EVOLUTIONARY QUANTITATIVE GENETICS: HOW LITTLE DO WE KNOW?

“However, the mechanisms maintaining this variation are not known, and are in general very difficult to discover for any quantitative trait ( Barton & Turelli, 1989 ).”

“Longerterm evolutionary dynamics, however, will depend on the influence of other genetic effects other than additivity (Barton and Turelli 1989).”

“Pleiotropy as the primary cause of genetic variation in sex ratio is more plausible when more loci are involved (Barton & Turelli, 1989).”

“genes than previously postulated for quantitative traits (28).”

“However, the narrow sense heritability (h2) of a trait represents the ratio of additive genetic variance (VA) to the total phenotypic variance (VP) and the heritability of traits largely affected by non-additive effects or environmental factors will thus by definition be low [25-28].”

“…from allozyme polymorphism, visible, or lethal mutations typically seem to be on the order of 10−5 or 10−6 whereas per-trait mutation rates are on the order of 10−2, which would seem to require an unrealistically large number of loci per trait or extensive pleiotropy (Barton and Turelli 1989).”

“The genetic correlation structure shapes macroevolutionary patterns by directing the evolution of mean phenotypes toward certain adaptive peaks and away from others (Barton and Turelli 1989; Schluter 1989; Price et al. 1993).”

“Barton & Turelli, 1989; Bulmer, 1989), the extinction of small populations (e.g.”

“Maintenance of genetic variation may have multiple causes (Barton and Turelli 1989; Roff 1997) such as heterozygote advantage (overdominance), antagonistic pleiotropy (Rose 1982), epistasis (Gimelfarb 1989), and spatial or temporal variation in selection coefficients (Hedrick et al. 1976; Hedrick…”

“Continuing advances in molecular technology may lead the way here, but some form of genetic testing is likely to be forever required.”

“Furthermore, such approaches would help to clarify whether temporally fluctuating selection could act to maintain adaptive variation in natural populations, an issue of longstanding theoretical and empirical debate 7–11 .”

“Two of the central questions in evolutionary genetics are: how much genetic variation exists within populations and how is that variation maintained (Lewontin 1974; Charlesworth 1987; Barton & Turelli 1989; Lynch et al. 1998; Charlesworth & Hughes 2000)?”

“…of differences in heritability among traits have emphasized differences in genetic effects, but differences in environmental effects may also be important (e.g., Barton and Turelli 1989; Price and Schluter 1991; Houle 1992; Weigensberg and Roff 1996; Merilä and Sheldon 1999; Kruuk et al. 2000).”

“This finding reinforces the importance of the question as to how genetic variance in a trait subject to directional selection can be maintained (Barton & Turelli, 1989; Stearns, 1992; Lynch & Walsh, 1998).”

“Hanging in the balance are key evolutionary questions, including the discoverability and evolutionary relevance of individual molecular variants (Barton and Turelli 1989; Rockman 2012; Travisano and Shaw 2013), the repeatability of molecular mechanisms in evolution (Stern and Orgogozo 2008; Martin…”

“If not, then predictions about trait evolution would depend upon how these matrices evolve (see also Barton & Turelli, 1989; Arnold, Pfrender & Jones, 2001).”

“It is now recognized that variable environmental effects on fitness, which were previously treated as evolutionarily irrelevant ‘‘noise,’’ may in fact slow or even prevent predicted selective outcomes (Barton and Turelli, ’89).”

“This pattern, as well as the fact that several of the detected SNPs were concentrated in a relatively small genomic region on chromosome 7, fit with the expected genetic architecture of an adaptive trait under stabilizing selection, migration and drift, assuming that the optimum value of the trait varies between popul…”

“Since the measurement error may lower our heritability estimates, these should be viewed as conservative estimates of parametric heritabilities which may be somewhat higher than observed here (Boag, 1983).”

“Life-history traits are often genetically negatively correlated (Barton & Turelli 1989) as the result of simultaneous selection on two correlated traits.”

“This observation suggests a possible paradox arising from the belief that these traits are influenced by stabilizing selection for an optimum, or simple directional selection to remove disease alleles, both of which should act to eliminate genetic variation (52,53).”

“Understanding how organisms respond to fluctuating environmental conditions is a fundamental goal of biology (Levins 1968; Barton & Turelli, 1989; Lande 2019).”

“Understanding how genetic variation is maintained in natural populations is a central issue in evolutionary biology ( Barton & Turelli 1989; Reed & Frankham 2001; Lenormand 2002; Hedrick 2006).”

“The big number of cases studying the relationships between environmental disturbance and population genetic diversity has allowed a good comprehension and has aided in the formulation of a theoretical approach to the biological mechanisms involved in this process (Barton and Turelli, 1990).”

“Quantitative genetic studies are also logistically demanding, and they generally require large sample sizes if they are to have reasonable power to detect biologically significant levels of variation (Klein et al., 1973; Barton & Turelli, 1989).”

“…on real stabilizing selection on all n traits, and the value of the selection coefficient is the total effect of such selections, given by s 5 aT a(1 2 2x)/4 (Appendix 1; cf. Bulmer21Vs 1985; Keightley and Hill 1988; Barton and Turelli 1989; Tanaka 1996), where x is the frequency of mutant allele.”

“This puzzling fact has been explained various ways (e.g., Barton & Turelli, 1989).”

“ters (Barton and Turelli 1989; Falconer and Mackay 1996). Heterozygote advantage on loci affecting a quantitative character can generate apparent stabilising selection (Robertson 1956). The ability of mutation-selection balance to explain quantitative genetic variation for peripheral characters is controversial with Z…”

“…on fitness components and phenotypic trait data of individuals to contribute to the evolutionary literature by estimating selection using the common currency provided by evolutionary quantitative genetics (Barton & Turelli, 1989; Hansen & P é labon, 2021; Lynch & Walsh, 1998; Walsh & Lynch, 2018).”

“These assumptions have been the subject of intense discussion and analysis (see Turelli 1984, 1988; Barton and Turelli 1989; Turelli and Barton 1994; Bürger 2000).”

“In the study of trans-acting regulatory variation, a priori a case of apparent 247 accelerated evolution of a pathway could be driven by a single mutation of large effect maintained by 248 drift in a species, as in any phenomenological analysis of trait evolution [13,34].”

“…inbred populations between a nondomesticated and a domesticated population (Eshed and Zamir, 1994) to identify traits with strong and repeatable phenotypes; however, we still do not know whether the number of loci responsible for most genetic variation is small or large (Barton and Turelli, 1989).”

“…pleiotropy concept in evolutionary biology (e.g., Caspari, 1952; Williams, 1957; Wright, 1968; Rose, 1985), little is known about particular pleiotropic genes affecting evolutionarily relevant traits (e.g., Barton & Turelli, 1989; van Tienderen, Hammad & Zwaal, 1996; Knight, Azevedo & Leroi, 2001).”

“Theoretical ( Barton and Turelli 1989; Goldstein and Holsinger 1992) and empirical (Denno et al. 1991) work suggests that if different genotypes exist and each is favored by different environments, then genetic variation can be maintained by spatially fluctuating environments.”

“Their lower heritability could be explained by a larger environmental variance rather than by a smaller genetic variance (Barton and Turelli 1989; Houle 1992).”

“This “variance effective number” is not necessarily the same as the effective (minimum) number of loci contributing to the difference between two lines or populations, estimated using crossing experiments (Lande 1981; Barton and Turelli 1989).”

“…the ultimate source of novel genetic variation upon which selection acts; and (2) can shift traits away from their optimum value, which can lead to the mean value of a trait differing from its optimum, and to the maintenance of genetic variance (Barton and Turelli 1989; Lynch and Walsh 1998).”

“The maintenance of genetic variation in traits that are under selection remains the most persistent and controversial problem in evolutionary biology (Barton and Turelli 1989; Tomkins et al. 2004; Johnson and Barton 2005).”

“However, it is difficult to detect low heritabilities because of their large sampling errors (Barton and Turelli 1989; Houle 1992).”

“Both QTL mapping experi- ments and tests for failure to complement support the more, proteolytic cleavage of Dl is related to down-regulation of Dl as well as Dl-Notch interactions (Klueg et al.hypothesis that DI and other candidate genes, initially identified through mutants of large effect on bristle 1998).”

“The ¢rst assumes a large number of genes with each having only small e¡ects on the trait, whereas the second assumes the existence of a few loci with major e¡ects, but possibly complemented by other loci with minor e¡ects (Barton & Turelli 1989; Orr & Coyne 1992; Mitchell-Olds 1995).”

“In addition, this approach can potentially be used to address the question of whether genetic covariance matrices are stable over space and time, a central question of evolutionary biology and quantitative genetics (Turelli, 1988; Barton and Turelli, 1989; Nichols and Butlin, 1989; Kirkpatrick and Ryan, 1991; Shaw, 19…”

“Discussion Compared to theoretical studies on the e↵ects of selection, migration and drift on gene frequencies (e.g. Haldane 1930; Wright 1931; Levene 1953), work on quantitative traits has a shorter history and remains less complete (Barton & Turelli 1989; Barton 1999).”

“However, evidence for epistasis affecting a quantitative trait is scarce (Barton and Turelli 1989; Barton and Keightley 2002) and the epistatic variance, when found, is often small relative to VA (e.g., Paterson et al. 1990).”