Why nature versus nurture? Surely it must be nature and nurture—we all know that traits develop over time through the interaction of the organism with genetic or hereditary and environmental or social influences. Indeed, the modern science of epigenetics, building on ever-increasing information about DNA sequences and how genes function, now shows us how chemicals from outside the cell can modify the activity of genes for the rest of an organism’s life and sometimes even into subsequent generations. Moreover, since the late 1800s—well before advances in molecular biology—developmental biologists have been studying the mechanisms through which a single cell divides into multiple cell types and gets arranged into tissues, organs, and the organism’s overall form (Gilbert 2013). The nature versus nurture in this book [Taylor 2014] is not, however, a matter of development of traits over time. A sketch of the history and current state of the study of heredity, development, and variation is needed to set the scene.
When it is said that a person resembles their parent, two aspects of heredity—the transmission of traits to offspring—are being raised: how does an offspring develop to have the trait in question at all, e.g., its eye color, and how does the outcome of the development at some point in the lifespan differ from that person to the rest of the family or population. Development and difference do not necessarily require separate lines of research. Biologists often study abnormalities in order to gain insight about typical processes of development. In Swyer syndrome, for example, a child with XY chromosomes has female, not male, external genitalia. The influence of estrogen and progesterone on development is illuminated by the finding that, if these hormones are administered at puberty, breasts can develop and regular menstruation can occur (Michala et al. 2008). Variation under a more or less normal range of conditions can also help biologists understand typical development. The age at which a baby comes to walk, for example, varies according to whether it sleeps on its back or stomach, has been swaddled or carried around in a sling, and so on, and this variation has been related to the timing and degree to which the baby uses its upper body muscles (Fausto-Sterling 2014).
Difference can, however, be studied without providing much insight about development. For example, the eyes of fruit flies, normally red, are sometimes white. Biologists identified the location on the chromosomes that corresponds to the white-eye mutation early in the 1900s and later investigated the pigment-formation metabolic pathway and the enzymes involved as fruit fly eyes develop the normal or mutant color. Such knowledge says little about how eyes develop. Even on the narrower question of how eyes get to have color, a lot has to be known already about the development of the eye as a whole to make sense of how and when during the fly’s development the enzyme produces color in the eye.
The study of difference separate from development—albeit with a promise of later returning to issues of development—characterized the emergence of the field of genetics (Sapp 1983). This approach has been very productive. Advances during the twentieth century include the cataloging of mutant genes and their location on chromosomes; associating different genes with different enzymes (molecules that modulate biochemical interactions); identifying the chemical basis of genes—DNA—and the mechanisms of replication and mutation; revealing, then manipulating, the processes by which genes influence traits first in viruses and single-celled bacteria, then in complex, multicellular organisms; mapping the specific DNA sequence of entire organisms; comparing sequences among taxonomic groups; and tracing where and when in development specific genes are active.
No catalog or database of genes and DNA variants for any organism remotely resembles a literal “blueprint” or “program” for its development. Nevertheless, with ever-improving knowledge about genes and technologies to manipulate them, the field of genetics is now involved, not only in explaining how one organism differs in a trait from another, but also in illuminating various steps in how an offspring develops to have the trait at all. When the use of genetic knowledge and technologies is seen as a productive strategy to work towards teasing open the complexities of development, the blueprint and program metaphors can be put aside; there is less need to depict DNA as the master molecule determining the traits an organism has and to assert the dominance of nature over nurture. The study of genes in development can then take researchers in many directions (Gilbert 2013). Some might continue to focus on the location of genes on the chromosomes, the mechanisms of their coding for enzymes, and how to engineer their action in new ways. Others might want to use knowledge of which genes are functioning and under which cues—from inside the organism or outside—to gain insight about mechanisms by which traits develop. Yet others might want to continue, in the long tradition of developmental biology, investigating how cells become arranged into tissues, organs, and the organism’s overall form, studying biophysical processes as well as the biochemical pathways linked to genes.
The study of difference separate from development also underlies a quite different branch of research on heredity. Very early on in the history of genetics, researchers such as Fisher and Wright saw the need to reconcile the discreteness of traits related to mutant genes with continuous variation in many observable traits, especially traits in agriculture of economic interest such as yield of plant and animal varieties or breeds. Discrete variants not clearly related to pedigrees of inheritance of a mutant gene also needed to be addressed. Using models of theoretical genes—idealized “genes,” not those later mapped and probed—and assuming that more of these genes are shared among relatives than in the population as a whole, data on the variation for a trait in a specific group or population could be analyzed so as to provide predictions of changes in the average value of the trait under selective breeding. The same methods of data analysis also partitioned the variation into components related to differences between the agricultural varieties, between the locations in which they were grown or raised, between specific variety-location combinations, and unsystematic variation. To choose a simple example, if the between-variety fraction of the variation—or heritability as this component came to be called—were zero, there would be no change expected under selective breeding.
Analysis of variation in ways that take genealogical relatedness into account without identifying inheritance of specific genes—the field of quantitative genetics—was soon extended from agricultural and laboratory breeding to analysis of variation in human traits, especially behavioral or psychological measures such as IQ test scores—the field of behavioral genetics. The emphasis here was not on selective breeding yet variation was still partitioned into heritability and other components… The usual shorthand for the between-variety and between-locations components of variation is “genetic” and “environmental.” Although these adjectives invite confusion [as discussed in the book], their acceptance is not surprising given that partitioning of trait variation has drawn from and fed into debates that pre-date heritability studies—indeed pre-date the field of genetics—about just how much a trait is nature versus how much is nurture. We all know that nutrition, care, and social interaction are needed for offspring to grow and thrive, but how much influence do we expect the environment or society to have on the eventual outcome—be it adult height or IQ test score—that results from that development?
Fausto-Sterling, A. (2014). “Letting go of normal.” Boston Review (March/April), http://www.bostonreview.net/wonders/fausto-sterling-motor-development (viewed 21 June 2014).
Gilbert, S. (2013). Developmental Biology. Sunderland, MA: Sinauer.
Michala, L., D. Goswami, et al. (2008). “Swyer syndrome: Presentation and outcomes.” BJOG: An International Journal of Obstetrics & Gynaecology 115(6): 737-741.
Sapp, J. (1983). “The struggle for authority in the field of heredity, 1900-1932: New perspectives on the rise of genetics.” Journal of the History of Biology 16(3): 311-342.
Taylor, Peter J. Nature-Nurture? No. Moving the Sciences of Variation and Heredity Beyond the Gaps. Arlington, MA: The Pumping Station, 2014 (details)