NOBLE · Novelties, Biases, and Landscapes in Evolution

Novelties, Biases, and Landscapes in Evolution

How predictable is evolution? Key components of evolution (mutation, recombination, and genetic drift) are all “random” processes [1,2]. Add chance in ecological events (like a particularly large asteroid impact) and it seems like evolution should be unpredictable. Despite this, similar traits (phenotypes) have evolved repeatedly across distantly-related groups. Examples of phenotypic convergence include the repeated evolution of multicellularity; the “tuna-shaped” body plan in fish, sharks, and ichthyosaurs; and ant-mimicry in arhtropods [3-5]. Convergent evolution is often attributed to natural selection. Lineages evolving under the same selection pressures may be pushed towards the same adaptive optima, resulting in a predictable phenotype. Evolutionary predictability is important for medicine (how will pathogens evolve?), conservation (how will species respond to environmental change?), and our understanding of evolutionary trends (how likely was the evolution of a particular trait?). In project NOBLE, we investigated three questions about evolutionary predictability: Q1: Can ancestral traits predict the appearance (or loss) of a novel trait in their descendants? Q2: Is convergent evolution driven by natural selection, or does it occur by chance? Q3: Does division of labor (splitting functions between cells, organs, or individuals) increase phenotypic convergence? We investigated these questions in phylum Bryozoa – a group of aquatic, colonial, suspension feeders. In particular, we focused on a novel trait found in cheilostome bryozoans called “avicularia” (Fig. 1). Avicularia are morphologically specialized colony members (like ant soldiers), and are thought to have a defensive function. Avicularia were chosen as a focal trait because the repeated evolution (and loss) of avicularia [6] provides a natural experiment to investigate the predictability of their evolution. Conclusions: We found that while the loss of a novel trait (avicularia) was related to ancestral traits (Q1), phenotypic convergence was less predictable and could have occurred by chance (Q2-Q3). Figure 1: Example cheilostome bryozoan colony. A: colony surface, showing colony individuals and avicularia (pink). B-C: Avicularia with closed (B) and open (C) mandibles. References 1. T. Lenormand, D. Roze, F. Rousset, Trends in Ecology & Evolution. 24, 157–165 (2009). 2. Z. D. Blount, R. E. Lenski, J. B. Losos, Science. 362, eaam5979 (2018). 3. K. J. Niklas, S. A. Newman, J Exp Bot. 71, 3247–3253 (2020). 4. R. Motani, K. Shimada, Sci Rep. 13, 16664 (2023). 5. J. D. Mclver, G. Stonedahl, Annual Review of Entomology. 38, 351–377 (1993). 6. A. H. Cheetham, J. Sanner, P. D. Taylor, A. N. Ostrovsky, Journal of Paleontology. 80, 49–71 (2006).

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