Sunday, December 8, 2024
This is the first article of a series on “The New Epigenetics”. The epigenetic perspective in biology and psychology provides a solution to the clunky, age-old nature-nurture issue. It says that the old view that pits nature against nurture (genes against environment) as rival explanations for how development happens is flawed. Genes and environments are partners in development; they can’t work without each other. This has radical implications for how we understand biology, development, and human nature. Because different political ideologies are build largely around different conceptions of human nature, it has deep implications for political life as well.
The old nature-nurture issue continues to be debated in everyday life. Which is more important – genes or environment? The positions that people take on this issue are not simply academic; they play a role in their political opinions and in how people understand their relationships with others in their communities. People who believe that genes are more important in determining who tend to be skeptical about the effectiveness of social programs designed to enhance human well-being. On the contrary, people who believe in the power of the environment tend to believe that people can be changed by social and cultural conditions. The former tend to be more conservative while the latter are more progressive.
The problem is that the nature nurture debate should by now be defunct. This is because it is based on a false premise, namely the idea that nature and nurture (genes and environment, biology, and culture) are separate and independent entities. They are not. Development is an epigenetic process – which means that nature and nurture are inseparable as causal processes in development. In this article, we explain what this means, and why it matters not only in explanations of human development, but also for the ways in which we approach political decision-making. If nature and nurture influence each other, it makes no sense to take sides on whether people are “born that way” or are “products of their social environments”.
Development is Both Constrained and Open-Ended
Not all people who carry the BRCA gene connected with breast cancer actually develop the disease. Why is this? How is it possible for someone to have a gene “for” breast cancer but never develop the cancer? The answer is that there is no such thing as a gene for breast cancer. In fact, there is no such thing as a gene “for” anything. Although genes make certain events (like breast cancer) more or less likely –there no “for” any given outcome. Multiple genes in addition to environments (including the biological environment of the genes themselves) work together to produce developmental outcomes. This is true whether we are talking about height, male or female genitalia, breast cancer, personality dispositions, or the capacity to sing or write poetry.
The idea that there are genes for this or for that is a result of the old and tired “nature versus nurture” debate. This question has been debated for centuries. This debate, however, is a fruitless enterprise. The question of the relative importance of genes or environment in the development of individuals is largely a meaningless one. Developments in the field of genetics- and particularly in the study of epigenetics– have produced a clearer picture of how human growth and development occur. Development is not merely a matter of nature versus nurture, genes versus environment, biology versus culture — it is a matter of the relations between the two: nature and nurture, genes and environment, biology and culture.
The New (and Old) Epigenetics
There are two senses in which scholars use the term epigenesis. In its broader sense, epigenesis refers to the idea that genes and environment are inseparable as causal processes in the development of an individual. This means that anatomical and psychological processes are neither predetermine nor preformed. Instead, they emerge over time through the ways in which genes and environment influence each other.
The error with the old nature-nurture debate was the belief that nature and nurture are separate and independent processes. They are not. There is no effect of “nature” that is independent of “nurture”. Nature influences nurture, and nurture influences nature. This broader notion of epigenetics – even though it flies in the face of the nature versus nurture debate – is one that has been around for a long time. It just hasn’t been more fully embraced until relatively recently – and especially with new discoveries about epigenetics in the more recent, narrow sense.
Epigenesis in the narrow sense refers to the ways in which chemicals in the biological environment of the genes function to regulate how genes express themselves[1]. We often think that DNA is the “blueprint” for the organism. That is, we tend to think that a person’s genotype exclusively determines their phenotype. In so doing, we think that DNA contains the “plans” that determine the structure of the organism – and even the “plans” that determine the nature of human behavior. But this is not exactly so. To be sure, DNA plays a central role in the development of the structure of the body. It not only influences the structure of our body but plays a role in everything that we do. However, DNA doesn’t work by itself. Although it is foundational, in development, portions of DNA are “turned on” and “turned off” by molecular events that are occurring in the physical environment of the cells that are developing! That is, events that occur “above” the level of the gene (the “epi” in “epigenesis”) influence the action and expression of genes!
And so, epigenesis involves the study how genetic expression is influenced by that which is “beyond genetics; it is the study of how the expression of genes can be changed without changing the physical sequence of DNA. Through epigenetic changes, genes — sections of the DNA that are involved in the development of particular physical structures — can be “activated” or “silenced” as part of the process of developing a particular outcome. For example, consider what happens mere days after a human life is conceived. Although every cell has the full genetic code for the organism, any particular set of cells only needs information relevant to the function it needs to perform. A brain cell doesn’t need access to the same genetic information a heart cell. About six days after fertilization, cells begin to specialize for the roles they will perform: as skin cells begin to form, they will only “pay attention” to the genes that are relevant to their functioning. Conversely, as brain cells develop, they only “pay attention” to the genes relevant to their functioning. Genes unrelated to the function are epigenetically “turned off” (Atlasi & Stunnenberg, 2017).
Figure 1. Not by Genes Alone: Different Outcomes of the Physical Development of Fruit Flies and Parasitic Wasps as a Function of Biological Environment
Figure 1 shows a several simple examples of epigenetic changes as it occurs in simple organisms. The top panel shows what happens to a fruit fly when ether is introduced into the embryo at a particular stage of its development, it develops a second pair of wings. A similar effect occurs in parasitic wasps. A parasitic wasp can lay its eggs in either a butterfly host or an adler fly host. Eggs laid in the butterfly host develop wings and other structures; those laid in the adler fly host do not develop wings. These examples show how the development of physical anatomy cannot be said to be under the singular control of genes. Information from the environment interacts with genes to produce the structures that actually emerge.
Which is more important in the development of the wasp’s wings – the wasp’s genes or the host in which the wasp develops? The question is a meaningless one. To understand development, we have to look at the specific ways in which specific genes interact with specific environments over time.
[1] To understand this narrower sense, it is helpful to first understand DNA – the genetic aspect of the nature vs. nurture debate. DNA – deoxyribonucleic acid — is an inherited macromolecule that resides in every cell of every living organism. Each strand of DNA consists of a sequence of four molecules adenine (A), cytosine (C), guanine (G) and thymine (T), guanine strung together in different combinations. DNA is organized in pairs of strands. For each strand, A pairs with T and C pairs with G. Each person’s complete sequence of DNA is called their genotype. The outcome of development – a person’s height, genitalia, cancer, personality disposition, or capacity to sing– are called phenotypes. The genotype is the person’s genetic code; the phenotype is the developmental outcomes for a person who has that particular genetic code. They are not the same.
References
Atlasi, Y., & Stunnenberg, H. G. (2017). The interplay of epigenetic marks during stem cell differentiation and development. Nature Reviews Genetics, 18(11), 643-658.
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