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Variation and Variability
Abstract
Variation is a central topic, both conceptually and historically in evolutionary biology. Phenotypic variation was Darwin's fundamental observation. Indeed, the first two chapters of On the Origin of Species deal explicitly with variation. Variation within and among species has certainly been as central to the thinking of Ernst Mayr (1963) as it was to the thinking of Sewall Wright (1968), two of the fathers of the modern synthesis. However, the study of variability or the propensity to vary, with few exceptions, has remained peripheral to study of the mechanisms of evolutionary change at any level of the biological hierarchy. Although implicit in virtually all research in the biological sciences, whether one is seeking understanding at the genetic, developmental, organismal, species, population, or ecologic/community levels, variation is seldom treated as a subject in and of itself. Variation is an extremely broad topic, and a modern treatment of this subject is not possible without a thematic focus. This chapter introduces this theme through both a hierarchical treatment and integrative approaches that point toward new directions of research.
... Evolution through natural selection is based on three main pillars: inheritance, selection, and variation (Darwin, 1872). Although remarkable advances in the understanding of inheritance and selection have been achieved, evolutionary variation still remains poorly understood (Andrade, 2009;Hallgrímsson, 2005). One of the difficulties in understanding evolutionary variation is that it is a multiscale phenomenon, that is, the source of variation is not only at the genetic level, but occurs at multiple levels of organization (Hallgrímsson, 2005). ...
... Although remarkable advances in the understanding of inheritance and selection have been achieved, evolutionary variation still remains poorly understood (Andrade, 2009;Hallgrímsson, 2005). One of the difficulties in understanding evolutionary variation is that it is a multiscale phenomenon, that is, the source of variation is not only at the genetic level, but occurs at multiple levels of organization (Hallgrímsson, 2005). As a result, although changes in DNA sequence or distribution of allele frequency in populations can produce morphological variation, many other factors can also promote or buffer variation. ...
In this review we examine the role of self-organization in the context of the evolution of morphogenesis. We provide examples to show that self-organized behavior is ubiquitous, and suggest it is a mechanism that can permit high levels of biodiversity without the invention of ever-increasing numbers of genes. We also examine the implications of self-organization for understanding the “internal descriptions” of organisms and the concept of a genotype-phenotype map.
... But, although less publicly recognized, intraspecific diversity is critical in decreasing the probability of extinction by increasing its ability to adapt to environmental changes [15][16][17]. On the contrary, reduced genetic diversity in a population or species decreases its ability to adapt to short (such as a new disease or a severe drought for example) and long term habitat changes (such as climate change for example) increasing the probability of its extinction [18,19]. In addition, intraspecific diversity plays a crucial role in the regulation of ecological processes [20], and is essential to ensure food security, and medicine development, to preserve cultural values and diversity and as a source of inspiration [16]. ...
On the edge of causing the sixth big mass extinction event, the development of positive attitudes towards the conservation of intraspecific diversity from early ages is essential to overcome the biodiversity crisis we currently face. However, there is no information available on elementary school students’ attitudes toward intraspecific diversity conservation nor is there a framework available to perform such analysis. For this study we designed, implemented, and evaluated an educational activity planned for third graders (8 to 13 years old) to explore the intraspecific diversity of vegetables and promote healthy eating habits. Additionally, a framework was developed to evaluate students’ attitudes towards intraspecific diversity conservation and applied to semi-structured interviews conducted with students before and after engaging in the educational activity. In this paper we present a reliable framework, developed aligned with the ABC model of attitudes, based on literature, and adapted to elementary school students’ responses, to evaluate students’ attitudes toward intraspecific diversity. Our results show that, before the educational activity, most students choose a non-biodiverse option, justifying this choice with the affective component of attitudes: mostly emotional factors but also aesthetic and social/cultural factors. After the educational activity, we observed a significant increase in the frequency of students that choose the biodiverse option and that justified it with the cognitive component of attitudes: mainly with biology and health knowledge factors, but also with economic and ethical knowledge factors. Our findings support the positive impact of educational activities that explore vegetable varieties on students’ attitudes toward intraspecific diversity conservation. This activity may also be used to foster education for sustainability and address socioscientific issues aligned with diverse sustainable development goals.
... Organismal development is a major determinant of phenotypic variation, and therefore is 42 fundamentally related to how organisms evolve (Hendrikse, Parsons and Hallgrímsson, 2007; 43 Hallgrímsson and Lieberman, 2008). Organisms are comprised of interrelated anatomical 44 elements whose morphology is patterned by shared genetic pathways (i.e., pleiotropic genes) and 45 often by the same developmental processes (Hallgrímsson and Hall, 2005;Murren, 2012). 46 Shared genetic and/or developmental processes lead to morphological integration, that is, the 47 tendency of sets of traits to covary more strongly internally than with traits in other sets 48 (Cheverud, 1996). ...
Bones in the vertebrate cranial base and limb skeleton grow by endochondral ossification, under the control of growth plates. Mechanisms of endochondral ossification are conserved across growth plates, which increases covariation in size and shape among bones, and in turn may lead to correlated changes in skeletal traits not under direct selection. We used micro-CT and geometric morphometrics to characterize shape changes in the cranium of the Longshanks mouse, which was selectively bred for longer tibiae. We show that Longshanks skulls became longer, flatter, and narrower in a stepwise process. Moreover, we show that these morphological changes likely resulted from developmental changes in the growth plates of the Longshanks cranial base, mirroring changes observed in its tibia. Thus, indirect and non-adaptive morphological changes can occur due to developmental overlap among distant skeletal elements, with important implications for interpreting the evolutionary history of vertebrate skeletal form.
Objectives:
The investigation of morphological variation in animals is widely used in taxonomy, ecology, and evolution. Using large datasets for meta-analyses has dramatically increased, raising concerns about dataset compatibilities and biases introduced by contributions of multiple researchers.
Materials and methods:
We compiled morphological data on 13 variables for 3073 individual mouse lemurs (Cheirogaleidae, Microcebus spp.) from 25 taxa and 153 different sampling locations, measured by 48 different researchers. We introduced and applied a filtering pipeline and quantified improvements in data quality (Shapiro-Francia statistic, skewness, and excess kurtosis). The filtered dataset was then used to test for genus-wide sexual size dimorphism and the applicability of Rensch's, Allen's, and Bergmann's rules.
Results:
Our pipeline reduced inter-observer bias (i.e., increased normality of data distributions). Inter-observer reliability of measurements was notably variable, highlighting the need to reduce data collection biases. Although subtle, we found a consistent pattern of sexual size dimorphism across Microcebus, with females being the larger (but not heavier) sex. Sexual size dimorphism was isometric, providing no support for Rensch's rule. Variations in tail length but not in ear size were consistent with the predictions of Allen's rule. Body mass and length followed a pattern contrary to predictions of Bergmann's rule.
Discussion:
We highlighted the usefulness of large multi-researcher datasets for testing ecological hypotheses after correcting for inter-observer biases. Using genus-wide tests, we outlined generalizable patterns of morphological variability across all mouse lemurs. This new methodological toolkit aims to facilitate future large-scale morphological comparisons for a wide range of taxa and applications.
On the edge of causing the sixth big mass extinction event, the development of positive attitudes towards the conservation of intraspecific diversity from early ages is essential to overcome the biodiversity crisis we currently face. However, there is no information available on elementary school students' attitudes toward intraspecific diversity conservation nor is there a framework available to perform such analysis. For this study we designed, implemented, and evaluated an educational activity planned for third graders (8 to 13 years old) to explore the intraspecific diversity of vegetables and to promote healthy eating habits. Additionally, a framework was developed to evaluate students' attitudes towards intraspecific diversity conservation and applied to semi-structured interviews conducted with students before and after engaging in the educational activity. In this paper we present a reliable framework, developed aligned with the ABC model of attitudes, based on literature, and adapted to elementary school students' responses, to evaluate students' attitudes toward intraspecific diversity. Our results show that, before the educational activity, most students choose a non-biodiverse option, justifying this choice with the affective component of attitudes: mostly emotional factors but also aesthetic and social/cultural factors. After the educational activity, we observed a significant increase in the frequency of students that choose the biodiverse option and that justified it with the cognitive component of attitudes: mainly with biology and health knowledge factors, but also with economic and ethical knowledge factors. Our findings support the positive impact of educational activities that explore vegetable varieties on students' attitudes toward intraspecific diversity conservation.
The importance of genetic diversity, especially within small populations of endangered species, is becoming increasingly apparent. Genetic engineering techniques that could potentially enhance diversity are now being developed. However, the use of such techniques would require social licence and yet within the general population there is limited understanding of the importance of variation within a species. This qualitative study explored the potential of introducing the concept of within-species variation to New Zealand primary school children (5 to 11 year olds) with the aim of preparing these students to make critical decisions about the use of these techniques in their futures. Eighty-four children participated in the study and their explanations for the impact that differences between individuals may have were analysed revealing their scientifically acceptable understanding of the concept. But findings also highlighted potential misconceptions that they may retain. These misconceptions include a belief that animals think and behave in a manner similar to humans or that variations within a species are for identification purposes only. Findings showed that the concept of within-species variation could be introduced to primary school children. However, to do this successfully curriculum documents may need to be reviewed and teachers may require specific professional development.
The study of desiccation resistance and its underlying traits is key to understanding species responses to changes in water availability, especially in the context of predicted increases in the frequency and severity of droughts due to climate change. We performed laboratory experiments using dung beetles, important ecosystem service providers, to investigate variations in physiological traits within species, both at population and individual levels. Desiccation resistance, water loss tolerance and water content were measured in lowland and mountain populations to investigate whether physiological traits vary (i) according to elevation across four species, and (ii) according to sex or male morphology (minor and major morphs) in two species, and if these responses were consistent across species. Our results showed that desiccation resistance of dung beetles varies both at individual and population levels. We found that desiccation resistance varied between lowland and mountain populations, but no differences were found for other traits such as water loss tolerance. Moreover, differences in individual physiological responses between females, major and minor males suggest that females were more resistant to desiccation than minors and majors, but these responses were species-dependent. Our analysis at two hierarchical levels, individual and population, emphasizes the importance of considering within-species variability in predictions of how species may respond to future climatic conditions. Predictions of the responses of species to environmental change may produce different conclusions if they rely on observations from single populations or take into account only a limited range of phenotypes per population.
Pacific giant salamanders (Dicamptodon) rank among the largest terrestrial caudates. Their ontogeny produces two distinct morphs—larval‐neotenic and metamorphosed—which differ in many morphological traits. We identified changes that are initiated by metamorphosis (distinguishing transformed from neotenic specimens) and also recognized age‐related changes occurring irrespective of transformation. During metamorphosis, specimens remodel the palate, rearrange the vomerine dentition, expand the maxilla, broaden the cheek, foreshorten the posterior skull table and develop specific serrated suture patterns in the dermal bones. Instead, large larvae grow a robust pterygoid sutured with a fully ossified trapezoidal vomer and a short maxilla. Small larvae are readily distinguished by tooth count, morphology and arrangement from more advanced larvae. Age‐related features, irrespective of metamorphosis, include pedicellate teeth, morphological differentiation of parasphenoid, enlargement of the orbitosphenoid, distal expansion of columella, and loss of coronoid teeth.
Variation in fitness across individuals is central to population growth, species coexistence and evolution by natural selection. Fitness variation associated with resource use is hugely consequential, but how this variation is generated and maintained within natural populations remains unclear. In particular, individual fitness may depend on many cumulative foraging decisions over time, but this hypothesis remains untested.
We used multi‐tissue stable isotope analysis to determine isotopic niche trajectories within species, populations and sexes of thin‐toed frogs and explored how this temporal dimension of diet affects individual reproductive investment, body condition and parasite load.
We found that individual frogs shifted their diets less than expected under a null model, likely due to functional trade‐offs that limit the incorporation of new prey types over time. However, within the observed range of diet shifts, individuals that modified their diet to a greater degree exhibited higher fitness, although this effect was sex‐dependent.
We suggest that these different relationships between isotopic niche trajectory length and fitness within thin‐toed frogs are driven by variability in the resource environment, negative density dependence and allometric constraints. These strong fitness effects suggest that the temporal dimension of diet change is a potential target of natural selection and, therefore, could drive correlated evolution in phenotypic traits underlying diet flexibility. Our findings add a new level of complexity to the understanding of ecological and evolutionary consequences of niche variation by demonstrating that temporal variation in foraging consistency within populations leads to different fitness pay‐offs.
A free Plain Language Summary can be found within the Supporting Information of this article.
The idea that physical processes involved in biological development underlie morphogenetic rules and channel morphological evolution has been central to the rise of evolutionary developmental biology. Here, we explore this idea in the context of seashell morphogenesis. We show that a morphomechanical model predicts the effects of variations in shell shape on the ornamental pattern in ammonites, a now extinct group of cephalopods with external chambered shell. Our model shows that several seemingly unrelated characteristics of synchronous, ontogenetic, intraspecific, and evolutionary variations in ornamental patterns among various ammonite species may all be understood from the fact that the mechanical forces underlying the oscillatory behavior of the shell secreting system scale with the cross-sectional curvature of the shell aperture. This simple morphogenetic rule, emerging from biophysical interactions during shell formation, introduced a non-random component in the production of phenotypic variation and channeled the morphological evolution of ammonites over millions of years. As such, it provides a paradigm for the concept of “developmental constraints.”
This study introduces a new multivariate approach for analyzing the effects of quantitative trait loci (QTL) on shape and demonstrates this method for the mouse mandible. We quantified size and shape with the methods of geometric morphometrics, based on Procrustes superimposition of fi ie morphological landmarks recorded on each mandible. Interval mapping for F(2) mice originating from an intercross of the LG/J and SM/J inbred strains revealed 12 QTL for size, 25 QTL for shape, and 5 QTL for left-right asymmetry. Multivariate ordination of QTL effects by principal component analysis identified two recurrent features of shape variation, which involved the positions of the coronoid and angular processes relative to each other and to the rest of the mandible. These patterns are reminiscent of the knockout phenotypes of a number of genes involved in mandible development although only a few of these are possible candidates for QTL in our study. The variation of shape effects among the QTL showed no evidence of clustering into distinct groups, as would be expected from theories of morphological integration. Fur ther, for most QTL, additive and dominance effects on shape were markedly different, implying overdominance for specific features of shape. We conclude that geometric morphometrics offers a promising new approach to address problems at the interface of evolutionary and developmental genetics.
Morphometrics has undergone a revolutionary transformation in the past two decades as new methods have been developed to address shortcomings in the traditional multivirate analysis of linear distances, angles, and indices. While there is much active research in the field, the new approaches to shape analysis are already making significant and ever-increasing contributions to biological research, including physical anthropology. Modern Morphometrics in Physical Anthropology highlights the basic machinery of the most important methods, while introducing novel extensions to these methods and illustrating how they provide enhanced results compared to more traditional approaches. Modern Morphometrics in Physical Anthropology provides a comprehensive sampling of the applications of modern, sophisticated methods of shape analysis in anthropology, and serves as a starting point for the exploration of these practices by students and researchers who might otherwise lack the local expertise or training to get started. This text is an important resource for the general morphometric community that includes ecologists, evolutionary biologists, systematists, and medical researchers.
This chapter reviews univariate and multivariate measures of variation, for both absolute and relative variation. Biologists almost never test variation appropriately; in particular, the F test should never be used for this purpose. There are, however, several useful and robust tests that apply, with modifications, to all useful measures of variation, including the multivariate coefficient of variation. Some of these applications are little known or new. A measure of the effective dimensionality of variation permits the relative variation of quantities such as volumes to be compared with that of linear measurements. Several measures exist for the tightness of a distribution in two or more dimensions, one of which generalizes this property of the correlation coefficient. Measurement error is part of observed variation and can be subtracted from it.
Canalization is the suppression of phenotypic variation. Depending on the causes of phenotypic variation one speaks either of genetic or environmental canalization. Genetic canalization describes insensitivity of a character to mutations, and the insensitivity to environmental factors is called environmental canalization. Genetic canalization is of interest because it influences the availability of heritable phenotypic variation to natural selection and is thus potentially important in determining the pattern of phenotypic evolution. In this paper a number of population genetic models are considered of a quantitative character under stabilizing selection. The main purpose of this study is to define the population genetic conditions and constraints for the evolution of canalization. Environmental canalization is modeled as genotype specific environmental variance. It is shown that stabilizing selection favors genes which decrease environmental variance of quantitative characters. Howev...