Biological Sciences
Phenotypic Variations
Phenotypic variations refer to the range of observable traits or characteristics displayed by individuals within a species. These variations can be influenced by genetic, environmental, and developmental factors, leading to differences in physical appearance, behavior, and other measurable traits. Understanding phenotypic variations is crucial in fields such as genetics, evolutionary biology, and ecology for studying adaptation and diversity within populations.
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11 Key excerpts on "Phenotypic Variations"
eBook - PDF- Matthew B. Hamilton(Author)
- 2021(Publication Date)
- Wiley-Blackwell(Publisher)
Even if there is only a single genotype, the pheno- type expressed by each individual in a population will vary somewhat depending on the environmen- tal conditions each individual experiences. For example, the biomass and fruit production of plants is impacted by the amount of sunlight and nitrogen each individual receives. Another example of envi- ronmental variation in quantitative traits is the role of diet and exercise in human disease, where better conditions tend to lessen the frequency or severity of disease phenotypes. Figure 9.3 shows how envi- ronmental differences among individuals contrib- ute to the continuous distribution of phenotypic variation. The left-hand panel of Figure 9.3 shows the five phenotypes produced by a trait due to two diallelic loci. If each phenotypic class expressed by a genotype is modified by environmental varia- tion, the distribution of phenotypes becomes both wider and smoother as shown in the right-hand panel of Figure 9.3. The environmental variation experienced by individuals is also likely to be a truly continuous variable, unlike the discrete categories of genotypes produced by multiple loci, which then causes continuous variation in phenotypes. Components of phenotypic variation Now that we have seen how discrete Mendelian genetic variation for multilocus genotypes combined with continuous environmental variation produces continuous phenotypic distributions, let’s represent the genetic and environmental causes of phenotypic variation in notation. In quantitative genetics, it is customary to symbolize expected quantitative trait variation with a V. The V variable always bears a subscript to indicate a specific cause of phenotypic variation. The total variation in phenotype is repre- sented by V P and examples of total phenotypic vari- ance are shown in Figures 9.1 and 9.4. This total phenotypic variation has both genetic and envi- ronmental causes.
eBook - ePub- Matthew Hamilton(Author)
- 2011(Publication Date)
- Wiley-Blackwell(Publisher)
Another primary cause of variation among individuals in quantitative traits is the environment. Even if there is only a single genotype, the phenotype expressed by each individual in a population will vary somewhat depending on the environmental conditions each individual experiences. For example, the biomass and fruit production of plants is impacted by the amount of sunlight and nitrogen each individual receives. Another example of environmental variation in quantitative traits is the roleFigure 9.3The effect of environmental variation on phenotypic variation. The phenotypic distribution on the left is produced by two Mendelian loci with all allele frequencies equal to ½s in Figure 9.2 . If the environment causes some variation in the phenotype expressed by each genotype, then the distribution of phenotypes produced by polygenic variation becomes both smoother and wider. In this illustration, environmental variation causes 50% of the individuals of each genotype to randomly increase or decrease one unit in phenotypic value. Although the average effect of environmental variation here is a zero change in phenotypic value, the phenotypic variance increases.of diet and exercise in human disease, where better conditions tend to lessen the frequency or severity of disease phenotypes. Figure 9.3 shows how environmental differences among individuals contribute to the continuous distribution of phenotypic variation. The left-hand panel of Fig. 9.3 shows the five phenotypes produced by a trait due to two diallelic loci. If each phenotypic class expressed by a genotype is modified by environmental variation, the distribution of phenotypes becomes both wider and smoother as shown in the right-hand panel of Fig. 9.3
eBook - PDFBiology Today
An Issues Approach
- Eli Minkoff, Pamela Baker(Authors)
- 2003(Publication Date)
- Garland Science(Publisher)
204 T he human species is highly variable in every biological trait. Humans vary in their physiology, body proportions, skin color, and body chemicals. Many of these features influence susceptibility to disease and other forces of natural selection. Continued selection over time has produced adaptations of local populations to the environments in which they live. Much of human biological variation is geographic; that is, there are differences between population groups from different geographical areas. For example, northern European peoples differ in certain ways from those from eastern Africa, and those from Japan differ in some ways from those from the mountains of Peru. Between these populations, however, lie many other populations that fill in all degrees of variation between the populations we have named, and there is also a lot of variation within each of these groups. Central to the study of human variation is the concept of a biological population, as defined in Chapter 5 (p. 151), and as explained again later. Both physical features and genotypes vary from one person to another within populations, but there is also a good deal of variation between human populations from different geographic areas as the result of evolutionary processes. How do populations come to differ from one another? How do alleles spread through populations? How do environmental factors such as infectious diseases influence the spread? Why are certain features more common in Arctic populations and other features more common in tropical populations? Why do we think of some of these variations as ‘races’? These are some of the questions that are explored in this chapter. There Is Biological Variation Both Within and Between Human Populations All genetic traits in humans and other species vary considerably from one individual to another. Some of this variation consists of different alleles at each gene locus; other variation results from the interaction of genotypes with the environment.
eBook - PDF- Fred W. Allendorf, Gordon Luikart(Authors)
- 2009(Publication Date)
- Wiley-Blackwell(Publisher)
Drosophila that differed phenotypically in natural populations could be brought into the laboratory for detailed analysis of the genetic differences under- lying the phenotypic differences. Similar studies were not possible for species with long generation times that could not be raised in captivity in large numbers. However, popula- tion genetics underwent an upheaval in the 1960s when biochemical techniques allowed genetic variation to be studied directly in natural populations of any organism. Molecular techniques today make it possible to study differences in the DNA sequence itself in any species. Projects are currently underway to sequence the entire genome of several species. However, even this level of detail will not provide sufficient information to understand the significance genetic variation in natural populations. Adaptive evolution- ary change within populations consists of gradual changes in morphology, life history, physiology, and behavior. Such traits are usually affected by a combination of many genes and the environment so that it is difficult to identify single genes that contribute to the genetic differences between individuals for many of the phenotypic traits that are of interest. This difficulty has been described as a paradox in the study of the genetics of natural populations (Lewontin 1974). We are interested in the phenotype of those characters for which genetic differences at individual loci have only a slight phenotypic effect relative to the contributions of other loci and the environment. “What we can measure is by definition uninteresting and what we are interested in is by definition unmeasurable” (Lewontin 1974, p. 23). This paradox can only be resolved by a multidisciplinary approach that combines molecular biology, developmental biology, and population genetics so that we can understand the developmental processes that connect the genotype and the phenotype (e.g. Lewontin 1999; Clegg and Durbin 2000).
eBook - PDF- Robert Jurmain, Lynn Kilgore, Wenda Trevathan, Russell Ciochon(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
But in the last 60 years or so, the emphasis has shifted to examining the differences in allele frequencies (and, more basically, DNA differ-ences) within and between populations, as well as considering the adaptive signifi-cance of phenotypic and genotypic variation. This shift in focus occurred partly because of the Modern Synthesis in biology. But now, armed with genome data sets for populations, biologists have an unprecedented opportunity to study and explain human variation and the role that evolutionary factors have played in producing it (Pritchard, 2010). In the twenty-first century, the application of evolutionary principles to the study of modern human variation has replaced the superficial nineteenth-century view of race based solely on observed phenotype . Additionally, the genetic emphasis has dispelled previously held misconceptions that races are fixed biological enti-ties that don’t change over time and are composed of individuals who all conform to a particular type . Clearly, there are visible phenotypic differences between humans, and some of these roughly correspond to particular geographical locations. But we need to ask if there’s any adaptive significance attached to these differences. Is genetic Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-300 THE CONCEPT OF RACE 417 drift a factor? What is the degree of underlying genetic variation that influences phenotypic variation? What influence has culture had in the past? These questions place considerations of human variation within a contemporary evolutionary, bio-cultural framework. Although, as a discipline, physical anthropology is rooted in attempts to explain human diversity, no contemporary scholar subscribes to pre–Modern Synthesis concepts of races (human or nonhuman) as fixed biological entities.
eBook - PDF- Robert Jurmain, Lynn Kilgore, Wenda Trevathan, Eric Bartelink(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
In its most common biological usage, the term race refers to geographically pat-terned phenotypic variation within a species. By the seventeenth century, naturalists were beginning to describe races in plants and nonhuman animals. They had recog-nized that when populations of a species occupied different regions, they sometimes differed from one another in the expression of one or more traits. But even today, there are no established criteria for assessing races of plants and animals, including humans. As a result, biologists now almost never refer to “races” of other species, but more typically talk about populations or, for major subdivisions, subspecies . Before World War II, most studies of human variation focused on visible pheno-typic variation between large, geographically defined populations, and these stud-ies were largely descriptive. But in the last 60 years or so, the emphasis has shifted to examining the differences in allele frequencies (and, more basically, DNA dif-ferences) within and between populations, as well as considering the adaptive sig-nificance of phenotypic and genotypic variation. This shift in focus occurred part-ly because of the Modern Synthesis in biology. But now, armed with genome data sets for populations, biologists have an unprecedented opportunity to study and explain human variation and the role evolutionary factors have played in producing it (Pritchard 2010). In the twentieth century, the application of evolutionary principles to the study of modern human variation replaced the superficial nineteenth-century view of race based solely on observed phenotype . Additionally, the genetic emphasis dispelled pre-viously held misconceptions that races are fixed biological entities that don’t change over time and that are composed of individuals who all conform to a particular type . Clearly, there are phenotypic differences between humans, and some of these dif-ferences roughly correspond to particular geographical locations.
eBook - PDF- David Briggs, S. Max Walters(Authors)
- 2016(Publication Date)
- Cambridge University Press(Publisher)
This notion of three types of variation, however, gives an oversimplified picture of the nature of individual variation. There is strong evidence for the proposition that the phenotype and behaviour of the plant are determined by interactions between genotype and environment. As we shall see, different genotypes react differently to a given set of environmental conditions, and plants of identical genotype produce different phenotypes under contrasting environmental conditions. Moreover, the interactions between genotype and environment are further complicated by the complex sequence of changes that occurs as a plant develops from an embryo to the mature fruiting state. Phenotypic variation In the growth of an organism from fertilised zygote the genotype of a particular plant plays a vital role in determining the characteristics of the mature Deficiency Original Breakage Altered From homologous chromosome Duplication Inversion Interchange translocation A B C D E F G H A B C D E F G H A B C D E F G H A B C D E F G H A B C D E F F F E D C H A B G F E D C H G B A FG H Eliminated A B C D E F H A B C D E T U V A B C D E F G H O P Q R S T U V O P Q R S F G H O P Q R S T U V Fig. 4.12. Diagrams to show how chromosome breakage and reunion can give rise to the four principal changes that chromo- somes may undergo. (After Stebbins, 1966.) Early work on the basis of individual variation 60 phenotype (perhaps a tree 30 m high) that is organised from raw materials drawn from outside the plant. There are complex close-knit interactions between genotype and environment at the level of the cell and of the whole plant. Concerning the genetic control of cellular processes, early ideas suggested that a gene provides information, which, in an appropriate environment, will contribute to a particular phenotype. However, accumulating evidence suggested that there was no certainty that a particular gene will always manifest itself.
eBook - PDF- Armando Caballero(Author)
- 2020(Publication Date)
- Cambridge University Press(Publisher)
54 3 Components of Phenotypic Values and Variances Suppose, now, that cuttings of different plants are grown in a greenhouse where the environmental conditions are kept as uniform as possible, controlling humidity, lighting, soil nutrient supply, and so on, so that the environmental conditions are as similar as possible for all plants. This is precisely what is sought in most experimental studies, since the source of environ- mental variability is usually the noise factor that masks the acquisition of genetic data. In this case, there will also be a phenotypic variation for the character (much greater than in the previous case) but this time it will be due almost exclusively to genetic causes (V P ≈ V G ), since the design tries to make the environmental variation null (V E ≈ 0). Therefore, for this population H 2 ≈ 1, even if it is the same trait and population under study as in the previous case. In fact, in this scenario H 2 can never reach the limit of 1 although it may get close, given that a total elimination of sources of environmental variation is not possible, as we will see in a later section. This shows that the definition of a quantitative trait includes both its description and the specification of the environ- ment in which it is evaluated. The previous reasoning reveals one of the possible methods of estimating the degree of genetic determination or broad-sense heritability. Using genetically identical individuals (such as the mentioned clones), an estimate of the environmental variance can be obtained which, subtracted from the phenotypic variance of a genetically variable population, allows for obtaining an estimate of the genotypic variance of this. This can also be done with highly inbred lines (which we will study in Chapter 4) as is the case with species whose natural form of reproduction is autogamy.
eBook - PDF- Robert Cliquet(Author)
- 2014(Publication Date)
- Masarykova univerzita(Publisher)
Two, and in the human perhaps even three, types of characteristics can be distinguished: Some characteristics are exclusively determined by genetic factors in the realisation of their phenotype. Well-known examples are the different blood group systems; Other characteristics are influenced both by genetic and environmental factors in their phenotypic expression. Most of these characteristics are continuously variable characteristics; It is possible there are behavioural patterns that are not at all dependent on the presence of certain genes, and are consequently exclusively determined by environmental circumstances. Nevertheless, even here one should beware of too simplistic a view. For instance, fashion variation that is obviously strongly determined by cultural factors, might be differentiated in a population partly on the basis of genetically influenced personality differences. INDIVIDUAL VARIATION AND INDIVIDUALISM /69 So far, only genetic processes that show a qualitative or alternative variability have been considered: one may be rhesus positive or rhesus-negative, one may be A, B, AB, or O in the ABO blood group system, one may have the predisposition to develop Huntington’s chorea or not, etc. This concerns characteristics that display clearly distinguishable alternative phenotypes resulting from the presence of different alleles of one gene. However, most biological characteristics – in particular most socially important performance characteristics such as physical performance abilities, emotional and cognitive personality characteristics, sexual and reproductive features, maturation characteristics, and many health characteristics – show a quantitative variability which may be of a continuous or discontinuous nature. Many of those quantitative variables show a more or less normal Gauss distribution. One shouldn’t conclude, however, that genetically influenced characteristics are controlled either by monogenes or by polygenes.
eBook - PDF- Tomar, Arun Kumar(Authors)
- 2021(Publication Date)
- Daya Publishing House(Publisher)
Chapter 18 Phenotypic Variance The phenotypic value and its causal components along with mean have been discussed in the last chapter. The population mean is used to compare the two or more populations for certain purpose. However, the mean value of a character is not sufficient to describe a population for a quantitative character because the population mean does not give any idea about the variability in phenotypic values of different individuals. The differences in phenotypic values of different individuals of a population are called as variation. The variation may be classified as group variation recorded on individuals o different groups, individual variation recorded on different individuals within a group and within individual variation recorded at different times on the same individuals. The degree or amount of variation is measured by various statistical measures viz . range, mean deviation, variance including the standard deviation, standard error and coefficient of variation. The variance is more useful population parameter because it has the properties of additivity and subdivisibility. The properties of variance are used topartition the total variance in to its causal components analogous to the partitioning of phenotypic value. These various componentsofphenotypic variance are very useful for their exploitationinbringing genetic improvementofa character. 18.1 Estimation of Variance The variance is taken as the mean of squares of the deviated values from their mean and referred as the mean of squared values (Mean square, M.S.). Thus, in estimating the variance, the phenotypic value of each individual is taken as its deviation from population mean, the deviated values are then This ebook is exclusively for this university only. Cannot be resold/distributed. squared and added, and the sum of squared valuesisdividedbythe number of individuals(on which the phenotypic values were taken).
- Evelyn Fox Keller(Author)
- 2010(Publication Date)
- Duke University Press Books(Publisher)
This criticism is of particular relevance to human behavioral genetics for the simple reason that such interactions are ubiquitous in the development of human behavior. Another common theme is the reminder that the significance of such measurements (when they can be meaningfully performed) is entirely dependent on the particular choice of population and the particular range of environments that the members of the population inhabit. For example, FROM INDIVIDUALS TO POPULATIONS 61 a finding of high heritability may allow one to conclude that most of the variation observed in the particular population under study is due to genetic variation, but that conclusion cannot be generalized to other popu-lations with di√erent ranges of genetic properties, or having available di√erent ranges of environmental contexts. Above all, such a finding does not provide an argument for genetic determination. Bluntly put, technical heritability neither depends on, nor implies anything about, the mecha-nisms of transmission (inheritance) from parent to o√spring. As an example, consider a trait that is known to be biologically inher-ited (i.e., repeated from generation to generation), such as, e.g., the num-ber of hands an individual has. We would normally say that hand number is a heritable trait. But what is its technical heritability? Answer: zero, or very close to it. And the reason is that, while there is phenotypic variance in the human population (not everyone has two hands), this variance is almost entirely due to accidents, not to genetics. The genetic variance relevant to hand number in the population at large is virtually nil. Or let us take, one last time, the example of pku . The expression of this disorder is clearly a heritable phenotypic trait, in the sense that the mutations responsible for that trait (most readily signaled by high levels of phenylalanine) are transmitted from parent to o√spring.
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