An adaptationist programme has dominated evolutionary thought in England and the United States during the past 40 years. It is based on faith in the power of natural selection as an optimizing agent. It proceeds by breaking an organism into unitary ‘traits’ and proposing an adaptive story for each considered separately. Trade-offs among competing selective demands exert the only brake upon perfection; non-optimality is thereby rendered as a result of adaptation as well. We criticize this approach and attempt to reassert a competing notion (long popular in continental Europe) that organisms must be analysed as integrated wholes, with Baupläne so constrained by phyletic heritage, pathways of development and general architecture that the constraints themselves become more interesting and more important in delimiting pathways of change than the selective force that may mediate change when it occurs. We fault the adaptationist programme for its failure to distinguish current utility from reasons for origin (male tyrannosaurs may have used their diminutive front legs to titillate female partners, but this will not explain why they got so small); for its unwillingness to consider alternatives to adaptive stories; for its reliance upon plausibility alone as a criterion for accepting speculative tales; and for its failure to consider adequately such competing themes as random fixation of alleles, production of non-adaptive structures by developmental correlation with selected features (allometry, pleiotropy, material compensation, mechanically forced correlation), the separability of adaptation and selection, multiple adaptive peaks, and current utility as an epiphenomenon of non-adaptive structures. We support Darwin’s own pluralistic approach to identifying the agents of evolutionary change.

The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme

An adaptationist programme has dominated evolutionary thought in England and the United States during the past 40 years. It is based on faith in the power of natural selection as an optimizing agent. It proceeds by breaking an organism into unitary 'traits' and proposing an adaptive story for each considered separately. Trade-offs among competing selective demands exert the onlv brake upon perfection; non-optimality is thereby rendered as a result of adaptation as well. We criticize this approach and attempt to reassert a competing notion (long popular in continental Europe) that organisms must be analysed as integrated wholes, with Bauplane so constrained by phyletic heritage, pathways of development and general architecture that the constraints themselves become more interesting and more important in delimiting pathways of change than the selective force that may mediate change when it occurs. We fault the adaptationist programme for its failure to distinguish current utility from reasons for origin (male tyrannosaurs may have used their diminutive front legs to titillate female partners, but this will not explain why they got so small); for its unwillingness to consider alternatives to adaptive stories; for its reliance upon plausibility alone as a criterion for accepting speculative tales; and for its failure to consider adequately such competing themes as random fixation of alleles, production of nonadaptive structures by developmental correlation with selected features (allometry, pleiotropy, material compensation, mechanically forced correlation), the separability of adaptation and selection, multiple adaptive peaks, and current utility as an epiphenomenon of non-adaptive structures. We support Darwin's own pluralistic approach to identifying the agents of evolutionary change.

/pdf/the-spandrels-of-san-marco-and-the-panglossian-paradigm-a-19fdlsieru.pdf

Rising atmospheric carbon dioxide (CO2), primarily from human fossil fuel combustion, reduces ocean pH and causes wholesale shifts in seawater carbonate chemistry. The process of ocean acidification is well documented in field data, and the rate will accelerate over this century unless future CO2 emissions are curbed dramatically. Acidification alters seawater chemical speciation and biogeochemical cycles of many elements and compounds. One well-known effect is the lowering of calcium carbonate saturation states, which impacts shell-forming marine organisms from plankton to benthic molluscs, echinoderms, and corals. Many calcifying species exhibit reduced calcification and growth rates in laboratory experiments under high-CO2 conditions. Ocean acidification also causes an increase in carbon fixation rates in some photosynthetic organisms (both calcifying and noncalcifying). The potential for marine organisms to adapt to increasing CO2 and broader implications for ocean ecosystems are not well known; both are high priorities for future research. Although ocean pH has varied in the geological past, paleo-events may be only imperfect analogs to current conditions.

/pdf/ocean-acidification-the-other-co-2-problem-4h18mw2cpj.pdf

Ocean Acidification: The Other CO 2 Problem

We review the biogeography of microorganisms in light of the biogeography of macroorganisms A large body of research supports the idea that free-living microbial taxa exhibit biogeographic patterns Current evidence confirms that, as proposed by the Baas-Becking hypothesis, 'the environment selects' and is, in part, responsible for spatial variation in microbial diversity However, recent studies also dispute the idea that 'everything is everywhere' We also consider how the processes that generate and maintain biogeographic patterns in macroorganisms could operate in the microbial world

/pdf/microbial-biogeography-putting-microorganisms-on-the-map-1k7e5do5pn.pdf

Microbial biogeography : putting microorganisms on the map

Darwin (1859, pp. 11-14, 143-150) stressed the evolutionary importance of covariation between characters in populations and its "most imperfectly understood" connection with correlated responses to artificial and natural selection. After the turn of the century the discoveries of pervasive pleiotropy and linkage of Mendelian factors revealed the underlying genetic mechanisms. Existing theory on the dynamics of correlated characters has been developed in the limited framework of practical plant and animal breeding. Methods of multivariate analysis, functional analysis and optimality criteria popular among evolutionists, do not account for dynamical constraints imposed by the pattern of genetic variation within populations. Consideration of phenotypic variation often does not suggest any clear mechanism connecting growth patterns or adult variation to interspecific evolution. When there is individual variation in development, no necessary correspondence exists between ontogenetic and adult variation in a population (Cock, 1966, pp. 148-15 1). It is also common for the pattern of adult variation within a species to differ from that at higher taxonomic levels (Simpson, 1953, pp. 25-29). An example which will be investigated here is the brain weight:body weight relationship. At various taxonomic levels, brain and body weights tend to follow the allometric equation

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1558-5646.1979.tb04694.x

Quantitative genetic analysis of multivariate evolution, applied to brain:body size allometry.

The prosogyrous shape and flattened lunule of a typical clam shell form a blunt anterior, the function of which is related to the forward-and-back rocking motion of the shell in burrowing. Analysis of movies has revealed that each rocking motion of a morphologically typical clam, Mercenaria mercenaria (Linne), involves purely rotational movement, with no translational component. The clam is able to burrow by "walking" its way downward only because the axis of backward rotation lies to the anterior of the axis of forward rotation. Ex- periments with burrowing robots show that the blunt anterior serves to shift the axis of back- ward rotation anteriorly, thus aiding in downward progress. The prosogyrous condition and the rotational mechanism of burrowing are fundamental adaptations of burrowing clams?and were apparently present in the ancestral bivalves of the Cambrian.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0094837300002189

Why clams have the shape they have: an experimental analysis of burrowing

Being of especially high quality, the Neogene fossil record of planktonic foraminifera offers special opportunities for assessing patterns of extinction and speciation. A variety of metrics indicates that within this group the mean duration of lineages has been much shorter (rate of extinction has been higher) for the globorotaliid clade than for the globigerinid clade. Furthermore, in the globorotaliid clade rates of extinction and speciation have not been closely linked to changes in diversity, but rather have been relatively high even at times when diversity has undergone little change. Thus, the globorotaliid clade has undergone more rapid evolutionary turnover than the globigerinid clade. Data for living species reveal that neither geographic range nor temperature tolerance is the primary factor controlling lineage duration. On the other hand, there is evidence that lineages marked by low abundance (small population size) are relatively short-lived. The reason that globorotaliid lineages have generally survived for shorter intervals, on the average, may be that their populations have been less abundant and less stable. Usually they live deeper in the water column, where food is often sparse, and many flourish only in areas of upwelling. Furthermore, the globorotaliids lack symbiotic algae for nutritional support. The same ecological factors may have accelerated speciation in the globorotaliid clade, by causing species to be patchily distributed. Thus, population size and structure have been more important than geographic range in determining rates of extinction and speciation. This parallels the situation for Neogene marine bivalves. For planktonic foraminifera, as for Western Atlantic Bivalvia, the normal pattern of extinction was reversed in late Pliocene time, apparently in response to climatic cooling. The globigerinids suffered a sudden pulse of extinction. The deeper dwelling globorotaliids fared better; probably many of their species benefited from elevation of the seasonal thermocline into the photic zone. At the same time, rate of speciation declined in the globorotaliid clade, which supports the idea, inferred from the evolutionary history of marine bivalves, that an increase in the size and stability of populations should depress both rate of extinction and rate of speciation.

Macroevolutionary differences between two major clades of Neogene planktonic foraminifera

Barnacles living along rocky shores provide the classic example of competitive dominance in the marine ecosystem: by means of firm attachment and rapid growth, balanoid barnacles commonly undercut and overgrow the genus Chthamalus, which is thereby restricted to the upper fringe of the intertidal zone, where balanoids are physiologically incapable of living. Today, after perhaps less than 50 Myr of evolution, balanoid barnacles are in the midst of rampant adaptive radiation, being represented by about 273 species, of which about half are free-living species of intertidal or shallow subtidal habitats. Chthamaloid barnacles, in contrast, are on the decline, having originated at least 70 Myr ago but today comprising only about 53 living species, approximately 40 of which occupy the uppermost intertidal. The remainder persist as localized, relict, and often disjunct populations. Through competitive exclusion, balanoid barnacles have apparently caused the ecological restriction and decline of the chthamaloids. The balanoids have an advanced feeding mechanism, but the most important adaptive breakthrough leading to their competitive success was probably the origin of a tubiferous wall structure, which affords rapid skeletal growth for the efficient monopolization of free space and for the destruction of chthamaloids.

Competitive exclusion in evolutionary time; the case of the acorn barnacles

For many decades the evolutionary history of the Bivalvia remained in relative obscurity. Now, for the first time, it is being elucidated in some detail through a variety of scientific approaches. Ironically, the bivalve fossil record is yielding a better conceptual picture of evolution than are the records of other groups that have been studicd much more intensively over the years. The reaso� for this paradox is that the great diversity of living bivalves offers an excellent opportunity for direct study of many adaptive features that have homologies and analogies among extinct taxa. Because of the nature of the fossil record, it is largely from shell form that the adaptations of extinct taxa are inferred. No effort is made here to review comprehensively the functional morphology of the bivalve shell. An earlier study, based on Western Atlantic bivalves (Stanley 1970), provided a start in this direction, and additional contributions have appeared since its publication. My present goal is to distill from the evolutionary record of the Bivalvia, via functional morphology, certain basic adaptive tendencies and patterns that seem, at our present state of knowledge, to be most fundamentally characteristic of the evolution of this important class of mollusks. For the sake of brevity, only a few examples are offered by way of illustration. Many of the adaptive themes discussed characterize, to varying degrees, the evolution of other animal taxa as well. Relevant concepts of functional mor­ phology are introduced at the outset as background for later discussion.

Adaptive Themes in the Evolution of the Bivalvia (mollusca)

Sexual reproduction predominates among organisms mainly because most evolutionary change is concentrated in speciation events, and asexual species cannot speciate in the normal sense. Asexual clones seldom diversify rapidly enough to overcome normal rates of extinction. In contrast, phylogenetic groups of sexual species (clades) commonly develop broad, heterogeneous adaptive zones rapidly enough to ensure survival.

Steven M. Stanley

Papers

Why clams have the shape they have: an experimental analysis of burrowing

Macroevolutionary differences between two major clades of Neogene planktonic foraminifera

Competitive exclusion in evolutionary time; the case of the acorn barnacles

Adaptive Themes in the Evolution of the Bivalvia (mollusca)

Clades versus clones in evolution: why we have sex.