Does natural selection alter genetic architecture? An evaluation of quantitative genetic variation among populations of Allonemobiussocius and A. fasciatus

To make long-term predictions using present quantitative genetic theory it is necessary to assume that the genetic variance–covariance matrix ( G ) remains constant or at least changes by a constant fraction. In this paper we examine the stability of the genetic architecture of two traits known to be subject to natural selection; femur length and ovipositor length in two species of the cricket Allonemobius. Previous studies have shown that in A. fasciatus and A. socius natural selection favours an increased body size southwards but a decreased ovipositor length. Such countergradient selection should tend to favour a change in G . In the total sample of eight populations of A. socius and one of A. fasciatus we show that there is significant variation in all genetic covariance components, i.e. V_A for body size, V_A for ovipositor length, and Cov_A. This variation results entirely from an increase in the covariances of A. fasciatus. However, although larger, these components are approximately proportionally increased, thereby leading to no statistically significant change in the genetic correlation. A proportional increase in the covariance components is consistent with changes resulting from genetic drift. On the other hand, the genetic covariance components are significantly correlated with the length of the growing season suggesting that the change in the genetic architecture is the result of selection and drift.     

Causes of covariation of phenotypic traits among populations

Morphological and life-history traits often vary among populations of a species. Traits generally do not vary independently, but show patterns of covariation that can arise from genetic and environmental influences on phenotype. Covariance of traits may arise at an among-population level when genetically influenced traits diverge among populations in a correlated manner. Genetic correlations caused by pleiotropy and/or gene linkage can cause traits to evolve together, but among-population covariance can also arise among traits that are not genetically correlated. For example, “selective covariance” can arise when natural selection directly causes correlated change in a suite of traits. Similarly, mutation, migration, and drift may also sometimes cause correlated genetic changes among populations. Because covariation of traits among populations can arise by several different processes, the evolution of suites of traits must be interpreted with great caution. We discuss the sources of among-population covariance and illustrate one approach to identifying the sources' using data on floral traits of Dalechampia scandens (Euphorbiaceae).     

Evolution of morphological differences with moderate genetic correlations among traits as exemplified by two flycatcher species (Ficeduia; Muscicapidae)

Theoretical work on multivariate evolution predicts that genetic correlations can act to constrain the rate at which new adaptive peaks are reached, but there is very limited empirical information available on this issue so far. To evaluate the importance of genetic correlations for evolutionary change, we studied the morphological differences between two flycatcher species (Ficeduia albicollis and F. hypoleuca) using both univariate and multivariate quantitative genetic models. Comparison of the results obtained using these different models revealed that even relatively low genetic correlations between traits will considerably increase the net selection forces needed for evolutionary changes in morphology. In particular, the divergence in wing and tail length, which are positively genetically correlated, would require a considerable amount of antagonistic selection. Because of the genetic correlations, strong selection will be needed to retain certain traits unchanged while others are changing. Based on these results, we argue that it is unlikely that small morphological differences such as between these two species could have evolved during a short (200 years) time period, i.e. the period of sympatry of these species in Sweden. These findings support the hypothesis that even relatively low genetic correlations may constrain short-term adaptive evolution in natural populations.     

Were neandertal and modern human cranial differences produced by natural selection or genetic drift?

Most evolutionary explanations for cranial differences between Neandertals and modern humans emphasize adaptation by natural selection. Features of the crania of Neandertals could be adaptations to the glacial climate of Pleistocene Europe or to the high mechanical strains produced by habitually using the front teeth as tools, while those of modern humans could be adaptations for articulate speech production. A few researchers have proposed non-adaptive explanations. These stress that isolation between Neandertal and modern human populations would have lead to cranial diversification by genetic drift (chance changes in the frequencies of alleles at genetic loci contributing to variation in cranial morphology). Here we use a variety of statistical tests founded on explicit predictions from quantitative- and population-genetic theory to show that genetic drift can explain cranial differences between Neandertals and modern humans. These tests are based on thirty-seven standard cranial measurements from a sample of 2524 modern humans from 30 populations and 20 Neandertal fossils. As a further test, we compare our results for modern human cranial measurements with those for a genetic dataset consisting of 377 microsatellites typed for a sample of 1056 modern humans from 52 populations. We conclude that rather than requiring special adaptive accounts, Neandertal and modern human crania may simply represent two outcomes from a vast space of random evolutionary possibilities.     

Genetic variation and asexual reproduction in the facultatively parthenogenetic cockroach Nauphoeta cinerea: implications for the evolution of sex

Asexual reproduction could offer up to a two‐fold fitness advantage over sexual reproduction, yet higher organisms usually reproduce sexually. Even in facultatively parthenogenetic species, where both sexual and asexual reproduction is sometimes possible, asexual reproduction is rare. Thus, the debate over the evolution of sex has focused on ecological and mutation‐elimination advantages of sex. An alternative explanation for the predominance of sex is that it is difficult for an organism to accomplish asexual reproduction once sexual reproduction has evolved. Difficulty in returning to asexuality could reflect developmental or genetic constraints. Here, we investigate the role of genetic factors in limiting asexual reproduction in Nauphoeta cinerea, an African cockroach with facultative parthenogenesis that nearly always reproduces sexually. We show that when N. cinerea females do reproduce asexually, offspring are genetically identical to their mothers. However, asexual reproduction is limited to a nonrandom subset of the genotypes in the population. Only females that have a high level of heterozygosity are capable of parthenogenetic reproduction and there is a strong familial influence on the ability to reproduce parthenogenetically. Although the mechanism by which genetic variation facilitates asexual reproduction is unknown, we suggest that heterosis may facilitate the switch from producing haploid meiotic eggs to diploid, essentially mitotic, eggs.     

Contrasting variation within and covariation between gender-related traits in autogamous versus outcrossing species: Alternative evolutionary predictions

We present several predictions concerning the expression of genetic variation in, and covariation among, gender-related traits in perfect-flowered plant taxa with different breeding systems. We start with the inference that the pollen:ovule (P/O) ratio in obligately autogamous species (in which the ovules in a flower are fertilized only by the pollen it produces) should be under much stronger stabilizing selection than in outcrossing taxa. Consequently, we predict that obligately autogamous taxa should exhibit lower genetic coefficients of variation in the P/O ratio. Nevertheless, genetic variation in both pollen and ovule production per flower might persist within autogamous as well as outcrossing populations. In autogamous taxa, genotypes with relatively few pollen grains and ovules per flower (but producing relatively high numbers of flowers) and genotypes with comparatively high numbers of gametes per flower (but producing relatively few flowers) could co-exist if lifetime flower production is selectively neutral. In contrast, in outcrossers, the maintenance of genetic variation in ovule and pollen production per flower might result predominantly from their ability to maintain variation in phenotypic and functional gender. Given genetic variation in primary sexual traits, we predict that the genetic correlation between investment in male and female gametes per flower should qualitatively differ between selfers and outcrossers. We predict a positive genetic correlation between pollen and ovule production per flower in obligately autogamous taxa, primarily because strong stabilizing selection on the P/O ratio should select against the gender specialists that would be necessary to effect a negative genetic correlation between mean pollen and ovule production per flower. Moreover, the fact that autogamous individuals are 50% female and 50% male means that gender-biased phenotypes cannot be functionally gender-biased, preventing selection from favouring phenotypic extremes. In contrast, in outcrossing taxa, in which functionally male- and female-biased genotypes may co-exist, the maintenance of contrasting genders could contribute to the expression of negative genetic correlations between pollen and ovule production per flower. We discuss these and a number of corollary predictions, and we provide a preliminary empirical test of the first prediction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Human genetic diversity,a critical resource for man's future

The human gene pool displays exuberant genetic variation; this is normal for a sexual species. Even small isolated populations contain a large percentage of the total variability, emphasizing the basic genetic unity of our species. As modern man spread across the world from its African source, the genetic basis for man's unique mental acuity was retained everywhere. Nevertheless, some geographical genetic variation such as skin color, stature and physiognomy was established. These changes were biologically relatively insignificant. Most of the genetic load in the genome has been carried throughout the history of the species. There is little hope of purging all of these harmful genes; we must accept them and continue to treat their syndromes medically. All populations carry extensive genetic variation due to genes that encode variations in quantitative traits. Of greatest importance among these is ubiquitous polygenic variability in brain function and intelligence. Mental acuity is what sets us apart from the rest of the biological world. Throughout our history, genetic recombination among the many genes involved in brain function has occurred. This has provided a genetic basis for the action of natural selection that favors intelligence in meeting the demands of the environment. As environments change in the future, this type of genetic variability will continue to be a crucial resource.This article is based on a contribution at the Session on Genetic Load chaired by Dr. Henretta Trent Band and presented at a meeting of the International Society for the History, Philosophy and Social Studies of Biology, Northwestern University, Evanston, Illinois in July 1991. The author is indebted to Professor Antonio Brito daCunha of the University of São Paulo, Brazil for his encouragement and comments.