Testing Weissman’s Lineage Selection Model for the Maintenance of Sex: The Evolutionary Dynamics of Clam Shrimp Reproduction over Geologic Time

摘要

One of the most perplexing questions within evolutionary biology is: “why are there so manymethods of reproduction?” Contemporary theories assume that sexual reproduction shouldallow long term survival as dispersal and recombination of genetic material provides apopulation of organisms with the ability to adapt to environmental change. One of the mostfrustrating aspects of studying the evolution of reproductive systems is that we have notyet been able to utilize information locked within the fossil record to assess breedingsystem evolution in deep time. While the fossil record provides us with information on anorganism’s living environment, as well as some aspects of its ecology, the preservation ofbiological interactions (reproduction, feeding, symbiosis, communication) is exceedinglyrare. Using both information from extant taxa uncovered by a plethora of biological andecological studies and the rich representation of the Spinicaudata (Branchiopoda:Crustacea) throughout the fossil record (from the Devonian to today), we address twohypotheses of reproductive evolutionary theory: (1) that unisexual species should be shortlived and less speciose than their outcrossing counterparts and (2) that androdioecy(mixtures of males and hermaphrodites) is an unstable, transitionary system that shouldnot persist over long periods of time. We find no evidence of all-unisexual spinicaudatantaxa (clam shrimp) in the fossil record, but do find evidence of both androdioecious anddioecious clam shrimp. We find that clades with many androdioecious species are lessspeciose but persist longer than their mostly dioecious counterparts. These data suggestthat all-unisexual lineages likely do not persist long whereas mixtures of unisexual andsexual breeding can persist for evolutionarily long periods but tend to produce fewerspecies than mostly sexual breeding.