Phenotypic selection in natural populations: what limits directional selection?

Studies of phenotypic selection document directional selection in many natural populations. What factors reduce total directional selection and the cumulative evolutionary responses to selection? We combine two data sets for phenotypic selection, representing more than 4,600 distinct estimates of selection from 143 studies, to evaluate the potential roles of fitness trade-offs, indirect (correlated) selection, temporally varying selection, and stabilizing selection for reducing net directional selection and cumulative responses to selection. We detected little evidence that trade-offs among different fitness components reduced total directional selection in most study systems. Comparisons of selection gradients and selection differentials suggest that correlated selection frequently reduced total selection on size but not on other types of traits. The direction of selection on a trait often changes over time in many temporally replicated studies, but these fluctuations have limited impact in reducing cumulative directional selection in most study systems. Analyses of quadratic selection gradients indicated stabilizing selection on body size in at least some studies but provided little evidence that stabilizing selection is more common than disruptive selection for most traits or study systems. Our analyses provide little evidence that fitness trade-offs, correlated selection, or stabilizing selection strongly constrains the directional selection reported for most quantitative traits.     

Trait selection in flowering plants: how does sexual selection contribute?

By highlighting and merging the frameworks of sexual selectionenvisioned by Arnold (1994) and Murphy (1998), we discuss howsexual selection can occur in plants even though individualsdo not directly interact. We review studies on traits that influencepollen export and receipt in a variety of hermaphroditic andgynodioecious plants with the underlying premise that pollinationdynamics influences mate acquisition. Most of the studies reviewedfound that phenotypes that enhance pollen export are in harmonywith those that enhance pollen receipt suggesting that in manycases pollinator visitation rates limit both male and femalefunction. In contrast, fewer traits were under opposing selection;but when they were, the traits most often were associated withenhancing the specific aspects of a given sex function. Ourreview helps clarify and illustrate why sexual selection canbe a component of trait evolution in hermaphrodite plants.     

What is hierarchical selection?

It has been proposed that natural selection occurs on a hierarchy of levels, of which the organismic level is neither the top nor the bottom. This hypothesis leads to the following practical problem: in general, how does one tell if a given phenomenon is a result of selection on level X or level Y. How does one tell what the units of selection actually are? It is convenient to assume that a unit of selection may be defined as a type of entity for which there exists, among all entities on the same “level” as that entity, an additive component of variance for some specific component F of fitness which does not appear as an additive component of variance in any decomposition of this F among entities at any lower level. But such a definition implicitly assumes that if f(x, y) depends nonadditively on its arguments, there must be interaction between the quantities which x and y represent. This assumption is incorrect. And one cannot avoid this error by speaking of “transformability to additivity” instead of merely “additivity”. A general mathematical formulation of the concepts of interaction and non-interaction is proposed, followed by a correspondingly modified approach to the definition of a unit of selection. The practical difficulty of verifying the presence of hierarchical selection is discussed.     

Kin selection in disguise?

In a recent article E.O. Wilson and B. H?lldobler (2005) describe an heuristic model for the evolution of eusociality. They present their model as an alternative to the standard model of kin selection, and describe the evolution of eusociality in terms of changes in frequency to an hypothetical eusocial allele. Here I build on sentiments of Foster et al. (2006) to suggest that the proposed model is not a clear alternative to the standard model, but appears to represent a special case of kin selection involving preferential interactions among individuals sharing the same altruistic gene. The model proposed by Wilson and H?lldobler is consistent with the ‘greenbeard’ model of kin selection, first proposed by W.D. Hamilton. Received 23 May 2006; revised 27 June 2006; accepted 5 July 2006.     

How strong is natural selection?

The strength of selection in nature has long been a controversial subject, partly because there were few quantitative measurements of phenotypic selection available until recently. In a new paper, Kingsolver and colleagues reviewed 63 studies and found that the median standardized directional selection gradient (a measure of the strength of phenotypic selection) was 0.16. Whether this means selection in nature is strong or weak depends both on one's point of view and on the error in selection estimates.     

Darwinian selection and “altruism”

Models are proposed for evolution at a single locus affecting altruistic behavior in which genotypic fitnesses are Darwinian and frequency (but not density) dependent. The fitnesses are composed, either in a multiplicative or an additive way, of factors which depend on the receipt and donation of altruistic behavior. The factors are determined from the matrices of conditional probabilities which describe the genotypes of relatives. Since selection occurs, these probabilities are in terms of genotype frequencies. The relationship between the risk to helper and benefit to recipient which allows altruism to evolve is shown to depend on the kinship coefficient between helper and helped, the particular fitness function proposed and the degree of dominance of the altruism. The commonly accepted criteria of W. D. Hamilton [J. Theor. Biol.7 (1964), 1–16, 17–52] apply only in the additive case. A second class of models of social cooperation independent of relationship and its evolutionary dynamics are discussed.     

Dimorphic male midshipman fish: reduced sexual selection or sexual selection for reduced characters?

In most taxa with male dimorphisms, some males are large inbody size with exaggerated secondary sexual characters (exaggeratedmorph), whereas other males in the same population are smalland have reduced secondary sexual characters (reduced morph).What selective pressures cause male dimorphisms? Reduced morphologiesmay result when a) some males develop a morphology that, inthe absence of sexual selection pressures for an exaggeratedmorphology, reduces energetic and developmental costs and/orb) some males opt for an alternative morphology that does wellat an alternative behavioral tactic such as cuckoldry. The 2mechanisms could act together, but each alone is theoreticallysufficient to drive dimorphisms. Here, we tested hypothesis"b" (sexual selection for reduced characters) in the plainfinmidshipman fish, Porichthys notatus. Behavioral plasticity betweenterritoriality and cuckoldry in an exaggerated male morph (typeI) allows for a direct comparison of cuckoldry by exaggeratedmorph males to cuckoldry by reduced morph (type II) males. Comparedwith type I cuckolders, type II cuckolders were able to remainnear the nest for longer periods before being chased by theterritorial type I male, suggesting that the reduced type IImorphology allows type II males to prolong the time before attackby territorial males. Combined with other studies showing arole of sexual selection in maintaining the exaggerated morph,the data support the "sexual selection for reduced characters"hypothesis and elucidate how sexual selection can act in differentways on different males to maintain 2 male morphologies withina single species.     

Disruptive selection and then what?

Disruptive selection occurs when extreme phenotypes have a fitness advantage over more intermediate phenotypes. The phenomenon is particularly interesting when selection keeps a population in a disruptive regime. This can lead to increased phenotypic variation while disruptive selection itself is diminished or eliminated. Here, we review processes that increase phenotypic variation in response to disruptive selection and discuss some of the possible outcomes, such as sympatric species pairs, sexual dimorphisms, phenotypic plasticity and altered community assemblages. We also identify factors influencing the likelihoods of these different outcomes.     

Does frequency-dependent selection optimize fitness?

In evolutionary biology, the axiom that natural selection tends ideally to maximize inclusive fitness of the individual or some other suitable quantity is often advanced (Cody, 1974; Maynard Smith, 1978; Krebs & McCleery, 1984; Houston et al., 1988). Moreover, the evolutionists generally distinguish two situations (Dawkins, 1980; Maynard Smith, 1982): one in which fitness is independent of the frequency of the phenotypes present in the population (frequency-independent selection), and one in which it does depend on this frequency (frequency-dependent selection). This led some authors such as Parker (1984), and more recently Parker & Maynard Smith (1990), to consider "a 2-speed optimization": frequency-independent selection should lead to a "simple optimum" at the end of the selective process, since all the individuals should have the same strategy and the mean fitness of the population should be maximized; frequency-dependent selection, formulated in terms of the theory of games, should lead to a "competitive optimum" even though the "evolutionary stable strategy" (or "ESS"; Maynard Smith & Price, 1973) characterizing the equilibrium "is not the strategy that maximizes fitness in a population sense" (Parker & Maynard Smith, 1990: 30). Our aim in this short communication is to criticize the concept of "competitive optimum" by Parker & Maynard Smith, as well as the general ability of natural selection to "maximize fitness", even in "phenotypic models" (Lloyd, 1977). These models, devoid of genetic constraints since each strategist is assumed to reproduce its own kind, are especially suitable for examining the ideal effect of natural selection.     

Sexual dimorphism in snake tail length: sexual selection,natural selection,or morphological constraint?

Male snakes typically have longer tails relative to body length than females, but the extent of this dimorphism varies among species. Three hypotheses have been suggested to explain tail dimorphism. The Morphological Constraint Hypothesis proposes that males have relatively longer tails to accommodate hemipenes and retractor muscles. The Female Reproductive Output Hypothesis proposes that females have relatively shorter tails as a secondary result of natural selection for increased reproductive capacity. The Male Mating Ability Hypothesis proposes that sexual selection favours relatively longer tails in males during courtship. These hypotheses make different predictions about the relationships among tail length, body size, male reproductive morphology, female reproductive output, mode of reproduction, and male mating behaviour among and within taxa. Predictions were tested using published data for 56 genera in the family Colubridae and original data for the water snake, Nerodia sipedon. Tail length dimorphism was more male-biased in tam having relatively short tails (r=–0.52, P < 0.001), hemipenes and retractor muscles occupied a greater proportion of the tail in taxa having relatively short tails (r=– 0.71, P < 0.00l and r=– 0.66, P = 0.001, respectively), and tail length dimorphism was more male-biased in taxa in which body size dimorphism was more female-biased (r=– 0.60, P < 0.001). These results support both the Morphological Constraint Hypotheses and the Female Reproductive Output Hypothesis. However, tests of other predictions, including those regarding patterns within N. sipedon , failed to support any of the three hypotheses. Comparisons among taxa suggest several species in which further tests of these hypotheses would be especially appropriate.     

Haldane’s view of natural selection

Among many things, J. B. S. Haldane is known for demonstrating how the principle of natural selection can be used to build a mathematical, and in particular quantitative, theory of evolution. However, to the end, he remained open to the idea of other evolutionary mechanisms. In his late writings, he repeatedly drew attention to situations in which natural selection did not operate, was hemmed in by constraints, or worked in a surprising manner. In this respect Haldane stands out among the architects of the Modern Synthesis.     

Natural selection: a phase transition?

Information has two aspects: a quantity to be called 'extent' and a quality which may be termed 'content' since it deals with meaning. The latter originates via selective self-organization, which can be described also in quantitative physical terms. A prerequisite is the reproducibility of the informational substrate forming the basis of selection. This paper focuses on selection being the analogue of a physical phase transition. In Section 1 the criteria for phase transitions are formulated. Section 2 introduces the concept of information space and describes information as selected points or regions in this space. In Section 3 selection is analyzed in terms of the criteria for phase transitions, and in Section 4 the concept is confronted with experimental data. The conclusion is reached that information content is generated via selection, which can be described as a phase transition in information space.     

Experimental studies of group selection: what do they tell us about group selection in nature?

The study of group selection has developed along two autonomous lines. One approach, which we refer to as the adaptationist school, seeks to understand the evolution of existing traits by examining plausible mechanisms for their evolution and persistence. The other approach, which we refer to as the genetic school, seeks to examine how currently acting artificial or natural selection changes traits within populations and focuses on current evolutionary change. The levels of selection debate lies mainly within the adaptationist school, whereas the experimental studies of group selection lie within the genetic school. Because of the very different traditions and goals of these two schools, the experimental studies of group selection have not had a major impact on the group selection debate. We review the experimental results of the genetic school in the context of the group selection controversy and address the following questions: Under what conditions is group selection effective? What is the genetic basis of a response to group selection? How common is group selection in nature?     

Does selection intensity increase when populations decrease? Absolute fitness, relative fitness, and the opportunity for selection

The variance in relative fitness, commonly called the “opportunity for selection,” is a measure of the maximum amount of selection that can occur in a population. I review the relation between fitness variance and population growth, showing that fitness variance is higher during periods of population decline. This is true both for survival and for commonly used models for variable descendant number (Poisson, negative binomial, generalized Poisson). Empirical evidence suggests that not just the opportunity for selection but also the actual selection occurring is commonly greater during such periods of population reduction.     

Strain selection for β-carotene production by Dunaliella

Four strains of Dunaliella were grown at 25°C and pH 8±0.5, with continous illumination at 200 W/m². Their maximum specific growth rates ranged from 0.093 day^-1 to 0.234 day^-1, nitrate yields from 3.0 to 7.8 g cells/g NaNO₃ and lipid contents from 3% to 6% of the dry wt, with carotenes 50 to 80% of the lipids. Of the carotenes, -carotene made up 7 to 19%; all-trans--carotene 32 to 52% and 9-cis--carotene 29 to 55%. There are, therefore, considerable intra-specific differences between strains of Dunaliella.