicariant explanations… [Thus] Our results indicate
[Oligocene or
younger] trans-Atlantic dispersal from Africa to the New World…” (Koenen et
al.,
2015). Again, this simply reflects the very narrow priors that were
imposed. The time
and place of origin of Meliaceae can be regarded as open questions.

Twenty years ago, most authors argued that species had their origin in the
Pleistocene (since 2.6 Ma). The argument that species could ever be
Cretaceous
(before 65 Ma), made on the basis of treating fossils as minimum clade ages
and on
biogeography (Craw et al., 1999), was controversial. Now, most authors
accept that
species may be as old as Miocene (5–20 Ma). However, recent molecular
studies have
proposed that species of plants, such as Gunnera herteri Osten (Bacon et
al., 2018),
and animals, such as petalurid dragonfly species and the frog Leiopelma
hochstetteri
(Ware et al., 2014; Carr et al., 2015), date back to the Cretaceous (65 Ma).
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