Peter Crnokrak - Academia.edu (original) (raw)
Papers by Peter Crnokrak
Evolution, 2002
Inbreeding depression, the reduction in fitness that accompanies inbreeding, is one of the most i... more Inbreeding depression, the reduction in fitness that accompanies inbreeding, is one of the most important topics of research in evolutionary and conservation genetics. In the recent literature, much attention has been paid to the possibility of purging the genetic load. If inbreeding depression is due to deleterious alleles, whose effect on fitness are negative when in a homozygous state, then successive generations of inbreeding may result in a rebound in fitness due to the selective decrease in frequency of deleterious alleles. Here we examine the experimental evidence for purging of the genetic load by collating empirical tests of rebounds in fitness-related traits with inbreeding in animals and plants. We gathered data from 28 studies including five mammal, three insect, one mollusc, and 13 plant species. We tested for purging by examining three measures of fitness-component variation with serial generations of inbreeding: (1) changes in inbreeding depression, (2) changes in fitness components of inbred lines relative to the original outbred line, and (3) purged population (outcrossed inbred lines) trait means as a function of ancestral outbred trait means. Frequent and substantial purging was found using all three measures, but was particularly pronounced when tracking changes in inbreeding depression. Despite this, we found little correspondence between the three measures of purging within individual studies, indicating that the manner in which a researcher chooses to estimate purging will affect interpretation of the results obtained. The discrepancy suggests an alternative hypothesis: rebounds in fitness with inbreeding may have resulted from adaptation to laboratory conditions and not to purging when using outcrossed inbred lines. However, the pronounced reduction in inbreeding depression for a number of studies provides evidence for purging, as the measure is likely less affected by selection for laboratory conditions. Unlike other taxonspecific reviews on this topic, our results provide support for the purging hypothesis, but firm predictions about the situations in which purging is likely or the magnitude of fitness rebound possible when populations are inbred remain difficult. Further research is required to resolve the discrepancy between the results obtained using different experimental approaches.
A_B_peace & terror etc. The computational aesthetics of love & hate blends world ... more A_B_peace & terror etc. The computational aesthetics of love & hate blends world politics with the aesthetics of computational data to create a powerful, pertinent, and spellbinding view of the modern world. As an intriguing collection of data, A_B_... reveals the quantitative contribution each of the 192 member states of the United Nations has made toward peace and terror in
Journal of Evolutionary Biology, 2001
The comparison of the degree of differentiation in neutral marker loci and genes coding quantitat... more The comparison of the degree of differentiation in neutral marker loci and genes coding quantitative traits with standardized and equivalent measures of genetic differentiation (FST and QST, respectively) can provide insights into two important but seldom explored questions in evolutionary genetics: (i) what is the relative importance of random genetic drift and directional natural selection as causes of population differentiation in quantitative traits, and (ii) does the degree of divergence in neutral marker loci predict the degree of divergence in genes coding quantitative traits? Examination of data from 18 independent studies of plants and animals using both standard statistical and meta-analytical methods revealed a number of interesting points. First, the degree of differentiation in quantitative traits (QST) typically exceeds that observed in neutral marker genes (FST), suggesting a prominent role for natural selection in accounting for patterns of quantitative trait differentiation among contemporary populations. Second, the FST – QST difference is more pronounced for allozyme markers and morphological traits, than for other kinds of molecular markers and life-history traits. Third, very few studies reveal situations were QST < FST, suggesting that selection pressures, and hence optimal phenotypes, in different populations of the same species are unlikely to be often similar. Fourth, there is a strong correlation between QST and FST indices across the different studies for allozyme (r=0.81), microsatellite (r=0.87) and combined (r=0.75) marker data, suggesting that the degree of genetic differentiation in neutral marker loci is closely predictive of the degree of differentiation in loci coding quantitative traits. However, these interpretations are subject to a number of assumptions about the data and methods used to derive the estimates of population differentiation in the two sets of traits.
Trends in Ecology and Evolution, 2002
Animal Behaviour, 1995
Alternate morphologies exist in a wide range of species. A commonly encountered dimorphism in ins... more Alternate morphologies exist in a wide range of species. A commonly encountered dimorphism in insects is wing dimorphism, in which one morph is winged (macropterous=LW) and flight-capable while the other has reduced wings (micropterous=SW) and cannot fly. GryllusJirmus is a wing-dimorphic cricket found in the southeastern U.S.A. Although trade-offs associated with wing dimorphism are well established in female crickets, no such trade-offs have been demonstrated in male crickets. Differences between morphs in male G.firmus in the likelihood of attracting a female were tested in the laboratory using a simple T-maze where females chose between an LW male and an SW male. Time spent calling for each male was recorded on the sixth day of adult life. SW males were more likely to attract a female and spent more time calling than LW males. A logistic regression of female choice against the absolute proportional difference in calling time between males revealed that, as the difference in calling time between males increased, the likelihood of a female choosing the longer-calling male also increased. Therefore it is concluded that there is a trade-off between macroptery and the likelihood of attracting a female, and that it mav be a primary factor in the maintenance of wing dimorphism in male G~jirmus.
Journal of Evolutionary Biology, 2003
Quantitative genetic theory assumes that trade-offs are best represented by bivariate normal dist... more Quantitative genetic theory assumes that trade-offs are best represented by bivariate normal distributions. This theory predicts that selection will shift the trade-off function itself and not just move the mean trait values along a fixed trade-off line, as is generally assumed in optimality models. As a consequence, quantitative genetic theory predicts that the trade-off function will vary among populations in which at least one of the component traits itself varies. This prediction is tested using the trade-off between call duration and flight capability, as indexed by the mass of the dorsolateral flight muscles, in the macropterous morph of the sand cricket. We use four different populations of crickets that vary in the proportion of macropterous males (Lab ¼ 33%,
Heredity, 1999
Despite its practical application in conservation biology and evolutionary theory, the cost of in... more Despite its practical application in conservation biology and evolutionary theory, the cost of inbreeding in natural populations of plants and animals remains to a large degree unknown. In this review we have gathered estimates of inbreeding depression (delta) from the literature for wild species monitored in the field. We have also corrected estimates of delta by dividing by F (coefficient of inbreeding), to take into account the influence that the variation in F will have on delta. Our data set includes seven bird species, nine mammal species, four species of poikilotherms (snakes, fish and snails) and 15 plant species. In total we obtained 169 estimates of inbreeding depression for 137 traits; 81 of those estimates included estimates of F. We compared our mammalian data (limited to those traits related to juvenile mortality) to the estimates for captive zoo species published by Ralls et al. (1988) to determine if, as predicted from the literature, natural estimates of inbreeding depression are higher than captive estimates. The mean delta +/- SE (significantly different from zero and not corrected for F ) for homeotherms was 0.509 +/- 0.081; for poikilotherms, 0.201 +/- 0.039; and for plants, 0.331 +/- 0.038. Levels of inbreeding depression this high in magnitude will be biologically important under natural conditions. When we limited our data set to mortality traits for mammals and corrected for F=0.25 (as is the case for the Ralls et al. data set), we found a significant difference between the two data sets; wild estimates had a substantially higher mean cost of inbreeding at F = 0.25: 2.155 (captive species: 0.314). Of the 169 estimates of delta, 90 were significantly different from zero, indicating that inbred wild species measured under natural conditions frequently exhibit moderate to high levels of inbreeding depression in fitness traits.
Journal of Evolutionary Biology, 2002
Wing dimorphism, where some macropterous long-winged (LW) individuals can fly whereas micropterou... more Wing dimorphism, where some macropterous long-winged (LW) individuals can fly whereas micropterous short-winged (SW) individuals cannot, is common in insects and believed to be maintained in part by trade-offs between flight capability and reproductive traits. In this paper we examine differences in whole-organism respiration rate between wing morphs of the sand cricket Gryllus firmus. We hypothesized that maintenance of the flight apparatus would result in elevated CO2 respired because of the high metabolic cost of these tissues, which, in turn, constrain resources available for egg production in females. As the trade-off involves calling behaviour in males, we predicted no equivalent constraint on organ development in this sex. We found female macropters (particularly older crickets) had significantly higher residual respiration rates than micropters. In males, we found only marginal differences between wing morphs. In both sexes there was a highly significant effect of flight muscles status on residual respiration rate, individuals with functional muscles having higher respiration rates. Both female and male macropters had significantly smaller gonads than micropters. Whole-organism residual respiration rate was negatively correlated with fecundity: macropterous females with high respiration rates had smaller gonads compared with macropterous females with lower respiration rates.
Animal Behaviour, 1998
We examined the effect of food limitation on fitness trade-offs between macroptery and time spent... more We examined the effect of food limitation on fitness trade-offs between macroptery and time spent calling in the wing-dimorphic cricket, Gryllus firmus. The results of previous studies have shown that, under optimal conditions, the proportionate time that males spend calling (with respect to neighbouring males) is directly associated with female attraction, and that micropterous (short-winged: SW) males call longer than macropterous (long-winged: LW) males , Animal Behaviour, 50, 1475-1481. Because crickets were examined under optimal conditions, these studies did not attempt to address how or whether fitness trade-offs change with the environment. In the present study, we allowed crickets ad libitum access to water, but restricted food intake to the minimum amount that would keep them alive for 20 days. On average, SW males called longer than LW males on 18 of 20 days in the stressed group, and 17 of 20 days in the control group. For both groups, SW males also attracted more females more often than did LW males. Although the absolute call durations decreased in the stressed group compared with the control group, the relative call durations remained approximately the same, as did the proportion of females moving towards the SW male. Cumulative call distributions showed that LW males called little after 10 days of adult life; the amount of time SW males spent calling seemed constant for the duration of the experiment.
Heredity, 1995
Strong directional, and to some degree stabilizing, selection usually erodes only additive geneti... more Strong directional, and to some degree stabilizing, selection usually erodes only additive genetic variance while not affecting dominance variance. Consequently, traits closely associated with fitness should exhibit high levels of dominance variance. In this study we compile a large number of estimates of dominance variance to determine if traits that are subject to strong selection and/or are closely associated with fitness have higher levels of dominance variance than traits less subject to selection pressure. Estimates were taken from the literature for both wild and domestic species and each group was treated separately. Traits closely associated with fitness (life history) had significantly higher dominance components than did traits more distantly related to fitness (morphology) for wild species. No significant differences were found between life history and morphological traits for domestic species. Traits that were known to have been subject to intense directional selection (morphological traits for domestic species) had significantly higher dominance estimates than did traits that were assumed not to have been subject to strong selection (morphological traits for wild outbred species). The results are discussed with respect to the maintenance of heritable variation and the bias introduced in the calculation of the full-sib heritability estimate by high levels of dominance variance.
Evolution, 2002
Inbreeding depression, the reduction in fitness that accompanies inbreeding, is one of the most i... more Inbreeding depression, the reduction in fitness that accompanies inbreeding, is one of the most important topics of research in evolutionary and conservation genetics. In the recent literature, much attention has been paid to the possibility of purging the genetic load. If inbreeding depression is due to deleterious alleles, whose effect on fitness are negative when in a homozygous state, then successive generations of inbreeding may result in a rebound in fitness due to the selective decrease in frequency of deleterious alleles. Here we examine the experimental evidence for purging of the genetic load by collating empirical tests of rebounds in fitness-related traits with inbreeding in animals and plants. We gathered data from 28 studies including five mammal, three insect, one mollusc, and 13 plant species. We tested for purging by examining three measures of fitness-component variation with serial generations of inbreeding: (1) changes in inbreeding depression, (2) changes in fitness components of inbred lines relative to the original outbred line, and (3) purged population (outcrossed inbred lines) trait means as a function of ancestral outbred trait means. Frequent and substantial purging was found using all three measures, but was particularly pronounced when tracking changes in inbreeding depression. Despite this, we found little correspondence between the three measures of purging within individual studies, indicating that the manner in which a researcher chooses to estimate purging will affect interpretation of the results obtained. The discrepancy suggests an alternative hypothesis: rebounds in fitness with inbreeding may have resulted from adaptation to laboratory conditions and not to purging when using outcrossed inbred lines. However, the pronounced reduction in inbreeding depression for a number of studies provides evidence for purging, as the measure is likely less affected by selection for laboratory conditions. Unlike other taxonspecific reviews on this topic, our results provide support for the purging hypothesis, but firm predictions about the situations in which purging is likely or the magnitude of fitness rebound possible when populations are inbred remain difficult. Further research is required to resolve the discrepancy between the results obtained using different experimental approaches.
A_B_peace & terror etc. The computational aesthetics of love & hate blends world ... more A_B_peace & terror etc. The computational aesthetics of love & hate blends world politics with the aesthetics of computational data to create a powerful, pertinent, and spellbinding view of the modern world. As an intriguing collection of data, A_B_... reveals the quantitative contribution each of the 192 member states of the United Nations has made toward peace and terror in
Journal of Evolutionary Biology, 2001
The comparison of the degree of differentiation in neutral marker loci and genes coding quantitat... more The comparison of the degree of differentiation in neutral marker loci and genes coding quantitative traits with standardized and equivalent measures of genetic differentiation (FST and QST, respectively) can provide insights into two important but seldom explored questions in evolutionary genetics: (i) what is the relative importance of random genetic drift and directional natural selection as causes of population differentiation in quantitative traits, and (ii) does the degree of divergence in neutral marker loci predict the degree of divergence in genes coding quantitative traits? Examination of data from 18 independent studies of plants and animals using both standard statistical and meta-analytical methods revealed a number of interesting points. First, the degree of differentiation in quantitative traits (QST) typically exceeds that observed in neutral marker genes (FST), suggesting a prominent role for natural selection in accounting for patterns of quantitative trait differentiation among contemporary populations. Second, the FST – QST difference is more pronounced for allozyme markers and morphological traits, than for other kinds of molecular markers and life-history traits. Third, very few studies reveal situations were QST < FST, suggesting that selection pressures, and hence optimal phenotypes, in different populations of the same species are unlikely to be often similar. Fourth, there is a strong correlation between QST and FST indices across the different studies for allozyme (r=0.81), microsatellite (r=0.87) and combined (r=0.75) marker data, suggesting that the degree of genetic differentiation in neutral marker loci is closely predictive of the degree of differentiation in loci coding quantitative traits. However, these interpretations are subject to a number of assumptions about the data and methods used to derive the estimates of population differentiation in the two sets of traits.
Trends in Ecology and Evolution, 2002
Animal Behaviour, 1995
Alternate morphologies exist in a wide range of species. A commonly encountered dimorphism in ins... more Alternate morphologies exist in a wide range of species. A commonly encountered dimorphism in insects is wing dimorphism, in which one morph is winged (macropterous=LW) and flight-capable while the other has reduced wings (micropterous=SW) and cannot fly. GryllusJirmus is a wing-dimorphic cricket found in the southeastern U.S.A. Although trade-offs associated with wing dimorphism are well established in female crickets, no such trade-offs have been demonstrated in male crickets. Differences between morphs in male G.firmus in the likelihood of attracting a female were tested in the laboratory using a simple T-maze where females chose between an LW male and an SW male. Time spent calling for each male was recorded on the sixth day of adult life. SW males were more likely to attract a female and spent more time calling than LW males. A logistic regression of female choice against the absolute proportional difference in calling time between males revealed that, as the difference in calling time between males increased, the likelihood of a female choosing the longer-calling male also increased. Therefore it is concluded that there is a trade-off between macroptery and the likelihood of attracting a female, and that it mav be a primary factor in the maintenance of wing dimorphism in male G~jirmus.
Journal of Evolutionary Biology, 2003
Quantitative genetic theory assumes that trade-offs are best represented by bivariate normal dist... more Quantitative genetic theory assumes that trade-offs are best represented by bivariate normal distributions. This theory predicts that selection will shift the trade-off function itself and not just move the mean trait values along a fixed trade-off line, as is generally assumed in optimality models. As a consequence, quantitative genetic theory predicts that the trade-off function will vary among populations in which at least one of the component traits itself varies. This prediction is tested using the trade-off between call duration and flight capability, as indexed by the mass of the dorsolateral flight muscles, in the macropterous morph of the sand cricket. We use four different populations of crickets that vary in the proportion of macropterous males (Lab ¼ 33%,
Heredity, 1999
Despite its practical application in conservation biology and evolutionary theory, the cost of in... more Despite its practical application in conservation biology and evolutionary theory, the cost of inbreeding in natural populations of plants and animals remains to a large degree unknown. In this review we have gathered estimates of inbreeding depression (delta) from the literature for wild species monitored in the field. We have also corrected estimates of delta by dividing by F (coefficient of inbreeding), to take into account the influence that the variation in F will have on delta. Our data set includes seven bird species, nine mammal species, four species of poikilotherms (snakes, fish and snails) and 15 plant species. In total we obtained 169 estimates of inbreeding depression for 137 traits; 81 of those estimates included estimates of F. We compared our mammalian data (limited to those traits related to juvenile mortality) to the estimates for captive zoo species published by Ralls et al. (1988) to determine if, as predicted from the literature, natural estimates of inbreeding depression are higher than captive estimates. The mean delta +/- SE (significantly different from zero and not corrected for F ) for homeotherms was 0.509 +/- 0.081; for poikilotherms, 0.201 +/- 0.039; and for plants, 0.331 +/- 0.038. Levels of inbreeding depression this high in magnitude will be biologically important under natural conditions. When we limited our data set to mortality traits for mammals and corrected for F=0.25 (as is the case for the Ralls et al. data set), we found a significant difference between the two data sets; wild estimates had a substantially higher mean cost of inbreeding at F = 0.25: 2.155 (captive species: 0.314). Of the 169 estimates of delta, 90 were significantly different from zero, indicating that inbred wild species measured under natural conditions frequently exhibit moderate to high levels of inbreeding depression in fitness traits.
Journal of Evolutionary Biology, 2002
Wing dimorphism, where some macropterous long-winged (LW) individuals can fly whereas micropterou... more Wing dimorphism, where some macropterous long-winged (LW) individuals can fly whereas micropterous short-winged (SW) individuals cannot, is common in insects and believed to be maintained in part by trade-offs between flight capability and reproductive traits. In this paper we examine differences in whole-organism respiration rate between wing morphs of the sand cricket Gryllus firmus. We hypothesized that maintenance of the flight apparatus would result in elevated CO2 respired because of the high metabolic cost of these tissues, which, in turn, constrain resources available for egg production in females. As the trade-off involves calling behaviour in males, we predicted no equivalent constraint on organ development in this sex. We found female macropters (particularly older crickets) had significantly higher residual respiration rates than micropters. In males, we found only marginal differences between wing morphs. In both sexes there was a highly significant effect of flight muscles status on residual respiration rate, individuals with functional muscles having higher respiration rates. Both female and male macropters had significantly smaller gonads than micropters. Whole-organism residual respiration rate was negatively correlated with fecundity: macropterous females with high respiration rates had smaller gonads compared with macropterous females with lower respiration rates.
Animal Behaviour, 1998
We examined the effect of food limitation on fitness trade-offs between macroptery and time spent... more We examined the effect of food limitation on fitness trade-offs between macroptery and time spent calling in the wing-dimorphic cricket, Gryllus firmus. The results of previous studies have shown that, under optimal conditions, the proportionate time that males spend calling (with respect to neighbouring males) is directly associated with female attraction, and that micropterous (short-winged: SW) males call longer than macropterous (long-winged: LW) males , Animal Behaviour, 50, 1475-1481. Because crickets were examined under optimal conditions, these studies did not attempt to address how or whether fitness trade-offs change with the environment. In the present study, we allowed crickets ad libitum access to water, but restricted food intake to the minimum amount that would keep them alive for 20 days. On average, SW males called longer than LW males on 18 of 20 days in the stressed group, and 17 of 20 days in the control group. For both groups, SW males also attracted more females more often than did LW males. Although the absolute call durations decreased in the stressed group compared with the control group, the relative call durations remained approximately the same, as did the proportion of females moving towards the SW male. Cumulative call distributions showed that LW males called little after 10 days of adult life; the amount of time SW males spent calling seemed constant for the duration of the experiment.
Heredity, 1995
Strong directional, and to some degree stabilizing, selection usually erodes only additive geneti... more Strong directional, and to some degree stabilizing, selection usually erodes only additive genetic variance while not affecting dominance variance. Consequently, traits closely associated with fitness should exhibit high levels of dominance variance. In this study we compile a large number of estimates of dominance variance to determine if traits that are subject to strong selection and/or are closely associated with fitness have higher levels of dominance variance than traits less subject to selection pressure. Estimates were taken from the literature for both wild and domestic species and each group was treated separately. Traits closely associated with fitness (life history) had significantly higher dominance components than did traits more distantly related to fitness (morphology) for wild species. No significant differences were found between life history and morphological traits for domestic species. Traits that were known to have been subject to intense directional selection (morphological traits for domestic species) had significantly higher dominance estimates than did traits that were assumed not to have been subject to strong selection (morphological traits for wild outbred species). The results are discussed with respect to the maintenance of heritable variation and the bias introduced in the calculation of the full-sib heritability estimate by high levels of dominance variance.