The nature of 2-locus epistatic interactions in animals: evidence from Sewall Wright's guinea pig data

Summary The nature of epistatic interactions affects covariance between relatives and the expression of heterosis in various crossbred genotypes. The investigation of these interactions for metric traits requires large data sets of a suitable type. Data from Sewall Wright's early work with guinea pigs are used to compare the goodness-of-fit of seven biological models of 2-locus interaction for the six out of eleven traits in which epistatic effects are apparent. The model equivalent to additive x additive epistasis gives the best general fit over traits, with an average transformed R² value significantly greater than that of the next best fitting model (P<0.05). This result is compatible with results from the one other study in this area, using data from mice. It is concluded that, based on results available to date, the additive x additive 2-locus model of epistatic interaction appears most suitable for reduced genetic models.     

Darwinism after Mendelism: the case of Sewall Wright's intellectual synthesis in his shifting balance theory of evolution (1931)

Historians of science have long been agreeing: what many textbooks of evolutionary biology say, about the histories of Darwinism and the New Synthesis, is just too simple to do justice to the complexities revealed to critical scholarship and historiography. There is no current consensus, however, on what grand narratives should replace those textbook histories. The present paper does not offer to contribute directly to any grand, consensual, narrational goals; but it does seek to do so indirectly by showing how, in just one individual case, details of intellectual biography connect with big picture issues. To this end, I examine here how very diverse scientific and metaphysical commitments were integrated in Sewall Wright's own personal synthesis of biology and philosophy. Taking as the decisive text the short final section of Wright's long 1931 paper on 'Evolution in Mendelian populations,' I examine how his shifting balance theory (SBT) related to his optimum breeding strategy research, his physiological genetics, his general theory of homogenising and heterogenesing causation and his panpsychist view of mind and matter; and I discuss how understanding these relations can clarify Wright's place in the longue durée of evolutionary thought.     

Organisms and their place in biology

Summary In this paper we review the concept of organism analysing the main ideas related to it in the context of present biological theories. The discussion is focused and developed according to four key issues: individuality, organisation, autonomy and reproduction. Once these basic connections are established, a spectrum of possible entities that fall under the label ‘organism’ is looked over, with special emphasis on limit or controversial cases. The aim is to see whether they all share a set of common features and, if they do, why it is so difficult to reach a consensus on the definition of the term. Finally, we try to release somehow the tension between those hierarchical schemes proposed to account for life as a global phenomenon and those approaches that take organisms as the central target of (theoretical) biology, suggesting a possible middle-ground solution open for further research.     

Sewall Wright and quantitative genetics

Some aspects of Wright's great contribution to quantitative genetics and animal breeding are reviewed in relation to current research and practice. Particular aspects discussed are as follows: the utility of his definition of inbreeding coefficient in terms of the correlation of uniting gametes; the maintenance of genetic variation in the optimum model; the inter-relations between past and present animal-breeding practice and the shifting-balance theory of evolution.     

The proper place of hopeful monsters in evolutionary biology

Hopeful monsters are organisms with a profound mutant phenotype that have the potential to establish a new evolutionary lineage. The Synthetic Theory of evolutionary biology has rejected the evolutionary relevance of hopeful monsters, but could not fully explain the mechanism and mode of macroevolution. On the other hand, several lines of evidence suggest that hopeful monsters played an important role during the origin of key innovations and novel body plans by saltational rather than gradual evolution. Homeotic mutants are identified as an especially promising class of hopeful monsters. Examples for animal and plant lineages that may have originated as hopeful monsters are given. Nevertheless, a brief review of the history of the concept of hopeful monsters reveals that it needs refinements and empirical tests if it is to be a useful addition to evolutionary biology. While evolutionary biology is traditionally zoocentric, hopeful monsters might be more relevant for plant than for animal evolution. Even though during recent years developmental genetics has provided detailed knowledge about how hopeful monsters can originate in the first place, we know almost nothing about their performance in natural populations and thus the ultimate difference between hopeful and hopeless. Studying the fitness of candidate hopeful monsters (suitable mutants with profound phenotype) in natural habitats thus remains a considerable challenge for the future.     

Hybrids, pure cultures, and pure lines: from nineteenth-century biology to twentieth-century genetics

Prompted by recent recognitions of the omnipresence of horizontal gene transfer among microbial species and the associated emphasis on exchange, rather than isolation, as the driving force of evolution, this essay will reflect on hybridization as one of the central concerns of nineteenth-century biology. I will argue that an emphasis on horizontal exchange was already endorsed by 'biology' when it came into being around 1800 and was brought to full fruition with the emergence of genetics in 1900. The true revolution in nineteenth-century life sciences, I maintain, consisted in a fundamental shift in ontology, which eroded the boundaries between individual and species, and allowed biologists to move up and down the scale of organic complexity. Life became a property extending both 'downwards', to the parts that organisms were composed of, as well as 'upwards', to the collective entities constituted by the relations of exchange and interaction that organisms engage in to reproduce. This mode of thinking was crystallized by Gregor Mendel and consolidated in the late nineteenth-century conjunction of biochemistry, microbiology and breeding in agro-industrial settings. This conjunction and its implications are especially exemplified by Wilhelm Johannsen's and Martinus Beijerinck's work on pure lines and cultures. An understanding of the subsequent constraints imposed by the evolutionary synthesis of the twentieth century on models of genetic systems may require us to rethink the history of biology and displace Darwin's theory of natural selection from that history's centre.     

Sewall Wright (1889-1988)

The genetic contributions of Sewall Wright is briefly reviewed with special reference to J.B.S. Haldane's work. These include his work in population genetics, statistics, and animal breeding.