Biology subjects

Bradler, S.

Publications and source records attributed to Bradler, S..

3 recordsLinked to original sources

bioRxiv2025

Sexual selection and flight as predictors of sexual size and shape dimorphism in stick and leaf insects

Sexual size dimorphism (SSD) and sexual shape dimorphism (SShD) are widespread in animals, yet their macroevolutionary drivers remain poorly understood. We analyzed 196 stick and leaf insect species (Phasmatodea), a clade with exceptional diversity in size, shape, flight capacity, and mating systems, using phylogenetic comparative methods to evaluate the roles of fecundity selection, sexual selection, and ecological factors. SSD was universally female-biased and varied over an order of magnitude. Allometric scaling supported the inverse of Renschs rule, and clade-level patterns matched predictions from quantitative genetic models in which stronger directional selection on females than on males explains female-biased SSD. However, interspecific variation in SSD was not explained by female fecundity. Instead, mating system and flight dimorphism best predicted variation in SSD and SShD: males engaging in short-term mate guarding (less than a few days) were larger and stockier than searching males or those exhibiting prolonged mate guarding, reducing SSD, whereas species with flight-capable males and flightless females showed increased dimorphism, largely due to smaller males. Habitat and climate had limited effects. Sex differences in growth rate and development duration contributed equally to SSD, with females growing faster and for longer, consistent with widespread protandry. These results indicate that while fecundity selection historically drove female-biased SSD, contemporary variation is primarily shaped by male-specific selection through mating system and flight-related ecological pressures.

evolutionary biology↗

bioRxiv2023

Divergence time and environmental similarity predict the strength of morphological convergence in stick and leaf insects

Independent evolution of similar traits in lineages inhabiting similar environments (convergent evolution) is often taken as evidence for adaptation by natural selection, and used to illustrate the predictability of evolution. Yet convergence is rarely perfect. Environments may not be as similar as they appear (e.g., habitats scored the same may be heterogenous to the organisms). And lineages can evolve in different ways even when submitted to the same environmental challenges, because responses to selection are contingent upon available genetic variation and independent lineages may differ in the alleles, genetic backgrounds, and even the developmental mechanisms responsible for the phenotypes in question. Both impediments to convergence are predicted to increase as the length of time separating two lineages increases, making it difficult to discern their relative importance. We quantified environmental similarity and the extent of convergence to show how habitat and divergence time each contribute to observed patterns of morphological evolution in stick and leaf insects (order Phasmatodea). Dozens of phasmid lineages independently colonized similar habitats, repeatedly evolving in parallel directions on a 26-trait morphospace, though the magnitude and direction of these shifts varied. Lineages converging towards more similar environments ended up closer on the morphospace, as did closely related lineages, and closely related lineages followed more parallel trajectories to arrive there. Remarkably, after accounting for habitat similarity, we show that divergence time reduced convergence at a constant rate across more than 60 million years of separation, suggesting even the magnitude of contingency can be predictable, given sufficient spans of time. Significance statementPhasmids (stick and leaf insects) exemplify the extraordinary power of natural selection to shape organismal phenotypes. The animals themselves are charismatic champions of crypsis and masquerade; and our characterization of their adaptive radiation reveals dozens of instances of convergence, as lineages adapted to similar changes in habitat by repeatedly evolving similar body forms. Our findings show that the similarity of environmental conditions experienced by the organisms - the closeness of the invaded niches - and the extent of elapsed time since divergence, both predict the strength of morphological convergence. The phasmid radiation reveals an evolutionary process that is surprisingly predictable, even when lineages have been evolving independently for tens of millions of years.

evolutionary biology↗

bioRxiv2021

A second view on the evolution of flight in stick and leaf insects (Phasmatodea)

The re-evolution of complex characters is generally considered impossible, yet, studies of recent years have provided several examples of phenotypic reversals shown to violate Dollos law. Along these lines, the regain of wings in stick and leaf insects (Phasmatodea) was hypothesised to have occurred several times independently after an ancestral loss, a scenario controversially discussed among evolutionary biologists. Here, we revisit the recovery of wings by reconstructing a phylogeny based on a comprehensive taxon sample of over 500 representative phasmatodean species to infer the evolutionary history of wings. We additionally explored the presence of ocelli, the photoreceptive organs used for flight stabilisation in winged insects, which might provide further information for interpreting flight evolution. Our findings support an ancestral loss of wings and that the ancestors of most major lineages were wingless. While the evolution of ocelli was estimated to be dependent on the presence of (fully-developed) wings, ocelli are nevertheless absent in the majority of all examined winged species and only appear in the members of few subordinate clades, albeit winged and volant taxa are found in every lineage. The disjunct distribution of ocelli substantiates the hypothesis on trait reacquisition and that wings were regained in Phasmatodea.

evolutionary biology↗