Evolution and Life Histories of Brown Algae

Life histories are of central importance in understanding evolution and phylogeny of brown algae. Like other hereditary traits, life history characteristics evolve by processes of natural selection, but because they are important determinants of biological fitness they have special evolutionary significance. Concepts of life history, as traditionally applied to brown algae, do not adequately reflect this, and they need to be broadened to include consideration of additional characteristics such as longevity and reproductive span. Life histories can be interpreted as adaptive strategies. Experimental evidence indicates that heteromorphic life histories probably evolved in response to seasonal change. Isomorphic life histories are possibly adapted to stable environments, although some may also possess certain features which are adaptations to seasonal change. Life histories that lack an independent gametophyte generation may have evolved through reduction of heteromorphic life histories. It is argued that a significant increase in the longevity of sporophytes may have been critical for the evolution of life histories lacking a free-living gametophyte, and also for the evolution of oogamy, phenomena which have occurred in several brown algal evolutionary lines. The common absence of asexual reproduction in advanced taxa probably indicates that its accessory ecological role in maintaining population size has become redundant, as well as reflecting the advantage of sexual over asexual reproduction. However, there is good evidence that sexual reproduction has been lost in a few species of brown algae, and the possible mechanisms and adaptive significance of this are discussed. exert a direct and immediate influence on reproducIt has been generally assumed that brown algae origtion (Istock 1982), and therefore play a large part in inated around the time eukaryotes first appeared on determining the biological fitness of a species, earth (Schöpf 1970, Schöpf et äl. 1973). The fossil evidence, however, is highly inconclusive, and recent ^ Concept of Life History rRNA studies (Lim et al 1986) have raised the possibility that the brown algae may be much younger The term 'life history' implies the sum of all develthan previously thought. Brown algae arose from opmental and reproductive events that occur during simple chrpmophyte ancestors, and have evolved allife in individuals, populations or species. The key most exclusively in a marine environment. This paper events in a life history are fertilization and meiosis, discusses the fundamental role of life history characand the associated processes of gamete and meiospore teristics in their evolution. Life history characteristics formation. Also important are any asexual and vegare of special importance in evolution because they etative reproduction of the gametophyte and sporophyte generations. These comprise the essential components of those skeletal flow diagrams which are 1 Paper presented at a Symposium 'Life histories of marine commonly used to summarize a life history. Life hisa£S±nE££££3£? £! £ "* TM *** *·— ·** *· that ** 1987. almost invariably based on very few culture studies Botanica Marina / Vol. 31 / 1988 / Fasc. 5 Copyright © 1'988 Walter de Gruyter · Berlin · New York 380 Clayton: Evolution and life histories of brown algae which rarely make an attempt to examine variation within populations, or the species as a whole. Therefore, the diagrams can be misleading. In addition to the essential features of a life history, there are several other very important attributes or characteristics that deserve close examination. They include the longevity of each generation, its reproductive span, the relative frequency of sexual and asexual or vegetative reproduction, and the seasonality of growth and reproduction. These characteristics have received little attention from phycologists although they have been pivotal to attempts to develop a general theory of life history evolution (Pianka 1970, Stearns 1976, Istock 1982). The possible significance of these additional life history characteristics in the evolution^ of brown algae is elaborated in this paper. The narrower, traditional view of brown algal life histories may well reflect the main interests of the phycologists that studied them. These interests have been concerned primarily with classification or with elucidating details of reproduction, rather than with trying to understand how species have evolved. In his comprehensive review of variation and selection in macroalgae, Russell (1986) drew attention to this deficiency, commenting that algal life history studies reflected a preoccupation with the number of different thallus forms and reproductive bodies, rather than the functional characteristics of thalli and the role of reproduction in maintaining populations. He remarked that, "It is not flow diagrams which evolve . but particular phenotypes" (Russell 1986, p. 343). Before it is feasible to construct reasonably convincing hypotheses of phaeophyte evolution and phylogeny, it will be necessary first to adopt a broader, more sophisticated concept of life history, one that is consistent with a sound understanding of evolutionary processes. Life History Patterns in the Brown Algae There are three kinds of sexual life history in brown algae, differing in the relative morphological elaboration of the gametophyte and sporophyte generations: i) an alternation of heteromorphic generations, ii) an alternation of isomorphic generations, and iii) life histories in which the sporophyte is the only free-living generation (Table I). One way of regarding a life history is as a complex adaptation, a successful solution to a particular ecological problem, or, to use fashionable 'paramilitary' jargon (Russell 1986), an 'evolutionary tactic or strategy'. The relative consistency of life history patterns within most orders of brown algae suggests that life Table I. Life histories in brown algae.