Morphological convergence among grasslandadapted clades of South American ghost spiders

NICOLAS MONGIARDINO KOCH; F. SARA CECCARELLI; EDUARDO M. SOTO; MATÍN RAMÍREZ

Portland, Oregon

Congreso; Evolution 2017; 2017

Society for the Study of Evolution (SSE), the Society of Systematic Biologists (SSB), and the American Society of Naturalists (ASN).

Adaptive morphological convergence is not only interesting as one of the most striking examples of evolutionunder natural selection, it is also key towards understanding the relative contribution of contingency inevolutionary biology. The repeated origin of equivalent solutions to similar problems illustrates the degree ofdifference that we should expect from multiple replays of life's tape, in Gould's famous metaphor. We explorethis topic by studying the origin of morphological adaptations that allow the exploitation of grassland nichesamong South American ghost spiders (Amaurobioidinae, Anyphaenidae). We infer a timecalibratedmolecular phylogeny for the clade, which confirms that grassland associated adaptations arose at least threeindependent times, as the rise of the Andes transformed vast areas of Patagonia from a forest to a grassdominated steppe. To better understand if these independent events of adaptation represent multiple findingsof the same peak in a macroevolutionary adaptive landscape, we compiled a comparativemorphometric dataset of ecologically relevant traits. We then explored the fit of several maximum likelihoodmodels of macroevolution in a multivariate framework, including several multi OrnsteinUhlenbeckmodels.After accounting for diverse sources of uncertainty, we find strong support for a macroevolutionary adaptivelandscape composed of one forest and two grasslandassociatedpeaks. This confirms that two grassadaptedgenera have converged completely, having found the same adaptive peak independently, while athird genus is best described as a different grassassociatedecotype. Although there is strong statisticalsupport for the fact that grassland adapted anyphaenids have acquired different morphologies, they all shareone trait: a reduced distance between the chelicerae. This slender configuration of the cephalothorax allowsspiders to extend the first two pairs of legs towards the front and fit within the blade of grass, and seems to bea necessary condition for the successful colonization of grasslands.