The scope of natural selection

Evolutionary theory puts together the Tree of Life claim, and the Selection principle; however, these two statements are not logically connected. We can imagine a possible world where there is selection and not one Tree of Life; Lamarckism gives a picture of the opposite possible world. The question then is the relationship between the two claims: to what extent is the Tree of Life accountable for by natural selection? This question was present but quite attenuated in Darwin since he thought of other mechanisms than selection (e.g., Lamarckian inheritance). However, it becomes urgent in the Modern Synthesis because it focuses on selectionist explanation in the forms and conditions outlined previously.

All the puzzles investigated in the preceding section concern the selectionist explanation in general, whether it is applied to speciation in a population and on a short time scale, or to what Mayr called "emergence of evolutionary novelties'' (1959a) (such as the transition of the protostomes to deuterostomes). However, there is a difference between those two objects, and this raises another question about the scope of the selectionist explanations under consideration up to now. For instance, it is plausible that Wright's SBT accounts for a lot of speciations on small time-space scales but that evaluating its validity on a wider scale may appeal to other criteria. Paleontologists distinguished after Goldschmidt (1940) micro-and macroevolution and wondered whether the same processes should be held responsible for the events in those two cases. Simpson (1944) argued that, even if macroevolution shows very different rhythms in different lineages, however, it implies the same processes as microevolution. The main objection, from a paleontological perspective, namely the lacuna in the fossil records on a large timescale, could be explained by purely geological reasons with no need to postulate special processes to account for them. But Simpson felt compelled to isolate a "mega evolution''—e.g., emergence of new lineages—that cannot so easily be interpreted along the lines of microevolution. Of course, Eldredge and Gould were the most convincing proponents of the difference between macro and microevolution, with the paleontological theory of punctuated equilibria. This theory is, first of all, a reading of the fossil records which claims that discontinuity is not a result of geological lacunae (as Darwin tried to establish in the Chapter IX of the Origin) when they show no major transformation for a very long period of time followed by sudden change. Here, the process accounting for this record is interpreted as a dual one, composed of fine tuning adaptation, which is a kind of stasis, and then a quick general transformation of the body plan giving rise to a new phylum. If the first process is explainable by selectionist explanations, such as the one I have considered up to this point, the second stage needs at least a change in the conditions under which natural selection can operate—if we still assume that no other process is needed.

No doubt challenges to Darwinian gradualism were numerous before Eldredge and Gould: before the Synthesis there were saltationists like De Vries and afterward came the "hopeful monsters'' proposed by the geneticist Goldschmidt (1940). As a result of this, Mayr (1965b) established that gradualism—meaning that no evolutionary change is due to a big mutation—is compatible with evolutionary novelties since any change (like exaptations of insect wings) or intensification (as in the evolution of eyes in some lineages) of function can account for many structural novelties. Punctuated equilibria is a really challenging theory because the difference in the form of the Tree of Life cries out for a difference in the nature or the conditions of processes. If we subscribe to the idea of Bauplane as an integrated set of constraints, as advanced by Gould and Lewontin (1978), then we might think that phases of stasis represent fine adaptive tuning of the existing Bauplane, whereas quick transformations represent the appearance of new Bauplane.

Nevertheless, this view rests on some orthodox considerations of selection: among the founders of the synthesis, Mayr (1965b) emphasized the stabilizing role of selection, which, given a particular environment, largely eliminates big mutations since, given the high degree of integration of most organisms they are probably deleterious and often likely to threaten functional integrity. Periods of stagnation are, therefore, to be expected, by the nature of selection. The crucial point, however, is the logical relation between large-scale and small-scale evolution. Founders of the synthesis, like Fisher and Wright, focused on microevolution. However, some assumptions defining such evolution become false when we jump to macroevolution: environments are no longer stable, and they can change quickly and intensively; and phenotypic variation available is not stable either since a very different range of variation will be available if the time scale is larger.

This second parameter is connected to Gould's other main concern, namely evolutionary theories of development, and the focus on heterochronies crucial to his Ontogeny and phylogeny. The question is: what are the constraints on the range of variation, and what constraints are about to change? Developmental constraints are likely to account for the restriction of available variation and then for the focusing of selection process upon fine adaptive tuning and finally for the puzzling outcome of stagnation in the evolutionary tree. If we want to understand the transformation phase, we have to turn to the modification of available variation and then to a possible change in constraints. To this extent, if a modification happens in developmental mechanisms, then we could expect an enlargement of phenotypic variation, a new field for selection and thus new evolutionary possibilities. This is because, if we consider that the features yielding this enlargement are deeply entrenched, we can understand that in this case selection will act upon many connected traits at many levels of the developmental process, and so a radical change of existing body plan is likely to result. This was Gould's (1977) point, following De Beer (1958) concerning heterochronies: a change in the timing of development, involving many subsequent and connected transformations in the life cycle is more likely to transform the body plan of a species than is change in an adult trait. This sets the agenda for other kinds of evolutionary research, including not only the taxonomy of different mechanisms able to affect development and thus yield evolutionary novelties,35 but also an attempt of causal accounting for them (an agenda which is a part of the Evo-Devo program). The important discovery of Hox genes developed in Lewin's studies on bithorax gene (1978; see Gehring (1998) for a historical account), which are homologous in arthropods and chordates, supports this thesis, since a slight replacement of such a developmental gene by the Antennapedia gene can give rise to a leg instead of an antenna in Drosophila. Setting aside the complexity of the cascades of interactions, the general idea is that great transformations of a Bauplan may be generated by slight modifications of some kinds of genes or of

35 Like in Gould (1977): heterochrony, paedomorphosis, neoteny are defined and exemplified.

36 Even if genes of this sort, such as Bithorax have been known since about 1915, a major stage in the emergence of Evo-Devo has been the molecular characterization of those genes in the 1980s, mostly by Gehring (see Gehring 1998). This revealed that homeobox genes are homologous across several phyla.

their expression channels (Arthur 1997) because the development and life cycles are affected at many levels. Whether this view will prove correct or will need a radical revision, such as DST claims, and no matter the range of biological cases to which they apply, its epistemological significance requires integrating developmental biology and evolutionary biology in order to assess the multiplicity of the processes needed to account for the varied features of the evolutionary tree.37

On large time scales, environments are very likely to change, not only due to the evolution of organisms and populations, but also because of general geological and meteorological shifts. This second dimension of mega evolution converges with the first one to present the philosopher (a) with an epistemological issue. It also inspired Gould in his stronger challenge to overall selectionism (b).

a. The epistemological issue is the following: when variation range and environment change, populations exhibit a response to selection constituted along parameters that were not previously relevant. It could be said, then, that populations and organisms are evolvable. But some features make them more evolvable than others. Hence, at this large evolutionary scale the question may no longer be the evolution of adaptations (with all the epistemological problems addressed previously concerning nature and limits of selectionist explanation) but rather the evolution of evolvability itself. Changing explananda, then, could shift interest toward other levels of selection than genes and individuals, for example, clades and populations, since some population-level traits, such as sex or polymorphism makes them obviously more evolvable (Gould, Williams, Sterelny). But it can also raise new questions, such as the evolutionary origins of those features of traits that make them easily evolvable: how, for instance, are we to explain the cohesion of genes in a chromosome (Keller 1999), modularity (Wagner 1995; Sterelny 2004) or redundancy? So shifting the scale in the Tree also shifts interest from epistemological and methodological issues proper to selection, drift and inertia, to a general concern with new objects, such as modularity.

b. In Wonderful Life, Gould tried to trace the philosophical conclusions of the recent analysis of the Burgess shale, particularly by Withington and Conway Morris. His verdict was that many phyla appeared with the Cambrian, of which only few survived; thereafter, very few new body plans and phyla were really ''invented'' through evolution. But this creativity in evolutionary novelty is somewhat puzzling and raises a concern for the new explananda stressed previously. With the famous metaphor of the film of life rerun, Gould suggested that the history of life was much too full of contingent events, such as the mass extinction that killed more than half of the Burgess phyla (plausibly after the fall of an asteroid, according to the Alvarez hypothesis). The punctuated equilibria claim was a weak challenge to an overall view

of selectionism since it can be reinterpreted as the necessity of defining two regimes of selection, the second one including the aforementioned concepts and concerns stemming from developmental theory. This latter view presents a strong challenge, since selection, and the adaptive capacities of individuals and species, cannot prepare them to face mass extinctions due to excessively strong changes of environment. Hence, the ones that survived did not owe their survival to their higher fitness, and the explanatory and predictive power of natural selection is very limited at this level of the history of life. Anomalocaris, for instance, seemed quite well fitted to its marine environment and was undoubtedly a strongly performing predator, surely no less well adapted than Pikaia, which seems to belong to the chordate phylum; it nevertheless disappeared. Thus, major events are contingent with regard to the parameters ordinarily involved in natural selection. This "contingency thesis'' heavily restrains the scope of natural selection.

The fate of this challenge rests on a lot of empirical elements that are not yet available. In particular, the diagnosis of the Burgess fauna is still debated since Conway-Morris himself revised his original judgment (1998) and estimated that many Burgess phyla are in fact ancestors of already known lineages.38 However, as Gould pointed out in his reply (Gould and Conway-Morris 1999), the point is not whether or not there are other mechanisms than natural selection, a conundrum that we are unable to solve, but whether there were many more new phyla in the Cambrian, a great part of which effectively disappeared.39 The contingency thesis relies on an affirmative answer to this question, which should be studied by paleontological and morphological means. So notwithstanding the strong challenge to selectionism, the important consequences for the interpretation of the history of life rely on empirical investigations. But the question is likely to be begged by methodological considerations involving disparity. If diversity means the variety of species, disparity means the heterogeneity of the body plans. Gould contends that whereas diversity may have increased, disparity decreased. But even if we knew what the Cambrian phyla were, this would not entail the ability to measure disparity (Sterelny 1995, 2000). Cladists mostly think that we can trace the genealogy of phyla, but not evaluate the distance or difference between two phyla, because the criteria are always instrumental. In this view, Gould's thesis would not be testable. The basic question, beyond the measure of disparity, is the

38 For example, Hallucigenia, once viewed as a quite unique species in its phyla, if turned upside down could enter into the phylum of the echinoderms (Conway-Morris 1998).

39 An argument against the contingency thesis would be convergence, if similar features appear several times in different lineages, they are more likely to appear even if we change some initial conditions of evolution (Sterelny 1995; Conway-Morris and Gould 1999). But such an argument makes use of excessively undefined notions of necessity and identity.

counting of body plans, hence the definition of body plans. Failing any consensus about that, the contingency thesis, whether or not empirically adequate, is not likely to be tested.

From a distance, the current state of evolutionary theory may in general be characterized as facing two kinds of challenges, weak and strong. Weak challenges imply, if successful, a revision of some part of the theory in order to integrate new methods and concepts; strong challenges entail giving up some major credos of the Modern Synthesis. In the case of Gould's punctuated equilibria and contingency thesis, those two challenges focus on the first Darwinian claim, the form of the Tree of Life. Here, the strong challenge would lead us to give up both gradualism and the hope of finding a general account of the history of life through one explanatory schema.

But the same situation arises in the case of the second Darwinian claim, concerning the process in evolution. Here, challenges are forged by developmental-ists. The weak challenge proposed by Evo-Devo involves a rethinking of the conditions and mechanisms of selection when it comes to development and the origin of evolvability. The strong challenge is formulated by DST proponents and entails giving up the concept of gene or its main role in inheritance and selection.40

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