Evolution of alternation of haploid and diploid phases in life cycles

Eukaryotic sex leads to an alternation of haploid and diploid nuclear phases. Because all multicellular animals are diploid, diploidy is often considered a 'biological success' and many arguments have been advanced to explain the evolution of a prolonged diploid phase. Nevertheless, among eukaryotes three basic situations are encountered, where the vegetative individuals are diploid or haploid or both. These three basic life cycles are widely distributed among kingdoms and in some taxa the occurrence of different life cycles within the same species has been reported. This article briefly summarizes the different hypotheses on the evolution of reproductive life cycles and underlines how possibilities of variation for this trait may open new perspectives for research.