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Life-History Strategies and the Effectiveness of Sexual Selection

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Opinion is intended to facilitate communication '&etween reader and author and reader and reader. Comments. viewpoints or suggestions arising from published papers are welcome. Discussion and debate about imponant issues in ecology. e.g. theory or terminology. may also be included. Contributions should be as precise as possible and references should be kept to a minimum. A summary is not required. Life-history strategiesand the effectivenessof sexual selection Kirk O. Winemiller, Environmental SciencesDivision, Oak Ridge NtIlional Labonllory, Oak Ridge, TN 37831-6036, USA Summary. The hypothesized relationship betWfttl the relati~e stage with the smallest average fitness (Arnold and streftlth of sexual seltdklll and life-history 5tr8tt1it5 is reexa- Wade 1984). I reexamine McLain's hypothesis and offer mined. The potential erredi~mess of sexual selection depends not only on tM relati~e 5Urvi~onhip of Immature staan, but two refinements to the problem. also on oChercomponmts of fitness. The effects or fecundity and McLain focused attention entirely on the effects of timina of maturation must be e~aluated toaeiMr with tbe sun-i- differential juvenile survivorship, and noted that the life ~orsbip In order to determine tbe reponsl~mess of alternati~e stage with lowest survivorship makes the largest contri- lire-history connaurations to the force of sexual selection. Mor~ o~er, tM r-K continuum is an inadequate model for compari- bution to total variation in lifetime reproductive success sons or lire-history strategies. A generai three-dimensional de- (LRS). According to Brown (1988), the overall variance mographic model pro~ides a more compre1ttnsi~e conceptual in lifetime reproductive success among breeders and rramtwork for lif~history comparisons. The thrft-p8ramtter nonbreeders can be partitioned into two components. demographic model is similar to se~eralearlier two-dimensional lire-history schemesand appears to describe a broad spectrum the proportion due to variance among individuals that of tM life-history strattaies exhibited In nature. Most hicher breed and the proportion due to complete failure to taxa tend to be dominated by only one or two of the three breed. These two components comprise the right half of endpoint stratqies: "equilibrium" (Iarae in~estmmt in rela- the equation, ti~ely rew indi~idual offspring). "opportunistic" (llnail size and rapid maturatkllll. and "ptriodk" (pulsed production of large numbers of small offspring). A sun-ey or teleost fIShes and Total variation in LRS - pV(LRS) + P(l-p) '[iS2. (I examples from se~tral other hlaher taxa supports McLain's (19911contention that the 5treDIih of sexual selection is iDnUtO- in which V(LRS) is the variance in lifetime reproductive ctd by life-history strategy. Conspicuous males are common amona relati~e equilibrium and opportunistic strategists, but succe.!!.:..£is the proponion of adults that actually breed. are essentially absent among speciesassociated wltb tM high- and LRS2 is the squared mean lifetime reproductive recundlty. periodic reproducti~e strattay. The absenceof sex- success. In this case. LRS is estimated from data in- ually selectedtraits In hlah-fecundlty broadcast spawners Im- volving breeding adults only. McLain illustrated this plies that differential sur~l~onhlp among Immature life staResis nonrandom in all cases. differential contribution to total variance in LRS using an example for a population in which some individuals fail to breed. where the contribution to total lifetime reproductive successdue to failure to breed is equal to -, Selection on life stages P(1-p)LRS- - -(l-p)[RS:-- P(1-p)[RS2 + pV(LRS) (l-p)LRS! + V(LRS) In a recent FORUM contribution. McLain (1991) pos- tulatcd that the relative strength of sexual selection may As p decreases.the relative contribution of nonbreeders depend upon the life-history strategy. Here sexual se- to total variation in LRS increases. lection j!; equated with differential mating successand For the purpose or establishing the argument that the its effect on the evolution of traits with no apparent relative strength of sexual selection should he innuen- survival value or even negative survival value. The idea ced by life-history attributes. McLain equates p with the seemsto have merit. becausedirectional selection on a proportion of individuals in a population cohort that phern)type is primarily associated with the life cycle survive to the age of maturation. Populations that ex- 3J~ 0{~...\2 CION hibit high mortality in the immature stages will have relatively small contributions to total variance in LRS from variation in breeding success(i.e., the opportunity (2) for sexual selection will be weak), because the greatest contribution to variance in LRS comes from differential where for a stablepopulation m, is age specificfecun- survivorship among the early life stages. This assumes dity, I, is agespecific survivorship. and w is the last age that genes responsive to sexual selection are in some classof active reproduction. In tllis form of the equa- way expressed during the immature stage. Using the tion, two componentsof reproductivevalue are added familiar r-K life-history continuum to advance the basic together: the current investmentin offspring (m,) and argument, McLain proposed that relatively r-selected the expectationof future offspring (residualreproduc- populations under predominantly density-independent tive value). Reproductivevalue is thereforeequivalent regulation should exhibit little response to sexual selec- to the lifetime expectationof offspring, and contains tion, because most of the failure to breed results from survivorship, fecundity, and timing components.The mortality in the immature life stages. In the balance intrinsic rate of population increasecan be approxi- between sexual selection and natural selection, natural mated as selection would have the greatest relative effect in those organisms exhibiting high mortality in large broods. InRo Sexual selection can contribute more toward variance in r- (3) LRS in K-selected species that have relatively high sur- T vivorship at the immature life stage. McLain also noted where Ro is the net replacement rate. T is the mean the possible effect of sexual selection as a directional generationtime. and (diversifying) force, one that may elevate rates of spe- ciation among K-selected species living in density-de- In 1:1,m, pendent settings (West-Eberhard 1983). Ro = 1:11mI, resultingin r = Two important points about life-history strategies T (4) were not covered in McLain's paper: 1) the effects of multiple components of fitness, and 2) the manner in Therefore, the relative rate of genotype or population which the unidimensional life-history model shapescor- increase (i.e., relative reproductive success)is directly roboration of the theory. Upon initial examination, the dependent upon fecundity, timing of reproduction, and first point seemsto pose problems for the theory. Yet, survivorship (Southwood et al. 1974, Southwood 1977, consideration of a multidimensional life-history contin- 1988, Michod 1979, Roff 1984, Sutherland et al. 1986). uum lends new empirical support for the general conclu- In terms of episodic selection, the strength of sexual sion that life-history strategies may indeed influence the selection is influenced not only by the negative correla- potential strength of sexual selection. tion between fecundity and juvenile survivorship, but also influenced by the age of maturation and the relative length of reproductive life. Components of fitness McLain (1991) dealt with fecundity (i.e.. clutch size) and survivorship as the sole components of fitness. Here I argue that inclusion of a third axis, timing of reproduc- Multidimensional life-history continuum tion, is essential to comparisons among general life- Given the multivariate nature of life-history strategies history strategies. I will not deal with quantitative ge- and the recognition that the r-K continuum is inade- netic measures of fitness (Lande 1982. Arnold and quate for discussions of the implications of life-history Wade 1984). and assume only that a sufficient genetic configurations, I will reexamine the relative effect of basis (e.g., heritable phenotypes. appropriate genetic life-history on the strength of sexual selection based on correlations in polygenic systems) and selective envi- a three-dimensional mode! of life-history evolution. If ronment are required for selection to occur. In life- two parameters and one dimension are insufficient for history theory, the demographic estimate of fitness is understanding the most basic life-history trade-offs. usually either V,. the reproductive value of an individ- then how many parameters and dimensions are re- ual or age class (Fisher 1958), or r. the intrinsic rate of quired? Three endpoint life-history strategies have been natural increase of a genotype or population (Birch identified in comparative studies of diverse taxonomic 1948. Cole 1954, Southwood 1988). Each of these esti- groups. including plants (Grime 1977. 1979). zooplank- mates contains three essential components: survivor- ton (Allan 1976), insects (Southwood 1977. Greenslade ship. fecundity, and the onset and duration of reproduc- 1983). fishes (Kawasaki 1979. Baltz 1984. Winemiller tive life. In the case of reproductive value. 1989). and reptiles (Tinkle et al. 1970. Dunham .lnd JIKOS /1:\1 (IW1) 319 mizes a disallows attainment of large body size and high fecundity. This early-maturation, small-size, low-fecun- dity tactic could
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