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Letters Response

Response to Kohler et al.: Impossible arguments aboutpossible species?

Jeremy E. Niven1,2

1 Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK2 Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa, Ancon, Panama, Republica de Panama

I welcome the response of Kohler et al. [1] to my recent

article [2], in which I discussed the controversy surround-

ing the interpretation ofHomo floresiensis, a fossil hominin

from the island of Flores [3]. Kohler and colleagues present

alternative arguments and interpretations of the evidence.

However, these arguments rely on assumptions that under

close scrutiny do not justify revising the status of H.

floresiensis.

The island rule describes the tendency of larger-bodied

species to become dwarfed whereas small-bodied speciesbecome enlarged relative to their mainland counterparts

when they are isolated on islands [4,5]. To this originalrule

Kohler et al. add changes in additional factors, including

energy metabolism, brain size and sense organs [1]. Several

of the factors (e.g. reduction of expensive locomotor beha-

viours, enhanced fat storage, increased lifespan) that they

incorporate into the island rule are based on observations

from a few species and, therefore, do not constitute a

demonstration of a general trend among all mammals on

islands. They state that because H. floresiensis does not

conform to their new set of island rules, it cannot be a valid

species. I have several objections to this argument whether

applied specifically to H. floresiensis or more generally.The original island rule is supported by empirical evi-

dence [4,5] but it remains a correlation in which there is

considerable variability among species and numerous

exceptions both at the level of single species and entire

mammalian orders. The declaration of Kohler et al. that

Island rules cannot be broken is simply not justified. Inter-

estingly, primatesdo conformto thisoriginal island rule and

the amount of reduction of body mass in H. floresiensis(assuming either H. sapiens or H. erectus as the ancestral

species) is consistent with the reduction in body mass

observed in other primates isolated on islands [6].

The island rule pertains to changes in body mass and

makes no specific predictions about which particular tis-sues should be affected [4,5]. Thus changes in body mass

could be achieved in many different ways, the amounts of

different expensive tissues such as brain, gut and kidney

being traded-off against one another [7]. Indeed, the pre-

cise phenotypic changes that occur after isolation on an

island would be expected to depend on the morphology,

physiology and behaviour of that species, the size and

geographical position of the island [8], and various

historical processes including founder effects andthe precise

order in which other species colonized the island [9]. All of

these factors will affect resource availability, predation risk

and competition on an island. The changes in traits result-

ing from these selection pressures will be expected to be

beneficial for the survival of an island mammal.

Kohler and colleagues mention two specific aspects ofH.

floresiensis morphology that they claim violate island rules

sense organ and brain size and limb morphology. Yet for

the reasons just discussed, we cannot predict specific

changes in brain volume and sense organs or in limb

morphology after isolation on an island, because this willdepend upon the specific selective pressures involved. This

is a particularly acute problem when considering brain

evolution, because we are only just beginning to under-

stand the relationships between energy consumption,

energy efficiency, neural processing and body mass [10].

Yet this is also a problem when considering limb

morphology, especially when no behavioural evidence for

the locomotory gait exists. Detailed comparative analysis

and modelling are essential before inferences can be made

about whether limb morphology could support particular

gaits. Indeed, a recent comparison of the wrist of H.

floresiensis with those of apes, humans and other fossils

suggests that it retains a primitive morphology [11].In short, Kohler and colleagues suggest that current

knowledge of mammalian evolution on islands is suffi-

ciently complete that we can exclude the existence of

species falling outside our expectations. Because we cannot

define the limits of evolutionary possibility, we cannot

consign a species to being impossible. Some possibilities

might seem remote (for discussion see Ref. [12]) but never-

theless lineages sometimes evolve remarkable innovations

under certain circumstances (for example, see Ref. [13]).

Kohler and colleagues deem H. floresiensis an impossible

species by assuming constraints on evolutionary possib-

ility for which they have little evidence.

Thus, as I stated in my original article, it seems too earlyto dismiss the claim that H. floresiensis is a new hominin

species. Indeed, recent fossil evidence suggests that there

might have been considerably more variability in hominin

body size than previously appreciated [14], emphasizing

the need to keep an open mind.

AcknowledgementsI thank Bill Eberhard and Mary Jane West-Eberhard for helpful

comments.

References1 Kohler, M.et al. (2008)Island rules cannot be broken. Trends Ecol. Evol.

23, 67DOI of original article: 10.1016/j.tree.2007.10.002.

Corresponding author: Niven, J.E. jen22@cam.ac.uk, nivenj@si.edu.

Update TRENDS in Ecology and Evolution Vol.23 No.1

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http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-mailto:nivenj@si.edumailto:nivenj@si.edumailto:nivenj@si.edumailto:nivenj@si.eduhttp://-/?-http://-/?-http://-/?-http://-/?-http://-/?-

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2 Niven, J.E. (2007) Brains, islands and evolution: breaking all the rules.

Trends Ecol. Evol. 22, 5759

3 Brown, P. et al. (2004) A new small-bodied hominin from the Late

Pleistocene of Flores, Indonesia. Nature 431, 10551061

4 Foster, J.B. (1964) Evolution of mammals on islands. Nature 202, 234

235

5 Damuth, J. (1993) Copes rule, the island rule and the scaling of

mammalian population density. Nature 365, 748750

6 Bromham, L. and Cardillo, M. (2007) Primates follow the island rule:

implications for interpreting Homo floresiensis. Biol. Lett. 3, 398400

7 Aiello, L.C. et al. (2001) In defense of the expensive tissue hypothesis.In Evolutionary Anatomy of the Primate Cerebral Cortex (Falk, D.

and Gibson, K.R., eds), pp. 5778, Cambridge University Press

8 Leigh, E.G. et al. (2007) The biogeography of large islands, or how does

the size of the ecological theatre affect the evolutionary play? Rev. Ecol.

(Terre Vie) 62, 105168

9 Fukami, T. et al. (2007) Immigration history controls diversification in

experimental adaptive radiation. Nature 446, 436439

10 Niven, J.E. et al. (2007) Fly photoreceptors demonstrate energy-

information trade-offs in neural coding. PLoS Biol. 5, 828840

11 Tocheri, M.W.et al. (2007) The primitive wrist ofHomo floresiensis and

its implications for hominin evolution. Science 317, 17431745

12 Authur, W. (2004) Biased Embryos and Evolution. Cambridge

University Press

13 Eberhard, W.G. (2001) Multiple origins of a major novelty: moveable

abdominal lobes in male sepsid flies (Diptera: epsidae), and the

question of developmental constraints. Evol. Dev. 3, 20622214 Spoor, F. et al. (2007) Implications of new early Homo fossils from

Ileret, east of Lake Turkana, Kenya. Nature 448, 688691

0169-5347/$ see front matter. Published by Elsevier Ltd.

doi:10.1016/j.tree.2007.10.004

Book Review

Biogeography emerging: provocative and integrativeperspectives in historical biogeographyBiogeography in a Changing World by Malte C. Ebach and Raymond S. Tangney, CRC Press, 2007. US$89.95, hbk (212 pages) ISBN

978 0 8493 8038 9

Mark V. Lomolino

Department of Environmental and Forest Biology, SUNY College of Environmental Science and Forestry, Syracuse, NY 13210, USA

Well, perhaps you can judge a book by

its cover, at least to some extent. Artist

Neal Adams was commissioned by the

editors of this collected volume of essays

to illustrate a still far from mainstream,

yet captivating, theory on the dynamics

of the earth the expanding earth

theory championed by a select group

of individuals (most notably S. WarrenCarey) from the 1950s to 1980s. The

theory remains an unaccepted, but perhaps unappreciated,

explanation for the dynamics of the continents and their

respective biotas, which drifted apart as a small, primor-

dial, continental earth expanded and ocean basins devel-

oped to fill in the gaps.

The cover illustration, thus, serves as a captivating

declaration that this collection of essays will be, if nothing

else, provocative and will challenge traditional views of

how regional biotas develop over time. The book is the

product of a symposium entitled What is Biogeography?,

which took place during the Fifth Biennial Meeting of the

Systematics Association in 2005. The stated goal of thesymposium was to present a broad-based perspective on

the nature of biogeography, offering historical perspectives

based on current understanding and methodological

advances, as well as what the future might hold. One

underlying theme for several essays in this volume is that

geographic variation among biotas is not only shaped by

geological dynamics but also that these biogeographical

patterns can inform and, at times, challenge our current

understanding in geology. Thus, although the volumes

title Biogeography in a Changing World might suggest

mistakenly to some a focus on climate change and recent,

anthropogenic modifications in landscapes and their de-

pendent biotas, it seems entirely appropriate within the

context of historical development of the earth and its

biotas.

In their introduction to this volume, the editors discuss

various definitions of biogeography and then briefly sum-

marize the history of the field, focusing on different

approaches for reconstructing the historical development

of regional biotas. As almost every student of historical

biogeography realizes quickly, the history of this field is

fraught with contentious debates among alternate schools,

which often degrade into contemptuous clashes among

their champions. Although controversy is of course part

and parcel of most, if not all, scientific crisis and revolu-

tions [1], a rapprochement among debating schools and a

reintegration of long divergent lines of study will be best

served if these debates are tempered and waged on scien-

tific and not personal grounds.

The first chapter, by David M. Williams, chronicles one

of these legendary clashes the debates between Ernst

Haeckel and Louis Agassiz and their students over theutility of the threefold parallelism (the synthesis of paleon-

tology, systematics and ontogeny) and the importance of

geographic variation in reconstructing genealogies. Lynne

Parentis chapter is the first in this volume to review and

critique the current state of the field and to discuss poten-

tial synthesis among its various camps, in particular,

cladistic and phylogenetic biogeography. Here, she pre-

sents a cogent and persuasive argument for achieving a

new synthesis, which Donn Rosen called for nearly three

decades ago: a revolution in the earth sciences an

integrated natural history of the geological and biological

systems. John Grehans chapter is a more specialized one,Corresponding author: Lomolino, M.V. (island@esf.edu).

Update TRENDS in Ecology and Evolution Vol.23 No.1

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http://dx.doi.org/10.1016/j.tree.2007.10.004http://0.0.0.0/mailto:island@esf.edumailto:island@esf.eduhttp://0.0.0.0/http://dx.doi.org/10.1016/j.tree.2007.10.004