It is often said that the timescales for evolution are too long to allow speciation to be studied experimentally. Consequently, researchers look to the fossil record to provide the evidence base. However, this also has its limitations. With fossils, molecular analyses are not possible because soft tissues decay rapidly. Furthermore, the drivers of speciation are often a matter of speculation. Nevertheless, by selecting a depositional environment that provides a sequence of stratigraphical horizons that allow analysis of environmental factors, some informative studies are possible.
"Long-lived lakes are virtually predetermined for these studies, because of their duration and relative stability, being therefore often called 'islands of evolution'. Many studies have proven this fact repeatedly, including the papers on the impressive morphological developments in the Middle Miocene Lake Steinheim planorbids, the Neogene Aegean freshwater gastropods, or the Recent Lake Tanganyika gastropods." (page 117)
Geographical setting of Lake Pannon (source here)
The research considered in this blog has gathered data from Lake Pannon. In the past, this covered parts of eastern Austria, Hungary, Slovakia, Croatia, Serbia, and western Romania. At its maximum, it covered 290,000 km2; it lasted from the Late Miocene to the Early Pliocene. A stratigraphy has been developed and its diverse fauna logged. The research paper considers the changing fortunes of the gastropod genus Melanopsis over successive stages of the lake's chronological development.
"The aim of this paper is to document and discuss the phenotypic evolution in a single evolutionary lineage by modern morphometric analysis. The investigated geological interval of c. 1.6 Ma provides an excellent opportunity to study alternating modes of evolution in the fossil record, including stasis, the proposed adaptive radiation, and the final extinction of one of the emerging branches. The integration of a comprehensive set of palaeoenvironmental data taken from the literature provides the necessary link between the observed phenotypic evolution and underlying parameters, potentially allowing evaluation of the taxonomic status of the phenotypes." (pages 117-118)
"Recent Melanopsis praemorsa (Linnaeus, 1758) is a generalist, living in a great variety of habitats. These include rivers, ponds, springs, shallow lakes with inundated marshes, mud and gravel shores of estuaries, irrigation canals, and oases. It tolerates high temperatures and brackish conditions. This species feeds variably on algae, detritus and carrion." (image source here)
The authors are very cautious about affirming that any particular phenotype is a species. They are aware that taxonomists have tended to declare a novel phenotype to be a species rather too easily. This is relevant to Melanopsis, and all the phenotypes observed have been given specific names by researchers. Those appearing in this study are: Melanopsis impressa, M. pseudonarzolina, M. coaequat, M. fossilis, M. rugosa, M. handmanniana and M. vindobonensis. All are illustrated in Figure 2 in the research paper.
"Moreover, the authors act on the a priori assumptions of the distinctiveness of species within a lineage. To retain objectivity, we refrained from any predefined species/subspecies delimitations, many of which were proposed in earlier studies, and refer to the different morphologies as phenotypes." (page 117)
Morphometric analysis of gastropod shells has many challenges. The authors explain their approach, which is based on defining the curves (or outlines) that make up the shape. This gave a quantitative dimension to the observed phenotypic changes that correlated well with the descriptive morphological work of taxonomists. Environmental interpretations draw on prior research:
"Both the samples used for the morphological analysis as well as the major part of the isotope samples studied by Harzhauser et al. (2007) originate from a geographically small area at the western margin of Lake Pannon. Based on the previously published isotope data a rather detailed model of palaeoenvironmental change is available for the time interval and area studied. This model is used to correlate changes observed in gastropod morphology with palaeoclimate shifts." (p.120)
The scenario that emerges has three phases.
Phase 1. Fluvio-deltaic but arid environment. The molluscs lived in flowing water and tended to be small and slender (i.e. the streamlined shape matched these conditions). This situation was stable and stasis is reported.
Phase 2. Humid climate with increased precipitation. The lake level rose and water flows became slow and sluggish. Stabilising selection for the streamlined morphology ceased, and the molluscs experienced a radiation of phenotypes. Several different morphologies emerged, finding their niche somewhere in the lake.
Phase 3. Climatic instability. The environment fluctuated, with variable river discharges, lake level changes, lake stratification with periodic eutrophic conditions involving algal blooms. The impact was felt primarily by shallow water animals such as Melanopsis. Gastropod phenotypes reduced during this phase until only one phenotype was left: a small form that is inferred to have coped with the range of environmental stresses experienced. There is some evidence that this morphology is adapted to higher water energies.
The authors are careful to avoid claiming too much. They are aware that they are unable to identify the mode of speciation. The term "adaptive radiation" is one they use tentatively.
"Nevertheless, the correlation between the phenotypic changes and the environmental alterations strongly argues for ecologically driven natural selection. Consequently, the strong phenotypic divergence is thought to reflect reproductive isolation. The argument for directional selection is corroborated by the fact that both shape and size traits show massive changes within the interval of the proposed adaptive radiation. Hunt (2007) suggested body size to be a more common subject to directional selection than shape traits." (p.124)
So, what has this research achieved? A sequence of morphological changes has been identified and a correlation has been made with palaeoenvironmental data. The authors suggest that an adaptive radiation occurred when the climate became more humid, with higher precipitation and higher lake levels. They note that stratigraphic resolution was coarse, so that conclusions were unable to be drawn as to whether speciation was gradual or punctuated.
It should be said that the clearest examples of adaptation are in phases 1 and 3, where the phenotypes can be regarded as tuned to environmental factors. Phase 2 is more a case of lifting constraints, so that a wider range of morphologies were viable. Whether or not they are adaptive does not yet appear to be established. We have a radiation, but it may be a case of populating the larger ecospace.
The authors see their work as contributing to a greater understanding of how morphological changes are related to evolutionary mechanisms. It is worth pointing out that the data does not take us very far in this direction. The morphologies can be described as examples of micro-evolutionary change. The Melanopsis gastropods are all members of the same genus. Whilst morphologies change, there are no evolutionary novelties. Indeed, there is no evidence here for anything more than multiple phenotypes emerging from the same genotype. The situation fits well with the concept of phenotypic plasticity (discussed here and here). A relevant comment comes from Luskin, pointing out the implications of the research of Austin Hughes:
"This leads to the question: How, then, do new traits arise? Rather than relying on positive selection, Hughes claims that one prevalent mechanism in producing new traits is the relaxation of purifying selection -- i.e. random genetic drift. But genetic drift, of course, is essentially a random process where mutations not only arise without respect to the needs of the organism, but also are preserved (or lost) without regard for the needs of the organism. In other words, it would have no reason to build complex traits."
There is nothing here to support the thesis that macroevolution is but the extrapolation of microevolution. Indeed, the argument can be made that the observed phenotypes in Melanopsis occur within limits: constrained by the Melanopsis genotype. There may be no requirement for new genetic information to produce the range of observed phenotypes. If this is the case, far too much emphasis is being placed on speciation. The real question for evolutionary biologists relate to the origin of novelty, of body plans, and ultimately of biological information.
Phenotypic evolution in a fossil gastropod species lineage: Evidence for adaptive radiation?
Thomas A. Neubauer, Mathias Harzhauser, Andreas Kroh
Palaeogeography, Palaeoclimatology, Palaeoecology, 370, 15 January 2013, 117-126.
Abstract: Detecting speciation in the fossil record is a particularly challenging matter. Palaeontologists are usually confronted with poor preservation and limited knowledge on the palaeoenvironment. Even in the contrary case of adequate preservation and information, the linkage of pattern to process is often obscured by insufficient temporal resolution. Consequently, reliable documentations of speciation in fossils with discussions on underlying mechanisms are rare. Here we present a well-resolved pattern of morphological evolution in a fossil species lineage of the gastropod Melanopsis in the long-lived Lake Pannon. These developments are related to environmental changes, documented by isotope and stratigraphical data. After a long period of stasis, the ancestral species experiences a phenotypic change expressed as shift and expansion of the morphospace. The appearance of several new phenotypes along with changes in the environment is interpreted as adaptive radiation. Lake-level high stands affect distribution and availability of habitats and, as a result of varied functional demands on shell geometry, the distribution of phenotypes. The ongoing divergence of the morphospace into two branches argues for increasing reproductive isolation, consistent with the model of ecological speciation. In the latest phase, however, progressively unstable conditions restrict availability of niches, allowing survival of one branch only.
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