In this blog post, we will look at why embryology was marginalized as a field of study and how the development of molecular biology brought about change.
The discussion of “development” has been a hot topic in the life sciences from ancient civilizations to the present day. In particular, the discussion of where and how life originated has existed since ancient Greek times, as can be seen in Aristotle’s book on biology, De Generatione Animalium.
At that time, the academic world caused a major stir by elucidating the development of animals and the flexible relationship between biological trees and evolution mainly through anatomy. This is important because it is a discussion that can serve as the basis for most fields of evolutionary biology. After Darwin published “On the Origin of Species,” the field of evolutionary biology was born, which merged with other fields of biological research, such as genetics and embryology, and began to generate numerous discussions.
Ironically, however, the mainstream discussion on development was dominated by genetics, and embryology was not readily accepted by the academic community. Until the development of molecular biology, embryology was a discipline that was difficult to prove mathematically, and because the phenomenon of “development” could not be scientifically proven, it was rejected as a discipline based on philosophical discussion.
However, does embryology really have no scientific value in discussions about development and evolution? With the development of molecular biology in the 1970s and the emergence of the principle of “ontogeny as a rule,” perceptions of embryology changed, and the direction of evolutionary biology research began to shift. In this article, we will first examine the differences in perspective between embryology and genetics and the resulting research directions, and then discuss the future direction of evolutionary biology. First, we will examine what information is contained in development, and then discuss the restoration of the reputation of embryology following the emergence of The Double Helix in the 1970s and the desirable direction of evolutionary biology research.
First, let us examine what information is contained in development and how the developmental process is controlled. According to most biology textbooks, the source of developmental information is genes. In fact, in the academic world of evolutionary biology, genes have been given a privileged position in developmental research and have become the main field of study. However, some argue that genes alone cannot be said to contain all the information necessary for development, as the process also requires other factors within the cell, such as organelles, the appropriate distribution of chemical concentrations within the cytoplasm, and DNA methylation patterns. Furthermore, they argue that these environmental factors are also inherited across generations like genes, and therefore also contain information about development and play an equal role in the developmental process alongside genes. In other words, they argue that it is not genes alone that are the units of evolution and resources for development, but rather the “developmental system,” which includes the intracellular environment. Based on these grounds, arguments have been raised since the mid-20th century to shift the academic trend of focusing solely on genes to a more comprehensive study of the developmental system. Considering that biology is a discipline that studies the interactions of various complex factors, this argument is quite convincing. However, embryology was unable to emerge as a major field of study in academia due to the dominance of geneticists and was eventually eliminated.
So what is the argument of geneticists? Geneticists argue that the information for development is contained in genes rather than external factors such as the surrounding environment. Let’s take a moment to look at the relationship between genes and phenotypes to clarify the source of developmental information. The relationship between genes and phenotypes is quite complex, but it can be divided into four main cases.
The two extreme cases are that phenotypes depend only on genotypes and are not influenced by the environment, and that phenotypes depend only on the environment and are influenced only by the acquired environment, regardless of the genotype. The other two cases are compromises. The first is a type in which the power of the environment cannot overcome differences in genotypes, but in the same environment, the phenotypes of two individuals with different genotypes differ consistently. The second is a type in which the influence of the environment plays a greater role, and even if the genotypes are different, the differences in genotypes can be reversed by the influence of the environment.
In the natural world, most cases fall under the first type of compromise, or “additive interaction.” In other words, the relationship that arises when genes are expressed as phenotypes is due to interactions between genes rather than other external environmental factors. Concepts of already identified phenotypes, such as “polygeny” and “pleiotropy,” also support the claim that phenotypes are expressed through interactions between genes.
Based on the above, we can conclude that genetic information is contained in genes. Of course, as mentioned earlier, the intracellular environment is used as a resource for normal development, but considering that the relationship between genes and phenotypes is determined by interactions between genes and that genes ultimately control development, we can say that the main genetic information is contained in genes.
To use an analogy, just as physicists analyze free fall motion by considering only gravity without considering other forces such as air resistance, it is possible to adequately explain development by considering only genes. We have looked at the differences in opinion between developmental biologists and geneticists.
It was simply a difference in perspective, but as mentioned earlier, genetics became the mainstream field of research in academia, which prevented integrated research in biology, especially in evolutionary biology. However, after the development of molecular biology in the 1970s and 1980s, both fields advanced rapidly, and the direction of research in biology entered a new phase.
At the end of the 20th century, advances in molecular biology led to the discovery of homeoboxes, genes that control the developmental processes of organisms. This had a tremendous impact on academia because, although not true in all cases, one of the homeobox genes, the Hox gene, showed remarkable compatibility even between distantly related species. Let’s look at an example of a gene that controls eye development. In fruit flies, a gene called eyeless controls eye development, while in vertebrates, a gene called Pax6 does the same. What would happen if the eyeless gene were transplanted into a mouse embryo or the Pax6 gene were transplanted into a fruit fly embryo? Surprisingly, normal mouse eyes would develop in the mouse embryo, and normal fruit fly eyes would develop in the fruit fly embryo.
This is because genes such as Pax6 and eyeless act as master control genes, which regulate cell differentiation in the early stages of embryonic development. This discovery provides a decisive clue to the most fundamental question in development: “Why can a single fertilized egg differentiate into various complex adult organisms?” With the advent of molecular biology, embryology also began to gain attention.
Previously, there was no scientific basis for explaining development, but with the discovery of developmental genes such as Hox genes through molecular biology, some of the mechanisms of development were elucidated. From the 1990s, a new field called Evo-Devo emerged, and development became part of mainstream evolutionary biology, leading to full-scale attempts to link development and evolution. This is very significant because it provided the motivation for embryology to be integrated into modern biology. In the early 20th century, when Darwin’s theory of natural selection was challenged by the claim that mutations could not be the cause of evolution, population genetics restored the credibility of natural selection, leading to the integration of natural selection and genetics, or modern synthesis. With the discovery of developmental genes and the emergence of Ibo-Dibo, developmental biology seized the opportunity to be integrated into modern integration.
In this article, we have seen how the fusion of genetics and developmental biology and the emergence of a new field called Ibo-Dibo came about through the discovery of the source of developmental information and the advancement of molecular biology. So, what direction should evolutionary biology take in the future? Until now, evolutionary biology has mainly focused on the study of genes rather than evolution and development. This is because, as mentioned earlier, genes contain most of the information about development and the evolutionary process. However, with the discovery of developmental genes and the emergence of iboido, it is now time to conduct integrated research without distinguishing between fields such as genetics, developmental biology, and natural selection theory. In addition to research on genes, even embryology, which was criticized as a metaphysical element based on past philosophical premises, must be further studied through iBodiBo. If we can complete the integration of embryology following the modern synthesis that integrated natural selection theory and genetics in the past, we will be able to find answers to various fundamental mysteries of evolutionary biology.
