Evolutionary transitions to selfing in Caenorhabditis nematodes and the life cycle of a laboratory-adapted C. elegans population. (A) Transitions in reproductive systems and breeding modes that three Caenorhabditis nematode species may have undergone in their history (black arrows). Like most extant Caenorhabditis species, the ancestral populations had a dioecious reproduction system until the appearance of a mutant female turned self-compatible hermaphrodite (grey arrow), which transformed them into trioecious populations. Females and hermaphrodites can only mate with males. Successful transitions to selfing imply invasion of hermaphrodites, loss of females and the maintenance of mixed selfing and outcrossing under androdioecy, or further loss of males and exclusive hermaphroditism (which is here defined as monoecy since we ignore mutation in sex determination). (B) The four-day discrete non-overlapping life cycle of a lab-adapted population, ancestral to all populations here constructed, experimentally evolved and characterised. With the exception of NaCl, the life cycle and population census sizes were the same as the experimental evolution here reported. Grey lines indicate the time points of experimental manipulation. Between 24 hours of the life cycle, when the first larval staged L1 individuals are seeded in solid NGM-lite plates, until 96 hours, when adults reproduce and give rise to the embryos that will constitute the next generation, NaCl concentrations in the media were 25 mM. Increased NaCl concentrations to 305 mM is here used as the novel environment.