Developmental instability is heritable - Wiley Online Library

0 Birkhtiuser Verlag, Base], 1997 J. evol. biol. IO (1997) IOIO-061X~97,'010069-08

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A. P. Moller’.* and R. Thornhill’ ‘Laborutoire d’Ecologie, CNRS URA 258, UniversitB Pierre et Murie Curie, B&. A, 7&w etuge, 7 quai St. Bernard, Cuse 237, F-75252 Puris Cedex 5, Frunce, e-muil: ctmoller@l~ull.snv.jussieu.JL ‘Department of Biology, University of New Mexico, Albuquerque, NM 87131, USA In our target paper we attempted to evaluate the hypothesis that measuresreflecting developmental instability (such as fluctuating asymmetry or phenodeviants) have a heritable basis. This hypothesis is not generally accepted as suggested by several reviews of fluctuating asymmetry (Palmer and Strobeck, 1986; Parsons, 1990) and discussionat the developmental stability workshop mentioned below. Furthermore, someworkers in developmental stability have claimed that there should not be any heritability of stability (e.g., Zakharov, 1989; Palmer and Strobeck, 1992, p. 59). We have read the commentaries addressing our paper on the heritability of developmental instability with great interest. Between the two of us we have published 340 papers and 7 books. We have received many pointed criticisms of our work in the spirit of promoting better science, but we have never received comments of hidden agenda or deception. The negative emotional tone of certain commentaries is interesting in itself, leading us to wonder what is actually at stake. Clearly the issue of heritability of developmental instability is more than just a scientific question to some commentators. In science, no one should feel threatened by others working in “their field”. We are newcomers to the study of developmental instability (only nine (A.P.M.) and seven (R.T.) years, respectively, working on this subject as a primary research interest) and have tremendous respect for the ground work laid by some commentators. Certain commentators claim to be experts on our motivations for doing the meta-analysis. They allege that we had an agenda of wanting to prove fluctuating asymmetry’s heritability in order to support a view that sexual selection operates by favoring individuals with genes that code for offspring viability (so-called good genes).Our motivation was scientific. At the workshop organized by T. A. Markow in Tempe, Arizona in June 1993 there was intense discussion about whether fluctuating asymmetry is heritable. Each side of the argument used one or two * Author

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studies to make their point in one direction or the other. We independently and simultaneously offered the possible solution of a meta-analysis during the discussion. Some of the workshop participants expressed interest in knowing the outcome of such an analysis. Neither of us had an a priori expectation about the genetic pattern of developmental stability, much less a preferred outcome. Regarding good genes sexual selection, it appears that some commentators don’t even want to consider the possibility that good genes might exist. Most of all we hope that the derogations of us by certain commentators don’t lead young investigators interested in developmental stability to fear that they may be attacked personally by certain established workers in the field upon publication of their work, or to fear that acceptance of their manuscripts will be significantly dependent on the whims of unobjective referees. There is real danger to a scientific field when established workers in the field view their colleagues as competitors and use innuendos and direct claims of malpractice to try to get an edge. Below we re-iterate why it is necessary to conduct a meta-analysis of the heritability of developmental instability (and any other unresolved hypothesis). It is to be expected that meta-analytic techniques will be controversial among biologists and that some will find the approach unreasonable. Meta-analysis is a relatively new tool in ecology and evolution (see Arnqvist and Wooster, 1995). Of course, new procedures are notoriously difficult for people who have become accustomed to other methods. Also, although non-quantitative, narrative or verbal generalizations are intuitively acceptable in how people reason, they are problematic for scientific understanding and far more so than quantitative generalizations. We would venture to say that the vast majority of all medical advice in Western medicine is based on the outcome of meta-analysis. Meta-analysis is highly respected and used widely in the medical and social sciences, but the same sort of criticism has been put forth in those fields as is seen in certain commentaries (see any general treatment of meta-analysis). We suggest that a major hurdle for the human mind is that it can’t always comfortably group together the different categories involved in meta-analysis such as different taxa (e.g., heritability of a trait in a plant or insect species and in a mammal species). This may reflect a mental rule that evolved because it promoted reproduction in human evolutionary history to keep certain organism categories distinct. If so, this evolved tendency will often lead biologists in the current environment of modern statistics to incorrect inference. Evolved reasoning, including how reasoning is constrained, is an active area of research in evolutionary ecology (e.g., Barkow et al., 1992). There is evidence for the existence of human mental or psychological adaptation for intuitive biology, and specifically relevant here, for intuitive biological taxonomy (Wilson, 1984; Pinker, 1994; Thornhill, 1996). Meta-analysis is fundamentally for dealing with statistical associations in data derived from very different classes of things, classes that may stretch our evolved capabilities to see as appropriate to combine quantitatively. Meta-analysis allows a stringent evaluation of hypotheses which are otherwise discarded as being invalid because of a low power of the statistical tests. A very large fraction of papers in ecology and evolution evaluate null hypotheses by concluding that there is no effect, because the null hypotheses cannot be rejected

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(e.g., Cohen, 1984). However, this conclusion is only warranted if the power of the statistical test is sufficiently high, usually when it exceeds 80% (Cohen, 1984). Many studies of the heritability of developmental instability have claimed that there is no heritability, when in actual fact they have shown that the null hypothesis of a heritability larger than zero cannot be rejected. If there is a statistically significant heritability this will have important implications for our understanding of a) the consequences of sexual selection in relation to developmental instability, b) the consequences of natural selection against asymmetric phenotypes, and c) models of development (Moller and Thornhill, 1997; Houle, 1997). A meta-analysis of a number of studies will allow a stringent test of the hypothesis that developmental stability is heritable (Glass, 1976; Hedges and Olkin, 1985; Arnqvist and Wooster, 1995). Meta-analysis allows calculation of an overall effect size, which basically corresponds to the amount of variance accounted for by the relationship in question (in itself an important parameter when assessing the magnitude of a particular effect (Cohen, 1984)), and provides a means for stringent evaluation of heterogeneity in the data set (Hedges and Olkin, 1985; Rosenthal, 1991). Effect size in our case was a transformation of heritability estimates or test statistics to a parametric correlation coefficient, with the proportion of the variance in the data set being accounted for simply being the squared correlation coefficient. The issue of heterogeneity in data sets is very important because traditional or narrative reviews of studies do not address this question, and therefore may attempt to explain heterogeneity that does not exist, or may not attempt to explain existing heterogeneity. Contrary to the statement of some of the commentators that meta-analysis is a poor method that only provides self-evident results, we emphasize that it is a stringent way for scientists to rigorously test for heterogeneity in the results of several studies and attempt to explain such heteogeneity. After all, we believe that it is the purpose of scientific enquiry to try to understand why different studies reach different conclusions, rather than stating that this study is right and that is wrong. They may all be right because some underlying factor may affect differet organisms in different ways. Whitlock and Fowler (1997) and Pomiankowski (1997) have suggested that meta-analyses assume that the distributions of the data in the different studies should be similar. This criticism is probably directly a manifestation of limitation in seeing the validity of combining distinct classes. The hypothesis in our target paper is about the possibility of a genenal developmental phenomenon that gives rise to heritability in developmental stability across taxa. This is the kind of idea that meta-analysis can legitimately test. It is not an assumption that the developmental phenomenon exists. Nor do we assume that the heritabilities of two species are the same quantity. It may be useful to keep in mind that every character will exhibit some heritability (Lewontin, 1983), perhaps with the exception of directional asymmetry (Lewontin, 1983). However, for reasons stated below, it is even questionable whether directional asymmetry has not got an additive genetic basis, and at least one study of direction asymmetry demonstrates significant heritability (Beardmore,

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1965). Moreover, it appears to be the case that developmental instability fulfills the rule of everything being heritable, as demonstrated by our meta-analysis. A very large number of loci are probably involved in controlling developmental stability. Several recent genetic studies of particular loci have demonstrated that asymmetry is associated with the action of specific alleles. This is the case for alleles associated with pesticide resistance in blowflies (Clarke and McKenzie, 1987) and mosquitoes (M. Raymond, pers. comm.), but also for ETS2-alleles with effects similar to those of Down’s syndrome (Sumarsono et al., 1996), the vascular endothelial growth factor gene (Carmeliet et al., 1996; Ferrara et al., 1996) and Hex-genes (Davis et al., 1995), We imagine that many different genes will contribute to the stable development of various aspects of the phenotype. A large number of such loci will invariably give rise to genetic variation in overall developmental instability, and relatives will obviously resemble each other with respect to measures of developmental instability. Therefore, it is no surprise that developmental instablilty has an additive genetic component. In all fields of biology there is progress in methodology, but this does not allow us to cast doubt on earlier studies on which we have based our research. Biologists working decades ago cannot be blamed for not having adhered to quality criteria or methodologies that have emerged recently. We have, for the first time ever, tested statistically whether adherence to such stringent quality criteria of studies affects our estimate of effect size. Our conclusion was that this was not the case. The average effect size of the heritability of developmental instability did not show any tendency to differ between studies that have adopted statistical tests for fluctuating asymmetry and found such asymmetry, and those studies that have not performed such statistical tests. Even if we only restrict our analysis to the data sets that adhere to the strictest definitions of fluctuating asymmetry, we will find a statistically significant overall effect size. Studies that have specifically tested for and found absence of directional asymmetry and antisymmetry, and have found low levels of measurement errors in asymmetry measures, are Moller (1996), Castro et al. (MS), M. Polak and J. Jaenike (unpubl.), Chenuil (1991), and Moller (1994, and unpubl.). If we restrict the meta-analysis to this subsample, we still find a significant heritability (mean (SE): 0.162 (0.063), N = 6, one-sample t-test: t = 2.59, df = 5, P = 0.049). Similarly, the average effect size is also significantly different from zero (average effect size = 0.203, z = 3.38, P = 0.0006). Interestingly, there is no statistically significant heterogeneity in this sample (Chi-square = 4.25, df = 4, N.S.). This later analysis excluded the study by Price et al. (1991) because we were unable to obtain a standard error for the heritability estimate from the publication. These estimates are very similar to (and certianly not significantly different from) the overall estimates reported by us in the original meta-analysis. The original conclusion is therefore upheld even when the analysis is restricted to studies fulfilling the most stringent current quality criteria. A similar conclusion was reached by Whitlock and Fowler (1997) in their analysis of the restricted data set fulfilling their criteria (all their studies by the way fall short of currently used criteria for fluctuating asymmetry studies (Palmer, 1994; Moller and Swaddle, 1997)).

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Palmer and Strobeck (1997) question whether heritability analyses are appropriate if characters analysed are size-related. This is a general problem not only related to studies of resemblance among relatives with respect to asymmetry (Falconer, 1989). We would claim that any character for which heritability has been estimated in one way or another is related to the size of another character, if for no other reason, than because underlying biochemical and physiological processes will affect the development of multiple traits. Pleiotropic effects and linkage disequilibrium will obviously also result in characters being phenotypically correlated with each other (Falconer, 1989). This well-known fact does not invalidate estimates of the heritability of a partifcular character, simply because predictions of the response to selection obviously will depend on the heritability and intensity of selection, as well as the indirect effects of selection on other traits that are phenotypically and genetically correlated with the train in question. Heritabilities may also be of interest in their own right because they estimate to which extent the phenotypic variance is partitioned between genetic and environmental variance components. We see no a priori reason why studies of the heritability of measuresof developmental instability should attempt to control for relationships with size. Furthermore, Palmer and Strobeck (1997) question whether our conclusion would be upheld if we include three studies that we have missed(Price et al., 1991; Blouw and Boyd, 1992; Fowler and Whitlock, 1994). The study by Blouw and Boyd only concerns fluctuating asymmetry in one population (Glenfinnan River; Blouw and Boyd, 1992, p. 35). There were no crossesamong individuals from this locality, and estimatesof the heritability of developmental stability were thus not available from the study. Only the first study fulfills the most stringent quality criteria for studiesof developmental instability (stringent tests for normality, deviations from a mean value of zero, tests for measurement error), and the heritability estimates of these studies (largest h* estimate = 0.06, Price et al., 1991, p. 527; only test for the homogeneity of variances in the paper by Fowler and Whitlock, 1994, p. 375) are low, but in agreement with our general conclusion reached above, as well as our conclusion of the overall meta-analysis (Moller and Thornhill, 1997). We do not want to go into detail concerning some of the minor points raised by the commentaries. However, we would like to emphasize that two studies (Beardmore, 1965; Tuinstra et al., 1990), which are claimed by commentators not to report on the heritability of fluctuating asymmetry, indeed do so as reported by us in our original paper (Moller and Thornhill, 1997). There is current controversy over whether only fluctuating asymmetry or also other kinds of asymmetry such as directional asymmetry and antisymmetry reflects developmental instability. Some authors suggest that only fluctuating asymmetry counts (Palmer and Strobeck, 1986, 1992), while a growing body of empirical and theoretical evidence suggeststhat other forms of asymmetry may also be important markers of developmental instability (McKenzie and Clarke, 1988; Leary and Allendorf, 1989; Markow, 1992; Graham et al., 1993; Moller, 1994a; Moller and Swaddle, 1997). Even though some of the studies cited in our meta-analysis may have concerned directional asymmetry and antisymmetry rather than fluctuating asymmetry, it is far from obvious why this u priori should have affected our

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conclusions. At least our meta-analysis does not suggest that use of tests for absence of directional asymmetry and antisymmetry and subsequent confirmation that traits demonstrate fluctuating asymmetry gives rise to statistically significant heterogeneity. This issue of whether only fluctuating asymmetry reflects developmental instability relates to the issues brought up by Houle (1997). Current models of the ontogeny of bilateral traits have abandoned the approach of the two sides developing independently of each other (Graham et al., 1993; a review in Moller and Swaddle, 1997). Negative or positive feedback mechanisms mediated through the circulatory, endocrine or nervous systems may render developmental processes relatively stable if morphogens affect a bilateral trait and each side of it. Interestingly, such processes are more consistent with current estimates of the heritability of developmental instability than the old models of independent development of the two sides of a bilateral trait (Houle, 1997). Although the relationships between mean and variance may differ for character size and asymmetry for the reasons reported by Houle (1997) and Pomiankowski (1997), we feel that our emphasis of the importance of genetic variation irrespective of size is warranted. The reason is that heritabilities of asymmetry are large, and that the two kinds of heritabilities are strongly positively correlated. The correlation was +0.91 and at P = 0.0125 statistically significant. A paired comparison of heritability estimates of character size and asymmetry (characters in Tab. 2 in Moller and Thornhill, 1997) did not reveal a significantly larger heritability for size (paired r-test: t = 1.75, df = 5, P = 0.14), although the power of this test is low. Finally, we would like to emphasize that additive genetic coefficients of variation for asymmetry and size in the study by Scheiner et al. (1991) can readily be calculated from the information in their Tables 1-2. Palmer and Strobeck (1997) point out that dissection of the variation in asymmetry in swallow tail feathers and flower petals reveals that poorly developed individuals exhibit antisymmetry and well developed ones exhibit fluctuating asymmetry. This is anticipated if fluctuating asymmetry is a reasonable correlate of phenotypic quality. Antisymmetry is underrepresentation of symmetrical individuals, which would be expected among low quality phenotypes. We seriously doubt the approach suggested by Palmer and Strobeck of dissecting a data set without u priori theoretical reason to see if antisymmetry exists somewhere within it, and then if found conclude that the sample does not reflect developmental instability. Also, as we have mentioned, antisymmetry may actually reflect developmental instability. In conclusion, we strongly believe that developmental instability as measured by fluctuating asymmetry, or other measures of developmental irregularity, has an additive genetic basis. Also, we strongly believe that meta-analysis is the proper way to look at the hypothesis of heritable developmental stability. Our conclusion is independent of a number of different ways in which the entire data set is restricted. Therefore, our conclusion that developmental stability has a significant additive genetic component remains solid. Interestingly, Swaddle (1997) is the single commentator that has addressed the issue of how research should proceed. We strongly encourage more studies of the

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heritability of developmental instability. Such studies should perhaps concentrate on traits that demonstrate relatively large degrees of asymmetry (e.g., secondary sexual characters or characters reflecting body size that are subject to persistent directional selection), rather than traits subject to intense stabilizing selection with relatively small levels of asymmetry (e.g., bristle number in Drosophilu) in order to reduce the effects of measurement errors. A number of traits should be studied, as suggested by Swaddle (1997), because that would ensure a better overall measure of the ability of individuals to control developmental processes. These future studies should also ensure that sample sizes are sufficiently large which results in sufficient power to be able to accept the null hypothesis of no significant heritability in case the null hypothesis is not rejected. Acknowledgements A.P.M. We thank

was supported by grants from the Swedish and Danish G. Arnqvist for useful discussion about meta-analysis.

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References (Only those not cited in Moller and Thornhill, 1997) Barkow, J. H., L. Cosmides and J. Tooby (Eds.). 1992. The Adapted Mind: Evolutionary Psychology and the Generation of Culture. Oxford University Press, Oxford, UK. Blouw, D. M. and G. J. Boyd. 1992. Inheritance of reduction, loss, and asymmetry of the pelvis in Pungirius pungitius (ninespine stickleback). Heredity 68; 33342. Carmeliet, P., V. Ferreira, G. Breier, S. Pollefeyt, L. Kieckens, M. Gertsenstein, M. Fahrig, A. Vandenhoeck. K. Harpal, C. Eberhardt, C. Declercq, J. Pawling, L. Moons, D. Collen, W. Risau and A. Nagy. 1996. Abnormal blood vessel development and lethality in embryos lacking single VEGT allele. Nature 380: 4355439. Davis, A. P., D. P. Witte, H. Hsieh-Li, S. S. Potter and M. R. Capecchi. 1995. Absence of radius and ulna in mice lacking /to.ra- I I and ho.~d- 11. Nature 375: 79 I -795. Ferrara, N., K. Carver-Moore, H. Chen, M. Dowd, L. Lu, K. S. O’Shea. L. Powell-Braxton, K. J. Hillan and M. W. Moore. 1996. Heterozygous embryonic lethality induced by targeted inactivation of the VEGF gene. Nature 380: 4399442. Fowler, K. and M. C. Whitlock. 1994. Fluctuating asymmetry does not increase with moderate inbreeding in Drosophila melanogaster. Heredity 73: 373-376. Houle, D. 1997. Comment on “A meta-analysis of the heritability of developmental stability” by Moller and Thornhill. J. evol. biol. IO: 17-20 (this issue). Leamy, L. 1997. Is developmental stability heritable? J. evol. biol. IO: 21 29 (this issue). Leary, R. F. and F. W. Allendorf. 1989. Fluctuating asymmetry as an indicator of stress: Implications for conservation biology. Trends Ecol. Evol. 4: 214-217. Lewontin, R. C. 1983. Gene, organism and environment, pp. 273285. In D. S. Bendall (Ed.), Evolution from Molecules to Men. Cambridge University Press, Cambridge, UK. Markow, T. A. 1992. Genetics and developmental stability: An intergrative conjecture on etiology and neurobiology of schizophrenia. Psychol. Med. 22: 2955305. Markow, T. A. and CT. M. Clarke. 1997. Meta-analysis of the heritability of developmental stability: A giant step backward. J. evol. biol. IO: 31-37 (this issue). McKenzie, J. A. and G. M. Clarke. 1988. Diazinon resistance. fluctuating asymmetry and fitness in the Australian sheep blowfly. Genetics 120: 2133220.

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A. P. 1994. Directional selection on directional asymmetry: Testes size and secondary sexual characters in birds. Proc. R. Sot. Lond. 8258: 147 151. Moller, A. P. and R. Thornhill. 1997. A meta-analysis of the heritability of developmental stability. J. evol. biol. 10: I - 16 (this issue). Palmer, A. R. and C. Strobeck. 1997. Fluctuating asymmetry and developmental stability: Heritability of observable variation vs. heritability of inferred cause. J. evol. biol. IO: 39-49 (this issue). Pomiankowski, A. 1997. Genetic variation in fluctuating asymmetry. J. evol. biol. 10: 51-55 (this issue). Pinker, S. 1994. The Language Instinct: How the Mind Creates Language. W. Morrow and Co., New York, New York. Price, T., E. Chi, M. Pavelka and M. Hack. 1991. Population and developmental variation in the feather tip. Evolution 45: 518-533. Sumarsono, S. H., T. J. Wilson, M. J. Tymms, D. J. Venter, C. M. Corrick, R. Kola, M. H. Lahoud, T. S. Papas, A. Seth and I. Kola. 1996. Down’s syndrome-like skeletal abnormalities in ETS2 transgenic mice. Nature 379: 5344537. Swaddle, J. P. 1997. On the heritability of developmental stability. J. evol. biol. IO: 57761 (this issue). Thornhill, R. 1996. Darwinian aesthetics. In C. Crawford and D. Krebs (Eds.), Evolution and Human Behaviour: Ideas, Issues and Applications. Lawrence Erlbaum, Hillsdale, New Jersey (in press). Whitlock, M. C. and K. Fowler. 1997. The instability of studies of instability. J. evol. biol. 10: 63 67 (this issue). Wilson, E. 0. 1984. Biophilia. Harvard University Press, Cambridge, Mass.