Which evolutionary processes influence natural genetic variation for phenotypic traits?

122

The relative importance of the roles of adaptation and chance in determining genetic diversity and evolution has received attention in the last 50 years, but our understanding is still incomplete. All statements about the relative effects of evolutionary factors, especially drift, need confirmation by strong demographic observations, some of which are easier to obtain in a species like ours. Earlier quantitative studies on a variety of data have shown that the amount of genetic differentiation in living human populations indicates that the role of positive (or directional) selection is modest. We observe geographic peculiarities with some Y chromosome mutants, most probably due to a drift-related phenomenon called the surfing effect. We also compare the overall genetic diversity in Y chromosome DNA data with that of other chromosomes and their expectations under drift and natural selection, as well as the rate of fall of diversity within populations known as the serial founder effect during the recent “Out of Africa” expansion of modern humans to the whole world. All these observations are difficult to explain without accepting a major relative role for drift in the course of human expansions. The increasing role of human creativity and the fast diffusion of inventions seem to have favored cultural solutions for many of the problems encountered in the expansion. We suggest that cultural evolution has been subrogating biologic evolution in providing natural selection advantages and reducing our dependence on genetic mutations, especially in the last phase of transition from food collection to food production.

Section: Resultsmentioning

confidence: 99%

Y chromosome diversity, human expansion, drift, and cultural evolution

Chiaroni

Underhill

Cavalli-Sforza

2009

122

“…Despite a large body of theory, the genetic basis of local adaptation is poorly understood. Key unanswered questions include the number and effect sizes of adaptive loci, whether locally favored loci reduce fitness elsewhere (i.e., fitness tradeoffs), and whether the adaptive potential of populations is limited by insufficient genetic variation (4)(5)(6)(7)(8)(9)(10)(11). The Fisherian view that adaptation is a function of many genes of small effect (12) has been challenged on theoretical grounds (4,6), and recent empirical work indicates that genes with both large and small phenotypic effects contribute to differentiation in putatively adaptive traits (e.g., refs.…”

mentioning

confidence: 99%

“…Because natural environments are variable in both time and space, temporally replicated experiments are particularly important for distinguishing whether adaptive alleles are conditionally neutral [i.e., favored locally but neutral elsewhere (7,15)] or are favored in one environment but disfavored in the other, reflecting an adaptive tradeoff (2,7,16). Evidence for conditional neutrality is far more common than that for tradeoffs (15), but this disparity may be attributable to the more rigorous statistical criterion for detecting tradeoffs (11), a hurdle alleviated in part by long-term field experiments.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Genetic mapping of adaptation reveals fitness tradeoffs in Arabidopsis thaliana

Ågren

Oakley

McKay

et al. 2013

166

320

Organisms inhabiting different environments are often locally adapted, and yet despite a considerable body of theory, the genetic basis of local adaptation is poorly understood. Unanswered questions include the number and effect sizes of adaptive loci, whether locally favored loci reduce fitness elsewhere (i.e., fitness tradeoffs), and whether a lack of genetic variation limits adaptation. To address these questions, we mapped quantitative trait loci (QTL) for total fitness in 398 recombinant inbred lines derived from a cross between locally adapted populations of the highly selfing plant Arabidopsis thaliana from Sweden and Italy and grown for 3 consecutive years at the parental sites (>40,000 plants monitored). We show that local adaptation is controlled by relatively few genomic regions of small to modest effect. A third of the 15 fitness QTL we detected showed evidence of tradeoffs, which contrasts with the minimal evidence for fitness tradeoffs found in previous studies. This difference may reflect the power of our multiyear study to distinguish conditionally neutral QTL from those that reflect fitness tradeoffs. In Sweden, but not in Italy, the local genotype underlying fitness QTL was often maladaptive, suggesting that adaptation there is constrained by a lack of adaptive genetic variation, attributable perhaps to genetic bottlenecks during postglacial colonization of Scandinavia or to recent changes in selection regime caused by climate change. Our results suggest that adaptation to markedly different environments can be achieved through changes in relatively few genomic regions, that fitness tradeoffs are common, and that lack of genetic variation can limit adaptation.O rganisms inhabiting different environments are often locally adapted, as demonstrated by experiments showing that local populations grown in their native sites outperform other populations (1-3). Despite a large body of theory, the genetic basis of local adaptation is poorly understood. Key unanswered questions include the number and effect sizes of adaptive loci, whether locally favored loci reduce fitness elsewhere (i.e., fitness tradeoffs), and whether the adaptive potential of populations is limited by insufficient genetic variation (4-11). The Fisherian view that adaptation is a function of many genes of small effect (12) has been challenged on theoretical grounds (4, 6), and recent empirical work indicates that genes with both large and small phenotypic effects contribute to differentiation in putatively adaptive traits (e.g., refs. 8 and 13). However, there are few direct estimates of the number of loci underlying fitness variation in native environments (14).Because natural environments are variable in both time and space, temporally replicated experiments are particularly important for distinguishing whether adaptive alleles are conditionally neutral [i.e., favored locally but neutral elsewhere (7, 15)] or are favored in one environment but disfavored in the other, reflecting an adaptive tradeoff (2, 7, 16). Evidence for conditi...

“…Local adaptation is common in both plant and animal species (2)(3)(4)(5), promotes the maintenance of genetic variation (6,7), and is a key component of models of speciation (8,9). However, the relative importance of traits contributing to adaptive differentiation and their underlying genetic architecture remain poorly known (1,7,10,11). One key question is whether local adaptation results from genetic tradeoffs (antagonistic pleiotropy) in which locally favored alleles reduce fitness elsewhere or is caused by alleles that are conditionally neutral (favored in the home environment but selectively neutral in other environments).…”

mentioning

confidence: 99%

Early life stages contribute strongly to local adaptation in Arabidopsis thaliana

Postma

Ågren

2016

121

171

The magnitude and genetic basis of local adaptation is of fundamental interest in evolutionary biology. However, field experiments usually do not consider early life stages, and therefore may underestimate local adaptation and miss genetically based tradeoffs. We examined the contribution of differences in seedling establishment to adaptive differentiation and the genetic architecture of local adaptation using recombinant inbred lines (RIL) derived from a cross between two locally adapted populations (Italy and Sweden) of the annual plant Arabidopsis thaliana. We planted freshly matured, dormant seeds (>180 000) representing >200 RILs at the native field sites of the parental genotypes, estimated the strength of selection during different life stages, mapped quantitative trait loci (QTL) for fitness and its components, and quantified selection on seed dormancy. We found that selection during the seedling establishment phase contributed strongly to the fitness advantage of the local genotype at both sites. With one exception, local alleles of the eight distinct establishment QTL were favored. The major QTL for establishment and total fitness showed evidence of a fitness tradeoff and was located in the same region as the major seed dormancy QTL and the dormancy gene DELAY OF GERMINATION 1 (DOG1). RIL seed dormancy could explain variation in seedling establishment and fitness across the life cycle. Our results demonstrate that genetically based differences in traits affecting performance during early life stages can contribute strongly to adaptive differentiation and genetic tradeoffs, and should be considered for a full understanding of the ecology and genetics of local adaptation.I dentifying the ecological and genetic mechanisms underlying local adaptation, defined as the local genotype outperforming nonlocal genotypes (1), is a central problem in evolutionary biology. Local adaptation is common in both plant and animal species (2-5), promotes the maintenance of genetic variation (6, 7), and is a key component of models of speciation (8, 9). However, the relative importance of traits contributing to adaptive differentiation and their underlying genetic architecture remain poorly known (1,7,10,11). One key question is whether local adaptation results from genetic tradeoffs (antagonistic pleiotropy) in which locally favored alleles reduce fitness elsewhere or is caused by alleles that are conditionally neutral (favored in the home environment but selectively neutral in other environments). Empirical studies have rarely detected genetic tradeoffs (12)(13)(14), but these studies typically include only part of the life cycle (3,5,15), possibly resulting in a systematic underestimation of the magnitude of adaptive differentiation and of the occurrence of genetically based tradeoffs.Early life stages can be expected to play a prominent role in local adaptation. First, selection can be particularly severe during the first part of the life cycle when organisms are vulnerable and suffer from high mortality rates (15)(16)(...