International Society for History, Philosophy, and Social Studies of Biology

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TUESDAY, JULY 7  /  11:00 - 12:30  /  DS-M340
Organized session / standard talks
From complex to simple: Evolution by simplification, and its philosophical implications

Maureen O'Malley (University of Sydney, Australia)

Evolution as a historical trend of complexification is usually taken for granted. We will challenge this assumption with several cases that show how major phylogenetic groups have evolved via decomplexification of genomes, intracellular structures and functional capabilities. Most of these cases will be taken from eukaryote evolution, because eukaryotes are standardly understood as comprising the most complex cells and organisms on this planet, and their evolution normally discussed as one of increasing complexification. The examples we will explore in depth are focused on the early evolution of eukaryotes, the evolution of metazoans (particularly the ancestors to metazoans), and the evolution of fungi. We will put these particular eukaryote examples into a larger context of ongoing processes of simplification in eukaryotes across the eukaryotic tree of life, and contextualize further with regard to prokaryote evolution, in which genome reduction and other simplification processes are very common. What are the philosophical implications of such cases? We will explore what they mean for understanding general trends in evolution, including relationships between simplification and complexification, and how evolutionary simplification can be explained. We will also discuss what simplification means for the standard default to the ‘most parsimonious’ explanation of an evolutionary outcome, when in most cases ‘parsimony’ is consistent with the idea that evolution proceeds via complexification not simplification. Evolution by simplification indicates this time-hallowed epistemic strategy may need revision too. We will conclude with thoughts on why scientists and philosophers have tended to focus on evolutionary complexification rather than simplification.

The early eukaryote garden of Eden and the loss of complexity

Maureen O'Malley (University of Sydney, Australia)

Eukaryotes contain undeniably complex cells even in their most recent forms, full of specialized compartments, possessing elaborate and often large genomes, and enjoying high-energy metabolisms. But despite these apparently complex extant states, the evolutionary trajectory of eukaryotes cannot be read as a story of increasing complexity from a primitive ancestor. It may be better described as a series of losses as eukaryotes were cast out of their original state of complexity. The five or six major eukaryotic lineages we know today are simplified descendants of a complex ancestor, and the very diversification of those groups was driven at least in part by a variety of decomplexification processes. The secondary simplification of eukaryotes as they diverged is a hugely important finding of recent eukaryote phylogeny. This interpretation suggests that macroevolution in general can be understood as a process of two conflicting dynamics of complexification and simplification: of neutral and adaptive elaboration of structures and functions (at various levels), and the secondary adaptive and neutral pruning of those structures and functions. These dynamic tensions are also at play in prokaryote evolution, which I will outline in order to give further context for the general identification of these processes. I will suggest some important explanatory adjustments evolutionary theorists need to make in light of these macroevolutionary dynamics.

The origin of Metazoa: A genomics and cell biology approach, or why things are never ever so simple

Iñaki Ruiz-Trillo (Institut de Biologia Evolutiva (CSIC-Universitat Pompeu Fabra), Spain)

The standard view of animal origins is shaped by decades of accumulated morphological, palentological and genetic data. This view emphasises an explosive origin of animals as shown in the fossil record by the well-known Cambrian fauna. This explosive origin and diversification of animals is seen as a result of competing factors: environmental (increase in oxygen levels), and/or ecological (arms race), and/or genetic reasons (gene regulatory networks). On the other hand, the morphological change from choanoflagellates to sponges that gave rise to animals is seen as a gradual, ladder-like event that proceeds from simple to more complex. These text-book views have been held by researchers for decades. Here I will present new genomic and cell biology data from several unicellular relatives of Metazoa that challenge previous views on animal origins. The data show that choanoflagellates lost many genes compared to more ancient protists. It also shows that the unicellular ancestor of Metazoa already had many genes and pathways relevant to multicellularity (i.e., genes involved in cell adhesion, cell communication and cell differentiation). All these data suggest a more gradual, although convoluted, acquisition of genes and pathways relevant to multicellularity in the unicellular-to-multicellular transition. On the other hand, the same data suggest a more explosive emergence of the different animal cell types, which was fuelled by a transition from temporal to spatial cell differentiation at the onset of Metazoa. Although the issue remains contentious, the new data tell a cautionary tale about our inherent search for parsimonious simple-to-complex explanations in evolutionary biology.

Learning from our mistakes: Convergent simplification and the kingdom Fungi

Jeremy Wideman (University of Exeter, United Kingdom)

Throughout the history of evolutionary theory, scientists have largely assumed that life evolves from simple to complex but very rarely from complex to simple. Thus, in the past, instances of convergent evolution through simplification have been overlooked in favour of similarity due to common descent. This disregard for simplification has led to a variety of misleading conclusions in the fields of taxonomy and phylogenetics. Recent advances in genome sequencing technology have allowed scientists to distinguish more easily similarity due to common descent from similarity due to convergent evolution, even when convergence is likely the product of secondary simplification. The kingdom Fungi is the best-studied simplifying lineage and contains several instances of simplification that converge with other instances of simplification across the tree of life. I will outline the general cellular simplifications that have occurred in the fungal lineage and show how analogous simplifications across the tree of life have led to confusion and errors in phylogeny and classification. This confusion likely occurred because scientists only ever considered complexification as they sought the most parsimonious explanations of evolutionary relationships. I will argue that the reason scientists and theorists tend to consider complexification as the most parsimonious evolutionary explanation is because of their general focus on plants and animals—lineages in which complexification appears to be the general rule. However, if scientists and philosophers alike take the full diversity of life into account, a more complete picture of evolutionary theory will emerge—a picture that includes mechanisms of simplification and trends toward convergence alongside mechanisms of complexification. This picture suggests a revision of Darwin’s famous quote: ‘From so complex a beginning endless forms most beautiful and simple’.