Showing papers by "Michael Lynch published in 1986"

Phenotypic evolution by neutral mutation

Abstract: A general model is developed for predicting the genetic variance within populations and the rate of divergence of population mean phenotypes for quantitative traits under the joint operation of random sampling drift and mutation in the absence of selection. In addition to incorporating the dominance effects of mutant alleles, the model yields some insight into the effects of linkage and the mating system on the mutational production of quantitative-genetic variation. Despite these additional and potentially serious complications, it is found that, for small populations, the simple predictions obtained by previous investigators using additive-genetic models hold reasonably well. Even after accounting for dominance and linkage, the equilibrium level of genetic variance is unlikely to be much less than 2NVm or to be more than 4NVm , where N is the effective population size and Vm is the new variance from mutation appearing each generation. The rate of increase of the between-line variance per generation ultimately equals 2Vm regardless of population size, although the time to attain the asymptotic rate is proportional to N. Expressions are presented for the rate of approach to the equilibrium level of genetic variance and for the expected variance of the within-population and between-population genetic variances. The relevance of the derived model, which amounts to a generalization of the neutral theory to the phenotypic level, is discussed in the context of the detection of natural selection, the maintenance of pure lines for biomedical and agricultural purposes, the development of genetic conservation programs, and the design of indices of morphological distance between species.

Measurement of the carbon balance in Daphnia

Abstract: Short term measures of assimilation and respiration rates of four species of Daphnia consistently lead to the prediction that the relation between body mass and net carbon intake, F(B),should be close to linear. Yet a more direct estimation of F(B)based on observed investments in growth and reproduction indicates that the true relationship is nonlinear with a roughly constant value of F maintained beyond the size at maturity. Several experiments demonstrate that our direct measures of F(B) are not greatly influenced by the experimental protocol. Hence, we conclude that short term measures of physiological parameters cannot be extrapolated to estimate growth and reproductive rates of Daphnia. Despite the existence of significant physiological differences between species, our results further reveal a conservative relation between age and the pattern of energy allocation to growth and reproduction in the genus and suggest that the evolution of Daphnia life histories is strongly regulated by one key character-the size at maturity. The intermediate position of cladocerans cific functions of clearance, assimilation, and in the food chain and their amenability to respiration rates from various sources into experimental manipulation have stimulatgeneral models of size-specific net energy ed a long tradition of research on their enintake (Threlkeld 1976; Hall et al. 1976; ergetics. A general assumption of this work Lampert 1977a; Lynch 1977). Such models is that the difference between short term have been used to generate expected patmeasures of assimilation and respiration can terns of size-specific variation in competibe extrapolated to provide more long term tive ability, vulnerability to starvation, and estimates of secondary production (growth life history expression and evolution. The plus reproduction). Unfortunately, research accuracy of the predictions of these models in this area has progressed in a rather disis obviously a function of the validity of the organized fashion. With the exception of the assumptions that interspecific variation in early work by Richman (1958) and Schindthe size specificity of energetics is not great ler (1968), virtually no one has simultaand that the pooling of data from different neously measured assimilation and respistudies (often with very different experiration. Controlled interspecific comparisons mental settings) is not of major conseare even rarer. Burns (1969), Downing quence. A third problem is the possibility (1981), and DeMott (1982) provide some that differences between short term assiminformation on clearance rates, but almost ilation and respiration measurements may no comparative data are available for asnot in fact be reflected in realized growth similation or respirationthe two energetic and reproduction. subcomponents that ultimately determine In order to examine these problems we the net energy intake of individuals. raised four species of Daphnia in compaThis lack of data has not discouraged sevrable environments and measured their sizeeral individuals from combining size-spespecific rates of feeding, assimilation, respiration, growth, and reproduction. Our results reveal that the complexities adSupported in part by NSF grant DEB 79-1 1773 to dressed in the previous paragraph are inM. Lynch and a fellowship from the Max-Planck-Indeed real and that future work in this area stitut to L. J. Weider. should proceed with much more attention * Permanent address: Department of Ecology, Etholto detail than it has in the past. We thank ogy, and Evolution, University of Illinois, 606 E. HeaB. Heckt and K. Spitze for assistance, and ley St., Shelford Vivarium, Champaign 6 1820. Permanent address: Department of Biology, UniW. DeMott and J. Jacobs for helpful comversity of Windsor, Windsor, Ontario N9B 3P4. ments.

Random drift, uniform selection, and the degree of population differentiation.

Abstract: Evolutionary biologists have long been interested in the forces that are responsible for the apparent degree of uniformity between conspecific populations. Since random drift will ultimately result in the differentiation of isolated populations, the prevention of divergence requires interpopulational gene flow and/or the operation ofsimilar selection pressures on the isolates. Ehrlich and Raven (1969) argued that uniform selection, not gene flow, is the primary cohesive force in evolution. Many authors have subsequently sided with them, although there now appears to be greater appreciation for the subtle ways in which gene flow and selection may interact (Levin, 1979; Levin and Kerster, 1974; Endler, 1977). ~ A recent paper by Cohan (1984) challenges the very roots ofthe uniform-selection idea, arguing that instead of resulting in convergent evolution, uniform selection operating on finite populations may often cause more divergence than expected under drift alone. In short, the expected probability of two populations fixing alternate alleles under random drift alone is 2p(1 p). Directional selection increases the probability of fixation of the favored allele while decreasing the probability of fixation of the alternate allele. Thus, since 2p(1 p) is maximizedatp = 0.5, selection will always decrease the probability of fixation of alternate alleles when the initial frequency ofthe favored allele is greater than 0.5. However, if p is small enough (the critical frequency depending on the intensity of selection), the probability that two populations under uniform selection will ultimately become fixed for alternate alleles will be greater than 2p (1 p). This is not the first suggestion that the dynamics of gene frequencies of finite populations under selection may be radically different than those of infinite populations. Robertson (1962), for example, demonstrated that selection for heterozygotes (which ensures a polymorphism in an infinite population) may actually accelerate the rate of fixation in small populations, provided the \"equilibrium\" gene frequency is sufficiently extreme. In contrast to Cohan's (1984) results for alleles that have an additive effect on fitness, however, uniform heterotic selection enhances the probability of fixation of the same allele in different populations. If the phenomenon pointed out by Cohan (1984) occurs frequently, it has significant implications for our ability to demonstrate the operation of directional selection in natural (including fossil) populations. One way to test for the operation of uniform selection on conspecific populations is to ask whether the population mean phenotypes are more similar to each other than expected for populations under random drift alone (see Lande [1977] for the case of divergent selection). This is not an easy task, since the latter process depends on effective population sizes and the time since isolation, neither ofwhich is readily determined in nature. IfCohan (1984) is right, however, even when such tests are possible, there would be little point in performing them because the results would be ambiguous. In the case of directional selection, the uniform selection hypothesis might be evaluated with time-sequence data that allow one to test whether the directional changes in mean phenotypes are more similar than expected by chance, provided common environmental effects can be ruled out. However, estimates of interpopulational variances would be uninformative. There are a few reasons why the mechanism proposed by Cohan (1984) may be of less general significance than he suggested. As can be seen in Figure 1, the total gain in probability mass caused by selection for low frequency alleles is always less than the loss of probability mass for high frequency alleles. Thus, in order for selection to cause a net enhancement of divergence at the allelic level, the initial frequencies of most favored alleles have to be less than 0.5 (considerably less than 0.5 for large Ns). Such circumstances are not impossible, and they might be expected for the constituent loci of a character that has recently been subjected to a change in the direction of selection. Cohan (pers. comm.) has also pointed out how periods of stabilizing selection punctuated by directional selection would be conducive to his hypothesis. Under these conditions, and with no overdominance, purifying selection might normally keep the most favorable allele at a locus near fixation. A new phase of directional selection would then often favor alleles that were kept at low frequencies during the previous period of stabilizing selection. However, contrary to Cohan (1984), it cannot be assumed \"that an allele never previously favored by selection will be in low frequency.\" Certainly, this will not be true in the case ofeffectively neutral alleles. In order to see the importance of the initial gene frequency distribution, consider the case of initially neutral alleles. This is appropriate since the null model is the case of pure random drift, and it may be of relevance to some ofthe studies in Cohan's (1984) table 1 that concern selection responses to completely novel challenges (pesticide and heavy metal tolerance). Provided there are a large number of possible alleles at a