The Demonstration of Speciation in Fossil Molluscs and Living Fishes
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Biological Journal of the Linnean Society (1985), 26: 325-336. With 3 figures The demonstration of speciation in fossil molluscs and living fishes G. FR Y E R , Freshwater Biological Association, Windermere Laboratory, The Ferry House, Ambleside, Cumbria P. H. GREENWOOD AND J. F. PEAKE British Museum ( Natural History), Cromwell Road, London Accepted for publication July 1985 Contrary to a recent assertion, freshwater (and marine) prosobranch gastropods and freshwater bivalves are subject to considerable variability. This, and the lack of a detailed understanding of the taxonomy of the forms involved, makes it difficult to accept that the changes documented by Williamson (1981) in a fossil sequence from Lake Turkana (Africa) represent speciation events. That 10 lineages, involving gastropods and bivalves, should change simultaneously, and the deviant forms should then simultaneously become extinct, can, we believe, be more plausibly attributed to ecophenotypic responses to environmental changes than to speciation. In revealing the pattern and process of evolution, both fossil and living forms are helpful, but in demonstrating the fine-scale events during and after speciation in living animals one can utilize techniques and observations that cannot be applied to fossil material. African cichlid fishes are particularly informative in this respect. Their current explosive radiation can be interpreted as a punctuational event in evolution. KEY WORDS: —Speciation - ecophenotypic changes - fossil molluscs - fishes - Lake Turkana. CONTENTS I n tr o d u c tio n ..................................................................................................................................................325 T he variability o f freshwater prosobranch g a s t r o p o d s ................................................................. 326 Speciation or variatio n ..................................................................................................................................331 Living fishes, fossil molluscs and the dem onstration o f sp eciatio n .................................................333 C o n c lu s io n s ..................................................................................................................................................335 References..........................................................................................................................................................335 INTRODUCTION While we admire the tenacity of Williamson in defending his interpretation of the assemblage of Caenozoic fossil molluscs at Lake Turkana against several critics, we are still unable to accept some of his conclusions, and find his defence against our criticisms (Fryer, Greenwood & Peake, 1983) unconvincing. Furthermore, by misinterpreting much of what we wrote he has brought up various points that are irrelevant to the general discussion. 325 0024-4066/85/120325 +12 $03.00/0 © 1985 The Linnean Society of London 326 G. FRYER E T AL. Williamson treats our criticisms under three headings. First he seeks to maintain the fiction that phenotypic variation in prosobranchs is small. He says nothing about bivalves, though in fact six of the 10 lineages on which the conclusions of his original paper (Williamson, 1981) are based are bivalves. He thereby circumvents the need to refute the well-established ecophenotypic variability of this group, to which we drew attention, as did Kat & Davis (1983). Second, he defends his claim that the forms present at the Suregei level are new species and not simply ecophenotypes, and finally he attempts to refute the suggestion that simultaneous changes in 10 lineages may indicate phenotypic changes rather than speciation. His disagreement about the limitations of fossil evidence and the relative value of fossil molluscs and living cichlid fishes as indicators of speciation, and his discussion of the punctuated equilibrium model of evolution are separate issues and are dealt with towards the end of this paper. In our original critique of Williamson’s interpretation of the Turkana fauna we outlined four lines of argument which seem crucial to a discussion of this interesting problem. (1) The considerable variability in shell form exhibited by freshwater prosobranch gastropods and by freshwater bivalves. (2) The problem of establishing a stable and unequivocal classification for such groups of organisms which has any biological meaning other than describing shell morphologies. Here, a major limitation is the absence of any taxonomic account establishing a baseline for Williamson’s study. One is available in his thesis (Williamson, 1980) but this differs from that employed in his publication in Nature (Williamson, 1981). (3) The availability of an alternative hypothesis, namely that the variation exhibited by the Turkana fauna could be explained as ecophenotypic or as under simple genetic control and not as speciation. (4) The belief that extant fishes of African lakes provide a far better example of rapid speciation (a punctuational phase of evolution), and therefore of the punctuated equilibrium model, than do the fossil Turkana molluscs. It should be noted that at no stage did we provide a critique of the punctuated equilibrium model. We noted the difficulties inherent in rigid adherence to any entrenched view, whether punctuationist or gradualist, but did not criticize either. THE VARIABILITY OF FRESHWATER PROSOBRANCH GASTROPODS A major objection to Williamson’s conclusions is that they are based to a large extent on a false premise, namely that variation in freshwater prosobranchs (and also bivalves) is not subject to environmental or simple genetic control. He therefore presumes that morphologically deviant populations at certain levels must represent genotypic divergence and, in turn, assumes this to be evidence of speciation. Those views are proclaimed unequivocally in his original paper (Williamson, 1981) in which the phenotypic stasis of the gastropods involved is stressed, and claims are made that, unlike basommotophorans, freshwater prosobranchs “are characterised by narrow phenotypic ranges [of morphological variability] in modern faunas”. It is maintained that this is “paralleled by the geographical stability of phenotype exhibited by their widely distributed modern representatives”, and “their current morphological stability SPECIATION IN FOSSIL MOLLUSCS 327 in a diverse range of modern environments”. This, it is proposed, precludes the possibility of phenotypic shifts reflecting any other phenomenon than speciation. In fact it is well established that these and other prosobranch molluscs are variable (see F ryer et al., 1983; Kat & Davis, 1983). Indeed, when discussing the species concept among the freshwater prosobranchs of Europe, Boeter (1982) refers to the taxonomic problems raised by “the exceptional variability of freshwater prosobranchs”. Coming from a student of living molluscs, and referred to in a context in no way concerned with this debate, this can hardly be considered an endorsement of Williamson’s claims. Variability among bivalves, to which we referred and cited references in our critique, is perhaps even easier to substantiate (e.g. Tevesz & Carter, 1980). Perhaps wisely, Williamson makes no attempt to refute what we stated there, though alleged speciation events in bivalves play an important part in his story. Williamson would distinguish two kinds of variability in prosobranchs. One is that defined by ‘Raupian’ parameters which are “generalized geometric descriptors of fundamental shell form”. Variations in shell size, thickness and colour, previously ignored, he now dismisses as “comparatively minor ecophenotypic variations” which he thinks are largely controlled by variations in water chemistry. “Major variations in fundamental shell geometry and sculpture” he believes “are not usually ecophenotypically varying characters in freshwater prosobranchs”. Central to his distinction between the two kinds of variability, between which we see no qualitative differences, is his a priori belief that changes in Raupian parameters are necessarily under genetic control, and must be associated with speciation. These assumptions are not established. They have been considered largely, but not solely, with reference to marine species by Vermeij (1980) in a paper significantly entitled ‘Gastropod shell, growth rate, allometry and adult size: environmental implications’. Here a wide range of evidence is presented for a relationship between growth rate and allometric changes in shell shape, with growth rate being markedly influenced by environmental factors. Probably most gastropods do not maintain a constancy of shell shape during growth. Quoting from Vermeij, “The effects of the environment on growth rate, shape, and colour of snail shells, and the postulated correspondence between growth rate and allometry, have obvious implications for gastropod systematics. Shell features may be so profoundly altered by the environment that the unwary taxonomist may regard variants as separate species when in fact the morphs are merely different phenotypes determined by different environmental regimes. The taxonomic question can be resolved only if the forms are grown reciprocally in each other’s environments”. This problem has been expanded in considerable detail by Gould and his co workers in a study of a highly variable genus of pulmonate land snails, Cerion (see Gould, 1984, for references). We appreciate that pulmonates are not prosobranchs,