Nature vs. Nurture and the Origin of Genetics
Is human behavior controlled by genes or by environmental influences? The “nature vs. nurture” controversy has raged throughout human history, eventually leading to the current antithesis between hereditarianism and environmentalism in biological research. These two schools of thought have shaped a dispute that is at once a difficult scientific problem and a thorny ethical dilemma. Many disciplines, chiefly genetics but also the cognitive sciences, have contributed to the scientific aspect of the discussion. At the same time, racist and sexist overtones have muddled the inquiry and inextricably linked it to the implementation of social policies. Nevertheless, the relative degrees of influence of genes and environment in determining the characteristics of living organisms is a legitimate and important scientific question, apart from any social or ethical consideration.
At the beginning of the twentieth century, scientists rediscovered the laws of heredity first formulated by Gregor Mendel
in 1865. Mendel understood a fundamental concept that underlies all genetic analyses: each discrete trait in a living organism, such as the color of peas, is influenced by minute particles inside the body that behave according to simple and predictable patterns. Mendel did not use the term “gene” to refer to these particles (he called them“factors”), and his pioneering work remained largely unknown to the scientific community for the remainder of the nineteenth century. Immediately following the rediscovery of Mendel’s laws in 1900, the Danish biologist Wilhelm Johannsen
proposed the fundamental distinction between “phenotype” and “genotype.” The phenotype is the ensemble of all physical and biochemical traits of a plant or animal. The composite of all the genes of an individual is its genotype. To some extent, the genotype determines the phenotype.
Reaction Norm: Environments and Genes Come Together
It was immediately clear to Johannsen that the appearance of a trait is the combined result of both the genotype and the environment, but to understand how these two factors interact took the better part of the twentieth century and is still a preeminent field of research in ecological genetics. One of the first important discoveries was that genotypes do not always produce the same phenotype; rather, the phenotype varies with the particular environment to which the genotype is exposed. For instance, if two genetically identical fruit flies are raised at two different temperatures, there will be clear distinctions in several aspects of their appearance, such as the size and shape of their wings, even though the genes present in these animals are indistinguishable.
This phenomenon can be visualized in a graph by plotting the observed phenotype on the y axis versus the environment in which that phenotype is produced on the x axis. A curve describing the relationship between environment and phenotype for each genotype is called a reaction norm. If the genotype is insensitive to environmental conditions, its reaction norm will be flat (parallel to the environmental axis); most genotypes, however, respond to alterations in the environment by producing distinct phenotypes. When the latter case occurs, that genotype is said to exhibit phenotypic plasticity. One can think of plasticity as the degree of responsiveness of a given genotype to changes in its environment: the more responsive the genotype is, the more plasticity it displays.
The first biologist to fully appreciate the importance of reaction norms and phenotypic plasticity was Ivan Schmalhausen, who wrote a book on the topic in 1947. Schmalhausen understood that natural selection acts on the shape of reaction norms: by molding the genotype’s response to the environment, selection can improve the ability of that genotype to survive under the range of environmental conditions it is likely to encounter in nature. For example, some butterflies are characterized by the existence of two seasonal forms. One form exists during the winter, when the animal’s activity is low and the main objective is to avoid predators. Accordingly, the coloration of the body is dull to blend in with the surroundings. During the summer, however, the butterflies are very active, and camouflage would not be an effective strategy against predation. Therefore, the summer generation develops brightly colored “eyespots” on its wings. The function of these spots is to attract predators’ attention away from vital organs, thereby affording the insect a better chance of survival. Developmental geneticist Paul Brakefield demonstrated, in a series of works published in the 1990s, that the genotype of these butterflies codes for proteins that sense the season by using environmental cues such as photoperiod and temperature. Depending on the perceived environment, the genotype directs the butterfly developmental system to produce or not produce the eyespots.
Quantitative Genetics of Heredity and Environment
An important aspect of science is the description of natural phenomena in mathematical form. This allows predictions of future occurrences of such phenomena. In the 1920s, Ronald Fisher developed the field of quantitative genetics, a major component of which is a powerful statistical technique known as analysis of variance. This allows a researcher to gather data on the reaction norms of several genotypes and then mathematically partition the observed phenotypic variation (Vp
) into its three fundamental constituents:
Vp
= Vg
+ Ve
+ Vge
where Vg
is the percentage of variation caused by genes, Ve
is the percentage attributable to environmental effects, and Vge
is a term accounting for the fact that different genotypes may respond differently to the same set of environmental circumstances. The power of this approach is in its simplicity: the relative balance among the three factors directly yields an answer to any question related to the nature-nurture conundrum. If Vg
is much higher than the other two components, genes play a primary role in determining the phenotype (“nature”). If Ve
prevails, the environment is the major actor (“nurture”). However, when Vge
is more significant, this suggests that genes and environments interact in a complex fashion so that any attempt to separate the two is meaningless. Anthony Bradshaw pointed out in 1965 that large values of Vge
are indeed observable in most natural populations of plants and animals.
The quantity Vg
is particularly important for the debate because when it is divided by Vp
, it yields the fundamental variable known as heritability. Contrary to intuition, heritability does not measure the degree of genetic control over a given trait but only the relative amount of phenotypic “variation” in that trait that is attributable to genes. In 1974, Richard Lewontin pointed out that Vg
(and therefore heritability) can change dramatically from one population to another, as well as from one environment to another, because Vg
depends on the frequencies of the genes that are turned on (active) in the individuals of a population. Since different sets of individuals may have different sets of genes turned on, every population can have its own value of Vg
for the same trait. Along similar lines, some genes are turned on or off in response to environmental changes; therefore, Vg
for the same population can change
depending on the environment in which that population is living. Accordingly, estimates of heritability cannot be compared between different populations or species and are only valid in one particular set of environmental conditions.
Molecular Genetics
The modern era of the study of nature-nurture interactions relies on the developments in molecular genetics that characterized the whole of biology throughout the second half of the twentieth century. In 1993, Carl Schlichting and Massimo Pigliucci proposed that specific genetic elements known as plasticity genes supervise the reactions of organisms to their surroundings. A plasticity gene normally encodes a protein that functions as a receptor of environmental signals; the receptor gauges the state of a relevant environmental variable such as temperature and sends a signal that initiates a cascade of effects eventually leading to the production of the appropriate phenotype. For example, many trees shed their leaves at the onset of winter in order to save energy and water that would be wasted by maintaining structures that are not used during the winter months. The plants need a reliable cue that winter is indeed coming to best time the shedding process. Deciduous trees use photoperiod as an indicator of seasonality. A special set of receptors known as phytochromes sense day length, and they initiate the shedding whenever day length becomes short enough to signal the onset of winter. Phytochromes are, by definition, plasticity genes.
Research on plasticity genes is a very active field in both evolutionary and molecular genetics. Johanna Schmitt’s group has demonstrated that the functionality of photoreceptors in plants has a direct effect on the fitness of the organism, thereby implying that natural selection can alter the characteristics of plasticity genes. Harry Smith and collaborators have contributed to the elucidation of the action of photoreceptors, uncovering an array of other genes that relate the receptor’s signals to different tissues and cells so that the whole organism can appropriately respond to the change in environmental conditions. Similar research is ongoing on an array of other types of receptors that respond to nutrient availability, water supply, temperature, and a host of other environmental conditions.
From an evolutionary point of view, it is important not only to uncover which genes control a given type of plasticity but also to find out if and to what extent these genes are variable in natural populations. According to neo-Darwinian evolutionary theory, natural selection is effective only if populations harbor different versions of the same genes, thereby providing an ample set of possibilities from which the most fit combinations are passed to the next generation. Thomas Mitchell-Olds pioneered a combination of statistical and molecular techniques known as quantitative trait loci (QTL) mapping, which allows researchers to pinpoint the location in the genome of those genes that are both responsible for phenotypic plasticity and variable in natural populations. These genes are the most likely targets of natural selection for the future evolution of the species.
Complex Traits: Behavior and Intelligence
The most important consequence of nature-nurture interactions is their application to the human condition. Humans are compelled to investigate questions related to the degree of genetic or environmental determination of complex traits such as behavior
and intelligence. Unfortunately, such a quest is a potentially explosive mixture of science, philosophy, and politics, with the latter often perverting the practice of the first. For example, the original intention of intelligence quotient (IQ) testing
in schools, introduced by Alfred Binet
at the end of the nineteenth century, was simply to identify pupils in need of special attention in time for remedial curricula to help them. Soon, however, IQ tests became a widespread tool to support the supposed “scientific demonstration” of the innate inferiority of some races, social classes, or a particular gender (with the authors of such studies usually falling into the “superior” race, social class, or gender). During the 1970s, ethologist Edward Wilson freely extrapolated from behavioral studies on ant colonies to reach conclusions about human nature; he proposed that genes directly control many aspects of animal and human behavior, thereby establishing the new and controversial discipline of sociobiology.
The reaction against this trend of manipulating science to advance a political agenda has, in some cases, overshot the mark. Some well-intentioned biologists have gone so far as to imply either that there are no genetic differences among human beings or that they are at least irrelevant. This goes against everything that is known about variation in natural populations of any organism. There is no reason to think that humans are exceptions: since humans can measure genetically based differences in behavior and problem-solving ability in other species and relate these differences to fitness, the argument that such differences are somehow unimportant in humans is based on social goodwill rather than scientific evidence.
The problem with both positions is that they do not fully account for the fact that nature-nurture is not a dichotomy but a complex interaction. In reality, genes do not control behavior; their only function is to produce a protein, whose only function is to interact with other proteins at the cellular level. Such interactions do eventually result in what is observed as a phenotype—perhaps a phenotype that has a significant impact on a particular behavior—but this occurs only in a most indirect fashion and through plenty of environmental influences. On the other hand, plants, animals, and even humans are not infinitely pliable by environmental occurrences. Some behaviors are indeed innate, and others are the complex outcome of a genotype-environment feedback that occurs throughout the life span of an organism. In short, nature-nurture is not a matter of “either/or” but a question of how the two relate and influence each other.
As for humans, it is very likely that the precise extent of the biological basis of behavior and intelligence will never be determined because of insurmountable experimental difficulties. While it is technically feasible, it certainly is morally unacceptable to clone humans and study their characteristics under controlled conditions, the only route successfully pursued to experimentally disentangle nature and nurture in plants and animals. Studies of human twins help little, since even those separated at birth are usually raised in similar societal conditions, with the result that the effects of heredity and environment are hopelessly confounded from a statistical standpoint. Regardless of the failure of science to answer these questions fully, the more compelling argument that has been made so far is that the actual answer should not matter to society, in that every human being is entitled to the same rights and privileges as any other one, regardless of any real and sometimes profound differences in genetic makeup. Even the best science is simply the wrong tool to answer ethical questions.
Key Terms
genotype
:
the genes that are responsible for physical or biochemical traits in organisms
heritability
:
a measure of the genetic variation for a quantitative trait in a population
phenotype
:
the physical and biochemical traits of an organism
phenotypic plasticity
:
the ability of a genotype to produce different phenotypes when exposed to different environments
quantitative trait locus (QTL) mapping
:
a molecular biology technique used to identify genes controlling quantitative traits in natural populations
reaction norm
:
the graphic illustration of the relationship between environment and phenotype for a given genotype
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