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Letters Response
Response to Kohler et al.: Impossible arguments about possible species?
Jeremy E. Niven
1,21 Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK
2 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 of Homo 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 species become enlarged relative to their mainland counterparts when they are isolated on islands [4,5]. To this original 'rule' 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, primates do conform to this original 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 and the 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 of H.
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 will depend 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 early to 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.
DOI of original article: 10.1016/j.tree.2007.10.002.
Corresponding author: Niven, J. E. ([email protected], [email protected]).
Available online xxxxxx
Acknowledgements
I thank Bill Eberhard and Mary Jane West-Eberhard for helpful comments.
www.sciencedirect.com 0169-5347/$- see front matter. Published by Elsevier Ltd. doi:10.1016/j.tree.2007.10.004
Please cite this article in press as: Niven, J.E., Response to Kohler et at.: Impossible arguments about possible species?, Trends Ecol. Evol. (2007), doi:10.1016/j.tree.2007.10.004
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