EVOLUTIONARY QUANTITATIVE GENETICS: HOW LITTLE DO WE KNOW?

“Testing the allelic effects on quantitative characters facilitates an understanding of the underlying genetics of these traits, primarily the number of polygenes and the magnitude of effect (Barton and Turelli 1989).”

“Firstly, the evolutionary outcome of any process depends on the unknown genetic details underpinning G (Barton & Turelli 1987, 1989; Slatkin & Frank 1990; Zhang & Hill 2002; Griswold & Whitlock 2003; Zhang et al .”

“The rate of phenotypic evolution will therefore depend upon the amount of additive genetic variance for a given trait,((9)) and the response to selection will also be influenced by factors such as the number of loci underlying the phenotype and whether the fitness of each genotype is linearly related to the trait valu…”

“Dissimilar phenotypic and genetic correlations are one of several reasons that individual and interspecific correlations may differ (Lande 1979, 1985; Turelli 1988; Zeng 1988; Barton & Turelli 1989).”

“This theoretical result is based on certain assumptions (Lande 1979, 1980; Turelli 1988; Barton and Turelli 1989), including the following: the absence of divergent selection; a constant input of additive pleiotropic mutations that maintain the mean phenotypes and the additive genetic variances and…”

“Another route to the establishment of symmetry might be spatially variable selection (Barton and Turelli 1989), but any resulting speciation would then no longer be a strictly sympatric phenomenon.”

“Also useful would be formal genetic models that incorporated the effect of genetic constraints such as mutation and pleiotropy (Barton and Turelli, 1989).”

“The distribution of mutational effects on fitness is a critical component of many mathematical models of evolutionary dynamics (Crow & Kimura, 1970; Lande, 1975, 1983; Turelli, 1984; Hill & Rasbash, 1986; Barton & Turelli, 1987), but there is very little quantitative information on this distribution ( Barton & Turelli…”

“These include longstanding questions such as the debate over micro- versus macromutation (Barton and Turelli 1989; Orr and Coyne 1992; Orr 1998; Barton and Keightley 2002; Orr 2005) as well as more current question regarding the relative roles of cis- versus trans-acting mutations.”

“Linkage maps provide a means of exploring one of the central questions of quantitative genetics: how many variable genes underlie observed variation of quantitative traits and what are their individual contributions ( Barton and Turelli 1989; Tanksley 1993; Thoday and Thompson 1976)? The greatest success has come from…”

“However, these findings have often been difficult to interpret, since it is generally difficult to distinguish between the effects of many loci of small effect and a few loci of major effect (Barton and Turelli 1990).”

“Moreover, heritabilities and genetic correlations might be tools to reconstruct patterns of natural selection in the past (Charlesworth, 1987; Barton & Turelli, 1989).”

“Essentially, the genotype with the highest relative fitness in asexual populations is favoured over all others, leading to the fixation of adaptive and nonadaptive traits ( Barton & Turelli, 1989 ).”

“It has been demonstrated, theoretically, that there are several evolutionary mechanisms that are capable of maintaining high levels of polymorphism (Charlesworth, 1987; Barton and Turelli, 1989).”

“Gene flow between populations divergent for LP phenotypes is common (Hagen, 1967; Bell & Richkind, 1981; Baumgartner & Bell, 1984; Withler & McPhail, 1985; Baumgartner, 1986) and may help maintain genetic variation for LP. Other potential mechanisms for maintenance of genetic variation (e.g., reviewed by Barton & Ture…”

“While the idea of polymorphisms evolving from temporally varying selection regimes is usually deemed unlikely (Barton and Turelli 1989; Roulin 2004a), spatial variation in the direction and magnitude of selection may play a significant role (Roulin 2004a).”

“Further progress in understanding the subtleties of evolution depend on understanding how genetic variances evolve under selection (Barton and Turelli 1989; Walsh and Blows 2009).”

“Currrent evolutionary thought assumes that most adaptive trait variation results from large numbers of additive gene loci, each with infinitesimal effects (Falconer 1981; Barton & Turelli 1989); although this paradigm recently has been challenged (Orr & Coyne 1992; Lai et al. 1994).”

“traits can be attributed to environmental noise and/or nonadditive genetic variance (107, 108).”

“Bradshaw (1984) has recently argued that many adaptive responses may indeed be limited by the availability of appropriate variation, and has coined the term genostasis to describe the situation in which a population has evolved as far as it can, and further evolution is constrained by the lack of the necessary additiv…”

“Although strong selection is predicted to erode genetic variance in any closed population (Barton & Turelli 1989), our results are likely to be most accurate for highly selfing and asexual populations.”

“The studies reporting lowtions or those experiencing reduced natural selection mutation rates and large average effects include a long-(e.g., Barton and Turelli 1989; Charlesworth and term mutation-accumulation experiment conductedCharlesworth 1998; Kondrashov 1998; Lynch et al. with the…”

“These estimates are based on the assumption that all QTLs have equal and additive effects (Fischer, 1918; Barton and Turelli, 1989 ).”

“At the same time, Bartonand Turelli (1989) reviewed theories and experimental results on evolutionary quantitative genetics.”

“The high potential for selective response in RMR can be explained by the trait being neutral with respect to fitness, being involved in antagonistic–pleiotropic relations or if the selection pressure varies in space and ⁄ or time (Barton & Turelli, 1989; Barton & Keightley, 2002).”

“Such cases of evolutionary parallelism suggest that pathways of evolutionary change in development may be more limited than classical evolutionary theory might suggest (Barton and Turelli, 1989).”

“…perception that development plays no major role in evolution logically requires, as it will be explained, three assumptions about the functioning of development and variation: (1) Additive genetic variation is normally available for most traits (Barton and Turelli, ’89; Falconer and Mackay, ’96).”

“The following five processes are generally thought (Barton & Turelli, 1989; Maynard Smith, 1998) to encompass the most likely candidates maintaining genetic variation in morphological traits: (1) frequencydependent selection, (2) temporal environmental heterogeneity and genotype by environment…”

“Classical genetics suggests that stress tolerance is generally a multigenic trait (Barton and Turelli, 1990; Hoffmann and Parsons, 1991) and therefore, it is difficult to obtain precise control of the traits that the host plant will express or consistent genetic stability in subsequent generations.”

“Temporally fluctuating conditions could select for the maintenance of genetic variance for critical phenotypic traits, and promote the evolutionary potential of those traits (Barton & Turelli 1989; Ellner & Hairston 1994; Turelli & Barton 1994).”

“Other potential mechanisms for maintenance of genetic variation (e.g., reviewed by Barton & Turelli, 1989; Grant & Grant, 2000) have not been investigated.”

“Small heritabilities may be the consequence of stabilizing or directional selection eroding additive genetic variation or due to large environmental variances (Barton & Turelli, 1989; Roff, 1997).”

“Recent studies have raised the question as to whether the majority of heritability for any particular trait is best explained by a small number of mutations of large effect (Johnson and Barton 2005) or a large number of small effect mutations (Barton and Turelli 1989; Barton and Keightley 2002).”

“However, as we have few (if any) conclusive estimates of gs in highly polygenic traits [46,47], the extent of constraint arising through low GP-redundancy will be difficult to assess without further directed study.”

“Although this approach has proven adequate for describing and predicting the response to artificial selection, remarkably little is known about the genetic architecture underlying complex traits (3), especially complex traits in humans.”

“…other SNPs showing strong associations, is likely to favor alleles that are maintained in contemporary populations through balancing selection where the optimal trait value varies among populations or that have recently increased in frequency (Barton and Turelli, 1989; Yeaman and Whitlock, 2011).”

“This point parallels the observation that differences in heritability among traits and studies may reflect differences in environmental effects rather than differences in genetic effects (Barton and Turelli 1989; Merilä and Sheldon 1999).”

“…& Marks, 1996; Barton & Charlesworth, 1998; Charlesworth & Charlesworth, 1998); (2) the maintenance of genetic variability at both the phenotypic (Barton & Turelli, 1989) and DNA levels (Charlesworth et al., 1993); (3) the fate of small natural or managed populations (Kondrashov, 1995; Lynch et…”

“-selection balance sufficient to explain the observed variability? This comparison of data and theory has, in the past, led to crucial insights both in classical and molecular genetics, for review, see [12, 60]. Apart from such equilibrium considerations, dynamical aspects have become more and more important. Can presen…”

“…thought to be at least partly explained by a balance between recurrent mutation, genetic drift, and selectionof enhancing the overall pattern of genetic correlation, although the usefulness of those correlations would be (Barton and Turelli 1989; Houle et al. 1996; Falconer and Mackay 1996, Chap.”

“1) are primarily pollinated by bumblebees (Bombus spp.; Beaudoin Yetter 1989).”

“Thus, selection for smaller size may be imposed by other selection pressures at the same or other life history stages (see Slatkin 1987; Barton and Turelli 1989).”

“Such gradients in environmental stress can be associated with divergent selection pressures that both maintain genetic variation through various forms of overdominance (Hedrick et al., 1976; Barton, 1989; Yeaman & Jarvis, 2006; Bergland et al., 2014) and phenotypic variation through life history trade-offs (Stearns, 1…”

“4) had previously been obtained by Latter (1960) and Bulmer (1972), who analyzed a diallelic multilocus model introduced by Wright (1935a,b), where at each locus there is a plus and a minus allele and all loci are equivalent.”