Saturday, February 20, 2010

Darwinian Evolution in Four Dimensions

People often question me about why I stress the importance of reading Charles Darwin's writings rather than relying on modern textbook presentations of evolutionary biology. After all, hasn't there been great progress in biological science since the publication of Darwin's Origin of Species a 150 years ago? This is certainly true. But it is also true that in many respects Darwin's original understanding of evolution is superior to the prevailing views of today's "Neo-Darwinians." Contemporary proponents of Darwinism like Richard Dawkins tell us that biological evolution is ultimately reducible to the natural selection of random genetic mutations. More and more critics are pointing out the flaws in such a genetic reductionist view of evolution, and they are arguing that we need to explain the complex interaction of multiple levels of evolution that cannot be reduced to the gene-centered view of Neo-Darwinism. But if one studies Darwin's writings, one notices that what these critics are proposing is actually a return to Darwin's original theory.

Consider Darwin's summary of his theory in the last paragraph of the Origin:

"It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing a Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."

Notice that Darwin states his "laws" of evolution in a very general and abstract way--the laws of reproduction, inheritance, variability, and struggle for life. If some entities can reproduce themselves, if these entities show heritable variation, and if some of this heritable variation affects their chances of surviving and reproducing in the competitive struggle for existence, then evolution by natural selection will occur. Stated in such a general way, these laws could apply as well to the evolution of human culture as to the evolution of animal anatomy. And, in fact, much of Darwin's Descent of Man is a study of the cultural evolution of human morality.

Notice also that Darwin thinks that heritable variation can arise from "the indirect and direct action of the external conditions of life, and from use and disuse." This means that Darwin embraces Lamarck's idea of the inheritance of acquired characters. So the common story that Darwin overthrew Lamarckianism is false.

In the 1930s, the "Modern Synthesis" of evolutionary biology combined a Neo-Darwinism that rejected all Lamarckianism with Mendelian genetics. It was assumed that genes were the only units of heredity, that variations in genes are random and not affected by the developmental history of the individual, and that selection favors individuals with genes that make them more adapted to their environment than others. Consequently, evolution was understood as some change in the genetic composition of some group of organisms.

In recent decades, empirical research and theoretical arguments have thrown this Modern Synthesis of Neo-Darwinism into doubt, because it seems that a gene-centered theory cannot fully account for the evolution of life. Some people are even saying that this is an intellectual revolution that will destroy Darwinism.

One of the best critiques of Neo-Darwinism is by Eva Jablonka and Marion Lamb, Evolution in Four Dimensions: Genetic, Epigenetic, Behavioral, and Symbolic Variation in the History of Life (MIT Press, 2005). But rather than overturning Darwinism, Jablonka and Lamb see their critique as a renewal of Darwin's Darwinism.

The argument of Jablonka and Lamb is that in evolution the genetic system of inheritance is only one of four dimensions of evolutionary inheritance, and that these multiple dimensions of inheritance show a Lamarkian evolution of acquired characters, just as Darwin believed.

To illuminate their general point, Jablonka and Lamb use an analogy to show how different systems of heredity can work along with the genetic system. We can think of a piece of music that is represented by a score, the notes written on paper. This score can be copied as it is passed on through the generations. Although a few mistakes in copying might occur over time, generally the score will be accurately transmitted. We might then see the relationship between the musical score and the musical performance as analogous to the relationship between a genotype and a phenotype in biology. Although mutations in the genotype will be transmitted to future generations, changes in the phenotype will not be transmitted, and so the Lamarckian inheritance of acquired characters does not occur.

But with the invention of new means of transmitting music--such as musical recordings and broadcasts--it becomes possible to transmit the musical performance (the "phenotype") by inheritance. It is also possible that popular performative interpretations of the music might bring notational changes in the score, and in this way the musical "phenotype" would change the musical "genotype."

In a similar way, Jablonka and Lamb argue, there are systems of inheritance beyond the genetic system that allow phenotypic variations to be transmitted across generations. All organisms have two systems of inheritance--the genetic system and the epigenetic system. Many animals have a third system--the behavioral system. Human beings are unique in that they have not only these three systems, but also a fourth--the symbolic system. The full complexity of evolution arises from the intricate interaction of these four dimensions of evolutionary inheritance, which correspond to various levels of complexity from the genome to cells to organisms to groups.

The genetic inheritance system is the foundation for the Neo-Darwinian theory of evolution. Jablonka and Lamb accept this as true, but they also criticize genetic reductionism and determinism for failing to see how gene action depends on the complexity of interacting causes within the genome, within cells, within organisms, within groups of organisms, and within ecological circumstances. Except for a few single-gene genetic disorders, "genetic astrology"--the idea that genes directly control specific traits--must be dismissed as foolish.

The epigenetic inheritance system is evident in the differences between specialized cells. Brain cells, liver cells, and skin cells are very different, although the nucleus of each cell has the same genome. Their differences are epigenetic, rather than genetic, because they have arisen through their developmental history in which there were different patterns of gene activation and interaction within the cell. This developmental information is passed on as these cells divide to produce more cells of the same kind. It is possible for evolution to occur through heritable epigenetic variation even without genetic variation. Just as a musical recording transmits interpretations in musical performances of a musical score, so does an epigenetic inheritance system transmit interpretations of the information in DNA, so that there is a Lamarkian inheritance of phenotypes instead of genotypes. One version of such inheritance that is now under active study is DNA methylation: strands of DNA are chemically modified during development, and these modifications can be transmitted through reproduction.

The behavioral inheritance system is the transmission of information among animals through social learning. For example, among some animals (including human beings) mothers transmit food preferences to their offspring, because information about what mother is eating is transmitted either in the womb or through suckling, so that the offspring inherits a preference for that food. More complex forms of social learning come through animal culture. For example, some chimpanzees can discover how to open nuts with a stone, and then pass on this practice within their group so that it becomes a social tradition. Different communities of chimps in Africa have different cultures based on distinctive profiles of traditional practices transmitted by social learning. As opposed to genetic evolution, cultural evolution is not blind but targeted to functional change.

The symbolic inheritance system is uniquely human because it shows the qualitative leap that defines our humanity as based on our capacity for symbolic thought and communication. Other animals can communicate through signs. But only human beings can communicate through symbols. The evolution of human language was probably crucial for the evolution of symbolism. Symbolic systems allow us to think about abstractions that have little to do with concrete, immediate experiences. Symbolic systems allow human beings to construct a shared imagined reality. These symbolic constructions are often fictional and future-oriented. Art, religion, science, and philosophy are all manifestations of human symbolic evolution. What Karl Jaspers called the Axial Age (600-200 B.C.) would be an example of a turning point in the symbolic evolution of humanity, in which Confucius, the Buddha, the Hebrew prophets, and Plato acted as agents of symbolic innovation that has been inherited across the generations for the last two thousand years.

Neo-Darwinian theorists can hardly deny the reality of symbolic or cultural evolution. So they have developed their own theories of how this works. Dawkins is famous for his theory of "memetics"--the idea that "memes" (units of cultural replication) can evolve as "viruses of the brain." The evolutionary psychologists (like John Tooby and Leda Cosmides) explain cultural evolution as a historical process of social learning constrained by the genetically evolved biases of the brain. Although Jablonka and Lamb see some partial truth in these approaches, they also see them as inadequate for explaining the uniqueness of symbolic evolution as shaped through the constructive activities of individual and social agents in history.

The four-leveled account of evolution could explain Aristotle's biological studies of political animals. Human beings are not the only political animals, Aristotle observed, but human beings are more political than the other political animals because the human capacity for logos--speech or conceptual reasoning--allows human beings to organize their political life around shared conceptions of the good and the just. Jablonka and Lamb might say that while human beings share with other animals a capacity for political culture based on behavioral inheritance, only human beings have a capacity for political symbolism that creates a shared symbolic meaning for political life.

I will be writing more posts on these four dimensions of evolution.


Anonymous said...

"strands of DNA are chemically modified during development, and these modifications can be transmitted through reproduction."

Isn't that a mutation?

Larry Arnhart said...

No, the chemical modification in epimutation is not in the DNA itself but in the chromatin marking. Methylated DNA has a small methyl group attached to some of its bases. The pattern of methylation influences which regions of DNA are expressed. The different kinds of cells in the human body have the same DNA, but the different patterns of methylation regulate the expression of DNA.

Anonymous said...

Darwin is an important historical figure, and an excellent English author. He's worth reading.

But if we're talking about doing science, Darwin alone doesn't cut it. No matter how Dr. Arnhart argues it, science is an ongoing process (a process in some ways contradictory to a conservative mindset). We shouldn't "revere" Darwin. We should revere the process by which we come to scientific understanding. That process is not through continual reading and re-reading of Darwin alone.

Darwinian Natural Right is going to have to be a Natural Right theory unlike any other because it's going to have to be based on the modern findings of the evolutionary sciences. It must therefore be flexible, open to change and modification, open to revision, and willing to regard truth as tentative. It must evolve. Otherwise, it will become a fossil.

The "truths" of science are a moving target, understood over time through a process. Darwin is just one figure in the history of that process. Perhaps Dr. Arnhart should change the name to "Evolutionary Natural Right."

Troy Camplin said...

I'm in the middle of reading this book myself and I have been finding it quite interesting. One could criticize them for not including all forms of epigenetics -- such as RNA editing -- but that doesn't detract from their work overall. As someone whose work involves spontaneous order theory, I am finding their ideas quite compatible with that theory of social evolution.

So far in the book they also do not talk about the fact that there are brain proteins that rearrange in the same way as immunoglobulins. That is an incredibly important fact that affects our ideas of intelligence, etc. and, therefore, the products of ou4r brains, meaning our social and symbolic environments.

Larry Arnhart said...


Jablonka and Lamb do include RNA interference as an "epigenetic inheritance system" (132-37). Is that what you had in mind?


Troy Camplin said...

No, RNA editing is very different from RNA interference. WIth RNA editing, there are RNA molecules which add nucleotides internally to mRNA, thus modifying the code. One could also bring up the addition of poly-A tails to mRNA as well, as the difference in tail length affect the number of copies of protein produced. Both are by definition epigenetic. I found it odd that neither were mentioned in what was otherwise a fairly exhaustive list.