[Taxacom] critique sweepstake colonization by mammals

John Grehan calabar.john at gmail.com
Sun Mar 31 10:53:08 CDT 2019

As I and others have periodically noted, the appeal to chance dispersal
renders a rather improbable and non-empirical view of evolution. Below is
the abstract of a recent analytical critique showing how the chance model
for mammal colonization of islands just does not add up - literally in
their quantitative assessment. Naturally there may be counter analyses, but
it is not sufficient to just say one believes otherwise regardless of the
analysis. Science is not about belief, its about the nature of evidence.

John Grehan

Mazza, P.A., Buccianti, A. & A. Savorelli. 2019. Grasping at straws: a
re-evaluation of sweepstakes colonization of islands by mammals. Biological
REviews doi: 10.1111/brv.12506
Natural rafting is an easy, non-evidence-based solution often used to
explain the presence of a variety of species on isolated islands. The
question arises as to whether this solution is based on solid scientific
grounds. It is a plausible
colonisation route only if intricate networks of variables are considered
and many different conditions satisfied. Thisreview provides a descriptive
account of some of the most critical issues underlying the theory of
natural rafting that
should be addressed by its supporters. These include: (i) biological
variables; (ii) characteristics of the vessels; and (iii) physical
variables. Natural rafting may explain the dispersal of poikilotherms with
low metabolic rates and low resource requirements that could withstand
trans-oceanic crossings, but explaining the transport of homeothermic
terrestrial mammals to oceanic islands is more problematic. Drifting at sea
exposes organisms to high concentrations of salt, high temperature and
humidity excursions, starvation, and above all to dehydration. A
sufficiently large group of healthy
reproductive individuals of the two sexes should either be transported
together, or be able to reassemble after separate crossings, to prevent
inbreeding, genetic drift and ultimately extinction. Any vessels of flotsam
occupied must minimally provide the animals they transport with sufficient
provisions to survive the journey, offer minimum friction and drag
through water, and be transported by appropriately directed, sustained,
high-speed currents. Thus, a ‘sweepstakes colonisation’ event would be the
result of a lucky combination of all, or at least the majority, of these
factors. Some
cases throw doubt on the use of a natural rafting model to explain known
animal colonisations, with one of the most striking examples being
Madagascar. This island is far from the nearest mainland coasts and the sea
currents in the Mozambique Channel are directed towards Africa rather than
Madagascar, yet, the island was colonised by terrestrial mammals (e.g.
extinct hippopotamuses, lemurs, carnivores, rodents and tenrecs) unable to
swim and to survive long journeys at sea. In order to assess the
feasibility of the natural rafting model in a case such as Madagascar,
tests were
performed using three variables for which enough information could be
obtained from the literature: length of survival without food, survival
without water, and sea current speed. The distributions of these variables
appear to be log-normal and multiplicative, or follow a power-law, rather
than being Gaussian. The tests suggest that a distributional analysis
is a more suitable approach than the use of geometric probability to
calculate the probabilities associated with the examined data. Such
non-linear and self-organising systems may reach a critical point governed
by different competing
factors. Mammals with high survival requirements, such as lemurs and
hippopotamuses, thus may have a virtually zero probability of reaching
distant islands by natural rafting. Our results raise doubts as to the
validity of a natural rafting model, and we urge a rethinking of the modes
in which numerous islands were colonised by land mammals and a careful
revision of past geological and phylogeographic work.

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