Sam Rosenbaum
Biogeography Literary Review #1
1/24/11
UFID: 7921-7401
Shai Meiri, Pasquale Raia, and Albert B. Phillimore. 2011. Slaying dragons: limited
evidence for unusual body size evolution on islands. Journal of Biogeography (J.
Biogeogr.) (2011) 38, 89–100
Meiri is with Tel-Aviv University, Tel-Aviv, Israel; Raia is with Universita` Federico II,
Naples, Italy; and Phillimore is from Imperial College, Ascot, UK.
The research problem or question:
How does statistical size distribution in insular vertebrates support either of
two hypotheses: 1) that islands promote evolution of extreme (very small or very
large) body size, and 2) that islands promote the evolution of medium body size.
General background knowledge leading up to the study:
Giant tortoises, huge flightless birds, dwarf elephants, and even pygmy
humans appear on inlands around the world. Islands have some of the most extreme
sized animals on the world. It is suggested that we may just happen to notice these
extreme sizes since they are already isolated for us on islands. But, there is a
supposed Island Rule hypothesis, which states that, “rather than showing size
extremes on islands, insular populations should be closer to the clade-wide median
body size than their mainland counter- parts.”
Methods used by investigators:
For size extremes, body mass and size data for species that had at least one
insular member and one continental member were taken from various sources.
They then assessed whether or not the data varied from the extreme size
expectation if insular status was randomly assigned to n individuals per taxon,
where n is the actual the number of insular species in that taxon. From that they
calculated the number of taxa for which the largest or smallest representative had
been randomly assigned insular status.
For mean size, they calculated the mean log10 body size of insular and
mainland species within each genus. Then they examined the slope of island versus
mainland taxon means along a normal regression line.
Results:
Evidence shows that only lizard families and possibly bird genera have a
favored evolution of extreme sizes in insular conditions and this is not shown at
other taxonomic levels. This also does not apply to mammals. The island rule applies
statistically to mammalian species within genera, though biologically there is little
evidence for large bodied small mammals, leaving us with only half the island rule.
Weaknesses of the Study:
There are many factors that go into the evolutionary size of insular animals
such as predation, competition, and size of the island. This study was done from a
statistical point of view with little physical habitat data.
What I learned from the study:
I learned that there is no simple way to explain the cases of extreme sizes in
insular animals. Through statistics, this study helped to it at various taxonomic
levels by looking at lizards, birds, and mammals. I never knew before reading this
study that there was a significant size difference in insular lizards compared to
continental lizards, and the same thing with some birds. I found it very interesting
that there is a prevalence in dwarfism in large insular mammals, but no evidence of
gigantism in small insular mammals.
Very good summary. Thanks.