We published a comprehensive phylogeny of a model spider group, the family Nephilidae, known for female and web gigantism and for extreme sexual size dimorphism with males over 100 times lighter than the females. These spiders are therefore model organisms for many biological studies, but prior phylogenies have been deficient and incomplete. Our paper adds a new phylogeny, based on modern phylogenetic analyses using 367 genetic markers and a new fossil time calibration. The work provides the foundation for understanding nephilid evolutionary history. Specifically, it finds no support for biological rules, such as Rensch's and Cope's Rules, as well as for sexually correlated size evolution.
COBISS.SI-ID: 44009773
Sexual size dimorphism (SSD) is one of the most striking animal traits. While male-biased SSD is common (particularly in vertebrates) and is readily explained by sexual selection for larger males, female biased SSD is not confined to particular clades but is evidently a more complicated outcome of gendered evolutionary drivers whose strength and direction are diverse. Extreme SSD (eSSD), which is defined as females twice or more the male size, has been identified in 16 spider clades. We propose drivers of SSD in spiders within the differential equilibrium model and defined the eSSD mating syndrome. We hypothesized that in the long run, the costs of female gigantism might become very high, and result in an evolutionary dead end.
COBISS.SI-ID: 5289551
We investigated whether smaller males of an exceptionally sexually dimorphic spider from South Africa, Nephilingis cruentata, a species that also exhibits a wide range of male sizes, indeed have an advantage in vertical climbing and bridging. If so, this could explain the maintenance of small males in the evolution of nephilid spiders that are notorious for sexual size dimorphism. In laboratory experiments, males of various sizes were subjected to climbing and bridging tests. Surprisingly, it was larger, not smaller males that were faster climbers, against the predictions for small male advantage. No differences in bridging ability was found across male sizes. These results suggest that locomotor abilities do not explain the evolutionary maintenance of small males in this sexually dimorphic species, and therefore, other selection pressures should be explored.
COBISS.SI-ID: 44831789