The synthesis is arguably the second most important historical event in evolutionary biology after the publication of the 'Origin of Species' As its name indicates the Modern Synthesis is not a dramatic new discovery. Like the 'Origin' itself it ties together many disparate aspects of biology and unifies them. Unlike the 'Origin' the Modern Synthesis was not the work of one person nor was there a single unifying event.
So what the heck was it? In the previous diary we discussed how the theoretical work of R.A. Fisher, Sewall Wright, and J.B.S. Haldane in the 1920s demonstrated how various evolutionary forces could act on the frequencies of alleles (particular versions of genes) in populations and cause those frequencies to change (i.e. evolution). At the time very few biologists had the mathematical background to appreciate these papers.
The synthesis involved
a) the dissemination of Fisher, Wright, and Haldane's findings into the broader biological community.
b) the realization that the disparate evolutionary phenomena studied in different subdisciplines could be unified by these findings
c) the realization that evolution over the short term (microevolution) and over the long term (macroevolution) had the same underlying processes.
Evolution (in general) in the early 20th Century
The last two diaries were largely devoted to genetics. The results of geneticists were not widely appreciated by other biologists for most of the first half of the 20th century. Richard Lewontin, probably the most influential post-synthesis population geneticist, describes being told as an undergraduate that genetics was a passing fad. This was in 1951! It is hard to imagine a biologist, 50 years after the rediscovery of Mendelism, making such a statement given the centrality of genetics to much of modern biology.
The repeated description of the study of evolution during the preWWII ear is of insularity and a widespread rejection of natural selection as a primary force in causing evolution. This is also pretty hard to imagine in the post-synthesis era. Lamarckian views of the inheritance of acquired characteristics were widespread in France in particular but were also held by prominent biologists in the US and elsewhere. Communication between specialists in different fields (paleontology, genetics, systematics, cytology (study of cells)) was at an extremely low level.
During the 1930s and 1940s this changed rather rapidly. As I mentioned above there wasn't a single major event. A number of biologists published influential books and by the late 1940s it was recognized that evolution was caused by natural selection and other evolutionary forces acting to change the frequencies of alleles in populations. The disparate phenomena being studied could all be linked.
The Start of the Synthesis
One of the striking characteristics of the modern synthesis is that it was dominated by American academics. Britain had long been the dominant nation in the study of evolution, dating back to Darwin and Wallace. While the synthesis was international in scope, the majority of the important figures were at American universities. Two of the most important were European expatriates - a common feature of American intellectual life at the time.
Not surprisingly the start of the synthesis was collaboration, either actual or indirectly, between the theoretical geneticists and empirical geneticists. R.A. Fisher worked with Edmund Brisco (E.B.) Ford (1901-1988) who was one of the first ecological geneticists. Ford worked extensively on selection in natural populations from the 1920s on. He focused on visible polymorphisms, meaning characteristics that occurred in several discrete forms in a given population. For example in a snail species he studied the shells could either coil to the left or the right. He also worked extensively on color polymorphisms in butterflies. This kind of system was very useful in a time when it was impossible to sample DNA or run electrophoresis gels. You see which genes an individual had by simply looking at it. Ford found evidence of strong selection maintaining these polymorphisms in nature (this is known as frequency dependent selection)
Theodosius Dobzhansky (1900-1975) was also a geneticist. Born in the Ukraine he emigrated to the US in 1927 and went to work in the lab of Thomas Hunt Morgan whom we encountered two diaries back. Dobzhansky became a Drosophila geneticist. Unlike Morgan he started studying wild flies, sampling different populations of the delightfully named species Drosophila pseudoobscura in the American West. He was able to detect genetic differences among the populations, mostly by looking a details of chromosome structure (remember this was before the discovery of the structure of DNA).
Dobzhansky was a crucial figure in the synthesis because he had a strong background in genetics that allowed him to at least partially understand the highly mathematical work of Fisher, Wright, and Haldane and he also had a strong interest in natural history and in putting the theory into an ecological context. He developed a model of speciation (the formation of species) in which genetic differences between populations increased over time such that the populations became reproductively isolated from one another. This is the basis of what eventually became known as the biological species concept. In 1937 Dobzhansky published his book 'Genetics and the Origin of Species' which is generally regarded as the first major work of the synthesis.
Ernst Mayr (1904-2005) was born and educated in Germany. He was an ornithologist and systematist (someone who studies the evolutionary relationships among organisms). In the late 1920s he was hired by the American Museum of Natural History to go to New Guinea to collect birds. A few years later, in 1931 he took a permanent job with the AMNH and moved to New York. In 1951 he took a position at Harvard where he remained until his retirement and beyond.
Mayr was a superb field naturalist and not at all a geneticist. He developed models of species formation based on his field observation. He built on the ideas of Dobzhansky to coin the biological species concept. His key observation was that species should not be defined by structure (appearance) but by the ability of individuals in populations to interbreed. This was laid out in his book 'Systematics and the Origin of Species' in 1942.
Mayr's views have been perhaps the most controversial of the primary synthesists. He was antagonistic to genetical theory - referring to it derisively as 'Bean Bag Genetics'. Like many highly empirical biologists Mayr didn't really understand the role of mathematical theory (see my comment at the end). The biological species concept has also been criticized repeatedly over time both for not being applicable to many forms of life (see the after the synthesis section) and also just for not being a generally practical way to distinguishing species. However it is an important and useful model for how species form. Mayr was also opposed to cladistics, which is now the dominant mode of doing systematics. In the mid-1990s when my wife was a post-doc she had the privilege of having dinner with Mayr along with a couple of other post-docs from her institution at the time. She couldn't believe that these young guys spent a lot of the dinner giving Mayr, then in his early 90s, a hard time about systematics. She wanted to hear stories about all the evolutionists that Mayr had known in his 70 year career (it should be noted she has zero interest in systematics).
Mayr developed a strong interest in the history and philosophy of biology and published many notable books in those areas in the latter decades of long career. His last two books were 'What Evolution Is' a popular introduction to evolutionary biology published in 2001 and a series of essays on the nature of biology published in 2004, the year he turned 100. Like Wright before him, Mayr had the last word on his contemporaries by living and remaining academically active for decades after the rest (Stebbins lived to 2000 but doesn't seem to have been nearly as active in retirement as Mayr).
Julian Huxley (1887-1975) was the grandson of Thomas Huxley (Darwin's bulldog whom we encountered several diaries back) and the brother of Aldous Huxley the novelist. Huxley is a fairly mercurial figure who did so many different things that he is hard to pin down. He was an early scholar in the field of Animal Behavior. In the early 20th century he was a strong advocate for the central role of natural selection in the British biological world. He was an important trainer of other British biologists including EB Ford mentioned above.
Huxley's had diverse interests. His passion for education led him to help found the Biology Department at Rice University where he worked until returning to Britain to serve in WWI. After the war he returned to Oxford where he spent another decade before leaving academia entirely. He was heavily involved in conservation and various progressive international causes. He also wrote popular books on biology and evolution. Huxley continued to write scholarly work on evolution and corresponded with the major figures of the day. His book, 'Evolution: The Modern Synthesis' was published in 1942 and coined the term.
George Gaylord Simpson (1902-1984) was an American paleontologist and probably the most important figure in his discipline in the mid-20th century. Stephen Jay Gould has spoken quite derisively of the state of paleontology before Simpson as being insular and lacking focus. Simpson's book, 'Tempo and Mode in Evolution' (1944) clearly places the fossil record in the context of evolutionary genetics and natural selection. Simpson was an expert on fossil mammals and published widely on the distribution and evolutionary history of these animals
G. Ledyard Stebbins (1906-2000) was the youngest and probably the least known of the major figures of the synthesis. Stebbins was an American geneticist and botanist. Stebbins studied how polyploidy could result in the formation of new plant species. Polyploidy refers to having more than two copies of each chromosome (diploid = two copies the 'normal' condition). Polyploidy is fairly rare in animals but is quite common in plants and Stebbins was able to show that new species could form basically instantly in plants through accidents resulting in polyploid offspring. Stebbins book, Variation and Evolution in Plants (1950) is the last major publication of the synthesis period.
So what we see is that experts in a variety of disciplines (genetics, ecology, systematics, paleontology, animal behavior, botany) all published major works that comprehensively recognize that disparate biological phenomena (variation in chromosome number and structure, differences between bird species, the fossil record of horses) are all the result of the same processes. Additionally there was an new emphasis - on how evolution could form new species. Darwin, despite the detail of his book, had not had a very good idea about how evolution would actually cause the splitting of one species into two (although he did recognize that this pattern of bifurcation was the source of biological diversity). Fisher, Wright, and Haldane had not given the issue of species formation a great deal of thought either (at least to my knowledge). The concept of reproductive isolation leading to the formation of new species was a key insight of the synthesis.
After the Synthesis
The conclusion of the modern synthesis happened around 1950. Another major event in the history of biology occurred just a few years later: the discovery of the structure of DNA. This juxtaposition of events is somewhat ironic for several reasons. The discovery of the double helix was in a sense an anti-synthetic event. It separated biologists into opposing camps for several decades. This period was the period in which much of the fundamental details of molecular biology were worked out. At least some molecular biologists were convinced that a thorough understanding of the molecular basis of biological process would render all other types of biology obsolete.
In contrast, many evolutionary biologists during this time had a disdain for mechanism (someone I knew once who shall remain nameless contrasted building foundations vs building castles - he went so far as to say that he assumed the foundation was OK and found building castles much more interesting). Mechanism (in the sense of how genes were influencing most characteristics of interest) was largely a black box for several decades following the synthesis.
This all began changing (slowly) as far back as the 1960s and the pace picked up a bit in the 1980s and since that time evolutionary biology and molecular biology have begun the process of being re-integrated.
Criticism of the Synthesis
The Modern Synthesis has been criticized from a number of perspectives over the years. One of the main criticisms has been that the view of evolution is animal-biased. With the exception of Stebbins all the major figures studied animals and there were no microbiologists. Another major criticism was that development and the action of genes were both largely ignored during the synthesis. A third is that natural selection is given too primary a role.
Botanists have criticized the Biological Species Concept for being animal centric. Many plant groups appear to have high rates of hybridization among species while the species still remain distinct. Oaks are good example of this, as anyone who has tried to ID oaks in the southern US will know.
Even more dramatically the microbial world doesn't fit the standard model of evolving populations. Bacteria do not have sexual reproduction - they reproduce asexually by fission. However they do have several mechanisms for incorporating external DNA into their genome. This can be accomplished by conjugation, which is superficially similar to mating but is not involved in reproduction. One individual transfers DNA to another individual but no new bacterium is produced. Bacteria can also take up DNA from the environment. This is known as horizontal gene transfer (vertical gene transfer means having offspring) and it gives the evolutionary history of bacteria a very different character than that of animals.
The synthesis also did not really deal with development. Developmental biology was highly descriptive at that time. Although some very elegant experiments had demonstrated control over parts of development at the level of cells, genetic control of development was a complete mystery, a black box. The big puzzle of development, which persisted into the 1970s and 80s, is how genetically identical cells inside an organism end up having different structures and function.
This was acknowledged as a major short coming at the time. It was apparent that the differences between species were largely the result of differences in development. Our genes control development to make us appear human while the genes of a rabbit control development to produce a rabbit.
Finally the synthesis ended up emphasizing natural selection as the most powerful evolutionary force. Field studies had found that selection was often much more powerful in nature than anyone had predicted. Stephen Jay Gould refers to this emphasis as the 'hardening' of the synthesis. In his view the synthesis started out being more pluralistic, allowing other forces such as drift an important role but eventually everything had to take a back seat to selection.
I will also point out that other evolutionary biologists were active at this time whose views fell outside of the synthesis. The most well known of these today is Richard Goldschmidt (1878-1958), a German/American geneticist. Goldschmidt originally worked in his native Germany and was interned in the US during WWI. He returned to Germany after the war but then permanently relocated to US after the Nazis came to power (he was Jewish). Goldschmidt worked on a number of topics but he most famous today for his ideas about macro mutation and his coining of the very catchy 'hopeful monster' phrase.
Goldschmidt felt that selection couldn't produce entirely new kinds of life. It could produce a bigger cat from a smaller cat or cause green lizards to evolve red coloration. But it couldn't produce something really new. So he proposed that major evolutionary changes were the result of very rare mutations that had major and favorable effects on the phenotype. Hence the 'hopeful' (beneficial change) and monster (a major change that produces something entirely new).
Gould later tried to resurrect this idea using the regulatory genes that control development. We will explore these ideas more fully in later diaries.
A Few Side Comments
1) As in the previous diary I will comment that the dominant figures in evolutionary biology were all white males. There certainly are notable female biologists during this time period but they tend to be either non-evolutionary or specialists in particular groups of organisms such as Miriam Rothschild rather than studiers of evolution per se. We will see this changing in future diaries. I will note that this diary has the first appearance of an evolutionary biologist known to be gay in the person of E.B. Ford. Unfortunately Ford is not a particularly good role model as he appears to have been quite misogynistic and vigorously campaigned not to allow women into his college at Oxford.
2) There is a fair amount of tension in biology between theoreticians and empirical biologists. When I was doing my MS a graduate student in my office 'published' a manifesto on real biologists. In his view, 'real biologists' worked outside in the real world. Mathematics was limited to statistical analysis of data. Data was everything and the theory papers were 'fake stuff'.
This is a misunderstanding of the role of theory. The idea of developing theory (at least in ecology and evolution) is not to develop complete mathematical models of evolution or ecology as they occur in the real world. Rather the goal of theory is to look at the behavior of simplified systems to provide information that allows empirical biologists to make more sophisticated tests. For example Fisher, Wright, and Haldane demonstrated that genetic drift is more powerful in small populations than in large populations and that drift tends to reduce variation within populations. This theory cannot exactly predict what will happen in and one population as drift is a random process and because other forces may also be acting. But it does make a general prediction about the genetic characteristics of large vs small populations which can be tested. Once tested this theory has proven to be extremely useful in any number of ways which will be discussed in future diaries.
The difficulty in understanding theoretical work in biology continues to this day. Most biologists (myself among them) lack the background to easily follow much of the advanced math in some papers. In the late 1980s, John Maynard Smith was given an honorary degree at my institution. Maynard Smith was a student of Haldane's and one of the most eminent British evolutionary biologists of the second half of the 20th century. He had published some quite mathematical papers applying game theory to evolutionary problems. My office mate who was a very mathematically proficient grad student asked Maynard Smith at the 'meet the grad students' gathering what he (Maynard Smith) thought of the work of one of our faculty who had really advanced the mathematical treat of evolution involving many genes at once over the previous decade.
Maynard Smith (in his late 60s at the time) said something to the effect of 'I have great admiration and affection for X but I do what any sensible person does when reading his papers. I read the introduction and the conclusions and skip everything in between.' That was a very encouraging thing for an overwhelmed grad student to hear. Not sure if it was good for me in the long run.
The Rest of This Series
As I see it the history of evolutionary biology since 1950 has consisted of two main phases. The period through the mid-1980s was a time of great theoretical expansion with the introduction of many new ideas. Since the mid-1980s we have seen an enormous increase in technology and in the sophistication of hypothesis testing. This period has enormously expanded our knowledge and understanding of evolution.
The last 60 years are not really suited to the same sort of linear progression we have seen so far. It would be too confusing as we would be constantly jumping back and forth between topics. Also the volume of material that evolutionary biologists have produced continues to expand exponentially. So trying to be comprehensive within diaries becomes more and more difficult.
In less than a week I am leaving for Ecuador (but not the Galapagos - sigh) and I'll be back in late May. At that time I will resume the series. Each diary will document the history of a particular topic (e.g. sexual selection) in evolutionary biology.