Two theories, Darwin’s Theory of Natural Selection and the Central Dogma, have dominated our thinking about evolution and genes for the last 50 years. The Theory of Natural Selection holds that variants arise in the population through random mutation, and that these variants are the substrate for selective pressures that enable new life forms to emerge. The Central Dogma holds that the basic unit of genetics is the gene. The combination of these two theories, which is called the New Synthesis, holds that changes in genes that arise through mutations, most commonly single base mutations, are the sources of the variation upon which natural selection operates. This view is very much centered on the primacy of the gene. Gene primacy is expressed in the famous maxim of Beadle and Tatum, “one gene, one enzyme”. The gene-centric view has led to searches for individual genes that cause diseases, the notion that individual genes can be selfish, and even the proposal that individual genes can account for specialized human traits such as language.
Several factors limit the rate at which evolution can proceed by this mechanism. One is the rate at which random mutations can create variants that have a selective advantage. This rate is likely to be low because virtually all mutations in existing genes are deleterious. A second factor is the rate at which selection operates, even when variants with advantage come into existence through mutation.
Evolution by the mechanism of the New Synthesis can operate advantageously when environmental pressures are low or increase gradually. These circumstances arise when the environment is stable. In this case, evolution takes place through competition between individuals. In this context, the New Synthesis mechanism of evolution, which is also called gradualism, is satisfactory.
However the environmental pressures are not always low, and in some cases can be strong and can increase rapidly. If the environment is rapidly changing, gradualism may evolve new life forms too slowly to ensure effective competition. Of course, changes in the natural environment, such a climatic changes, can create new selective pressures. However, one novel example of a rapidly changing selective pressure is societal pressures that are exerted through the rapid pace of human social and cultural evolution. As humans evolved from societies of hunting and gathering to agricultural ones, and then to urban societies, the demands upon the brain and nervous system increased immeasurably. Brain evolution through gradualism, which takes place gene by gene and is not directed towards acquisition of function, may well have been inadequate to meet this challenge.
An alternative mechanism for evolution, the Lamarckian mechanism, holds that characteristics acquired from the environment can contribute to inheritable changes and hence to evolution. The Lamarckian mechanism was discounted on the basis of the Central Dogma and our modern understanding of how genes function. Weismann posed the major criticism of Lamarckian theory. Weismann’s dogma stated that gametes and somatic cells are separate. Therefore the somatic cells cannot influence the gametes. Without such an influence, the characteristics acquired by the soma cannot be transferred to the gametes and hence cannot be inherited.
New evidence however has opened a window within which Lamarckian evolution can operate. This window arises from our modern understanding of epigenetics and mechanisms of human development. The science of epigenetics holds that heritable changes in gene function and expression can arise without the occurrence of mutations of the sort that fuel Darwinian evolution. Epigenetic changes involve the placement of small modifications of DNA base structure called marks, which alter DNA function without changing the DNA base sequence. Epigenetic marks can also involve chemical modifications of the histone proteins that package DNA. These marks do not alter the structure of the protein that is encoded by a gene. Instead they alter the level of expression or the circumstances under which the gene is expressed. Thus they do not suffer the limitation of Darwinian evolution imposed by its reliance on random mutation, which is that most mutations are deleterious. Also epigenetic mechanisms can affect the expression of many genes of an organism in a single generation, unlike Darwinian evolution in which genes alter one at a time. Epigenetics therefore provides a new view of the gene. The gene is not the primary unit of genetics; instead it is but one component of a larger system that controls organism function.
During development, specifically during gametogenesis or the early stages of embryogenesis, the soma of the mother can potentially influence the genes of the mothers’ gametes or the genes of the developing embryo epigenetically.
Barbara McClintock has demonstrated changes in the genome occur that do not involve changes in the structure of the genes themselves.
In gene imprinting, epigenetic mechanisms alter the level of expression of the maternal gene copy relative to the expression of the paternally inherited gene. Through imprinting, a parent is able to influence the level of expression of the gene copy that it provides to its offspring separately from the regulation of the same gene from the other parent. Catherine DuLac of Harvard has suggested that imprinting may represent a competition between the two parents. Each parent is struggling to optimize the offspring for its own purposes. The father may through imprinting exert a pressure that leads to greater numbers of offspring, while the mother may use imprinting to exert a pressure that makes the development or nurturing of the embryo compatible with the health of the mother. Imprinting is epigenetic because it imposes heritable changes on gene expression without altering the primary DNA sequence of the gene.
What sorts of factors could lead to epigenetic changes transmitted from the environment of the mother to the gene expression program of the offspring and its descendents? One is stress, a second is levels of nutrition, a third is the influence of drugs, and another is the health of the mother, for example whether the mother has suffered viral or bacterial infections during gestation. Stress and disease trigger can release powerful hormones that may influence the developing embryo.
Give here the Second World War example from Holland of the effects of starvation on first trimester developing embryos and their offspring.
Another is the ability of administration of cocaine to the mother to impose heritable behavioral changes on the offspring that can even be traced to the third generation.
Another is the effect of obesity of the mother on the caloric intake of the offspring and its descendants.
This notion of epigenetic, heritable reprogramming, which is Lamarckian at heart, suggests a new relationship between environment, mother and developing child. The mother may use epigenetic mechanisms to reprogram the development of the child and its descendants to prepare them for adverse environments. The Central Dogma and Gradualism distorted our view of disease, evolution and even social responsibility by holding such a mechanism impossible. They also have had a large impact on social policy. Epigenetics place a new light on the consequences of the environment for the well being of future generations. Now that we appreciate that hazardous or deleterious environments can have long-term consequences for future generations, such conditions can no longer be tolerated.