On the Evolution of Complex Life Cycles in Plants: A Review and an Ecological Perspective

Abstract

Complex life cycles and alternation of generations are characteristic of many plants, a diploid sporophyte typically alternating with a haploid gametophyte. The prominence of each generation varies greatly among taxa. Purely phylogenetic or morphogenetic explanations of these differences are unsatisfying, as are those based solely on population fitness. Existing adaptational explanations seek selective advantages in diploidy and in sexual reproduction, but these explanations leave much to be explained-i.e., the existence of asexuality and of haploid organisms. Much of the existing variation in life cycles can be explained by selection on reproductive rates to meet the ecological problems of dispersal, colonization, niche preemption and exploitation, and mate competition. Placement of complex life cycles of plants in an ecological framework will, I hope, encourage specific studies exploring possible adaptive aspects and limitations on the evolution of life cycles in plants with different life histories. A prominent feature of most textbook introductions to various major groups of plants is the description of reputedly typical life cycles of selected forms. Attention is focused on the alternation of haploid and diploid phases (or generations) of the cycle, the relative dominance of one phase or another, the timing of meiosis and fertilization, the occurrence of asexual reproduction, and which phase is the dispersal or resting stage. Most of the available published literature on plant life cycles is directed toward elucidating developmental, morphological or physiological and, finally, phylogenetic patterns. A rather small number of papers (see below) addresses the possible selective advantages or disadvantages of reproducing sexually or asexually, of being haploid or diploid, or of having more than one morphological generation. Here, I use the word generation to refer to both the alternating diploid and haploid entities and to any intervening, asexually produced, progenies. So far as I can determine, virtually nothing has been written on the ecology (aside from genetics) of complex life cycles in plants. Several botanical reviewers have claimed that all of the ideas summarized here regarding the ecology and evolution of plant life cycles are old hat. They should be, perhaps, but no one has shown me evidence that they are. This review is presented in hopes of stimulating discussion and research that include an ecological perspective. I begin by summarizing some apparently classic life cycles of selected plants (broadly defined to include fungi, but excluding prokaryotes). There follows a review of available hypotheses attempting to explain the evolution of such cycles in plants and a critique of these hypotheses. Finally, I review some of the liter