Happy Darwin Day everyone. It is fitting and serendipitous that my second diary in this series should fall on Darwin's 203rd birthday as it deals with Darwin himself and the establishment of evolutionary biology as a formal discipline with the publication of 'On the Origin of Species'.
I am extraordinarily gratified that the first diary in this series was so well received and thank everyone for their kind words. I am also enormously impressed (and I must confess somewhat intimidated) by the great range of knowledge exhibited by my fellow kossacks. My one regret is that due to personal time constraints and the large number of comments I was not able to express my appreciation personally to all who commented. I am appending some general comments about the series to the end of this diary. Jump over the orange adaptive landscape to read about Darwin.
In the previous diary we saw how in the early 19th century the fossil record and other geological evidence supported both the idea that the earth was ancient and that life on earth had changed over time. A number of scholars had proposed biological evolution and some had even proposed natural selection. So then why is Charles Darwin (1809-1882) celebrated above all other biologists and why is the publication of his work, 'On the Origin of Species' considered the most momentous event in the history of biology?
Creativity is only one characteristic of a successful scientist. Darwin was certainly creative. He came up with the idea of natural selection and the idea of a tree of life. But equally importantly, Darwin thought clearly and worked his ideas through to logical conclusions, he synthesized concepts and data from disparate areas and he carefully and painstakingly amassed huge amounts of data to support his ideas. He really did it all, science-wise.
One advantage that Darwin had over previous natural historians was field experience. In the 19th Century global travel was incredibly slow and uncomfortable by our standards but it was routine in a sense that it had not been previously. For the first time educated men had the opportunity to see the world. And what a revelation this world would have been to a European. Europe is, biologically, pretty depauperate. The Mediterranean and the Alps both serve to restrict the north and south movement of organisms. So glacial periods (ice ages) have had a more devastating impact in Europe than they have in North America or eastern Asia where warm adapted species could more easily disperse to the south in cold periods and then move back to the north.
The extraordinary richness and variety of life in the tropics and the variation in life from one place to another across the planet was revelatory to 19th century natural historians who had the opportunity to travel. Darwin, Wallace, and others were highly stimulated by their experiences and many new ideas arose as a consequence, evolution by natural selection being the most notable but only one of many.
Events Leading to the Origin of Species With the preparation for and publication of 'The Origin' that evolutionary biology as a discipline really began. Darwin put in 20 years gathering data and refining his ideas. He was able to address a huge number of criticisms and questions before they even arose. There were others he as not able to address. Both his successes and his 'failures' laid the foundation for many scientists work over the years to come.
Charles Darwin spent five years in the early to mid 1830s traveling the world on HMS Beagle. The purpose of the Beagle's voyage was to chart the coastline of South America and Darwin was hired as the "Captain's Companion". This post provided the Captain of a ship a person of equal social status with whom he could talk over the long periods of isolation. The Beagle spent much of the five years along the coast of South America but did circumnavigate the globe at the end of the voyage. The voyage gave Darwin the experiences and knowledge to become a well known natural historian who published on a wide range of topics over the next couple of decades. The experiences also gave him the idea of evolution by natural selection. Based on his notes it appears that Darwin had thought out and become committed to his idea by the late 1830s. Knowing that it would be controversial he delayed publication for years while gathering more and more evidence. Although academically ambitious Darwin was a rather retiring individual who shrank from being the center of public controversy. He also wished to spare his devoutly religious wife any grief.
Alfred Russell Wallace (1823-1913) was a professional collector of biological specimens for museums who had spent years in both the Amazon and southeast Asia. He independently came up with the idea of natural selection while in a malarial fever in a hut in Asia in the late 1850s. He decided to write it down and send it to Darwin, not knowing that Darwin had been working on that very idea for 20 years. Darwin was not at all happy to receive Wallace's letter and might well have destroyed all of his work had his colleagues not dissuaded him. They convinced him to write a paper with Wallace as a co-author which was then read to the Linnean Society and published in their journal in 1858 (the reading of this paper at a society meeting was a decidedly inauspicious event - Darwin was not present and Wallace, still in southeast Asia was entirely unaware of what was happening - we will discuss Wallace more in the next installment). Darwin was spurred to publish and 'The Origin of Species' came out the next year, advertised as an 'Abstract' of a forthcoming longer work that never was published. The book caused enormous controversy in the general public and caused educated people to rethink their ideas about the place humans and human society in the natural world. This diary is not about any of that. We are concerned with 'The Origin' as the start of the formal study of evolutionary biology.
Ideas in 'The Origin'
Ernst Mayr (1904-2005) a noted evolutionary biologist and historian of evolutionary biology has pointed out that 'Darwin's theory' is really five theories. In other words, the Origin has five major ideas. Let's explore them and see how they relate to ideas of the time and to current knowledge of evolution.
1. Evolution has happened. This is the most basic of the ideas. Life on earth is not the same as it was in the past and this has occurred through changes in species over time. Necessary for this to be true was the supporting idea that the earth was much older than had been thought based on religious beliefs. This was not an idea that was original to Darwin as we saw in the previous diary Today this would be described as the fact of evolution and the evidence for it is overwhelming.
2. All life on earth had a common ancestor. This was a revolutionary notion (although as noted last week Darwin's own grandfather had held a similar view) and probably one of the most important from a religious/social point of view. Of all Darwin's ideas this one had the least support at the time. There was little evidence uniting plants and animals - virtually nothing was known of biochemistry or cellular biology. Darwin apparently proposed this idea because he thought the origin of life to be so difficult that he couldn't imagine it happening more than once.
3. The diversity of life on earth has been generated through a pattern of change over time combined with splitting to produce a 'tree of life'. I think this is the most over-looked and original aspect of Darwin's thinking, despite the fact that he famously drew an evolutionary tree in his notes and wrote 'I think' beside it. A lot of misunderstandings of evolution are due to a failure to adopt 'tree-thinking' which we will discuss in more detail in a later diary. This idea of evolution as a combination of changes over time and splitting of lineages seems to be generally correct with a few major exceptions such as the evolution of mitochondria and chloroplasts. Darwin saw the connection between this idea and Linnean system of classification. pointing out that the classification is a summary of the tree of life.
4. Natural Selection is the primary mechanism driving evolutionary change. Wallace and Darwin came to this conclusion independently and, again, as we saw in the previous diary a number of other scholars had previously hit upon the same idea without really following through with it. What set Darwin apart was his ecological knowledge. Natural selection is as much an ecological idea as it is an evolutionary one. He could easily imagine how the 'struggle for existence' could be influenced by variation among individuals and how successful variants could pass those traits onto their offspring.
5. Evolutionary change happens gradually over time. Darwin viewed evolution as difficult and that vast periods of time would be necessary for slow, incremental changes to accumulate. This has been a topic of contention among evolutionary biologists repeatedly over the history of the discipline. It seems clear that evolution happens at different rates at different times and in some cases it can be quite rapid.
Evidence in 'The Origin'
Darwin didn't just present ideas in his 'abstract', he amassed a great deal of evidence from diverse sources. Some would have been obvious from our previous diary: the geological evidence for the age of the earth, the fossil record, homologous structures in comparative anatomy. Darwin also brought in information on the geographic distribution of organisms, particularly organisms on oceanic islands that had never had contact with continents. His observations in the Galapagos first gave him the idea of evolution and island species are among the most potent evidence in his work. Oceanic islands such as the Galapagos and the Hawaiian islands have many unique species found no where else on earth. Typically they are related to but clearly distinct from species from the nearest continents. There are often unique species on each island in a chain with older islands having the most species.
Darwin also looked at data from contemporary populations of organisms, viewing evolution as an ongoing process. He documented the fact that variation in characteristics is ubiquitous, that for almost any thing you can measure in a plant or an animal there is variation among individuals. This is necessary for the idea of natural selection to work. He also carefully documented how the many varieties of domesticated plants and animals have been formed through selective breeding which he saw was analogous to natural selection in the wild.
Darwin's Problems
Darwin faced a number of difficulties with his ideas. Some of these involved characteristics that were hard to explain in the light of natural selection. One of these was the existence of social insects such as bees and ants. Most individuals in a colony never reproduce or even attempt to do so, spending their lives helping to rear the offspring of a handful of reproductive individuals. Natural selection depends on traits that make individuals successful being passed on to their offspring. Not having offspring by definition, makes individuals evolutionarily unsuccessful. The answer to this puzzle requires some moderately sophisticated genetics and ecology and it wasn't really addressed systematically until over a century after the 'Origin'. We'll get back to it in future installments as this 'problem' has stimulated a great deal of research.
A second problem was the existence of traits such as the tail of a peacock or the bright plumage of a male cardinal. These traits did not appear to aid in the production of offspring and were likely to be detrimental to the survival of individuals bearing them. Darwin proposed his theory of sexual selection to explain these types of traits. It is a fairly simple and intuitive idea. Sexual selection acts on traits that are involved in reproductive competition. Even if they hinder survival these traits will be favored if they increase the likelihood of mating. Darwin noted that these kinds of traits were more common in males and that males tended to compete for matings with females rather than the other way around. He proposed two different mechanisms for sexual selection: male-male competition in which males fight for access to females giving rise to traits such as antler and horns and female choice in which females choose mates based on characteristics such as color and ornamentation. He discusses sexual selection in the 'Origin' and then published more extensively on the topic a dozen years later. I'll discuss it further in the next diary and in future installments as it has been a very active area of research.
The most fundamental problems that Darwin faced were a lack of knowledge of the nature of inheritance and the maintenance of variation in populations. Both of these are vital features of evolution acting through natural selection. Without variation there is nothing for natural selection to act upon and without inheritance there will be no evolutionary change. In the 19th century the widespread view of inheritance was what is now called 'blending inheritance' in which an offspring is a blend of its parents. Sort of like mixing black and white paint to get grey paint. The problem is that you cannot reconstitute the gray paint to get black and white out of it again. So blending inheritance in and of itself is going to reduce variation. Natural selection will also reduce variation. And so evolution should rapidly come to a halt.
Darwin had a very difficult time answering this line of attack and in later editions of the 'Origin' proposed a kind of Lamarkian inheritance to provide a suitable source of variation. These problems persisted for decades after the rediscovery of Mendelian genetics in 1900 as we shall see.
Appendix
I realized that this material would have been good to include in the first diary and so I will append it here. This is mostly a statement of where I am coming from (i.e. my biases) in writing these diaries and is certainly not a directive about the directions others should take in their comments.
I am a biologist and not a historian. My focus is largely on the biology and not so much on the cultural context although I will comment on that from time to time, especially with regard to the participation in evolutionary biology by scientists who are not white males. I am really trying to document the types of issues that are of interest and have sparked controversy in my discipline (evolutionary biology) as a contrast to the social controversy over evolution.
And finally a note on evolution as it relates to human beings. I will comment on this from time to time but it is not going to be a major focus of the series. I'm not going to discuss how Darwin's work revolutionized our view of ourselves. Nor am I going to comment on religion any further than I have already. I am going to discuss eugenics mostly to point out its flaws as a science and to point out that scientists who were advocates of eugenics did in fact make important contributions in other areas of evolutionary biology.
In a later diary I will discuss attitudes of evolutionary biologists towards the study of certain areas of human evolution as they are widely disparate. I'm not talking about paleontology or population genetics but rather studies of the adaptiveness of human behavioral characteristics (i.e. evolutionary psychology). Many evolutionary biologists have the attitude which I will illustrate by paraphrasing my wife - it is really really hard to understand these kinds of characteristics in fruit flies where you can do huge controlled experiments, it is sloppy science to draw conclusions about humans from much weaker data in a system where the environment (i.e. culture) is likely to be so much more important than in flies. Others have the reverse attitude and say that it is ridiculous and muddle-headed to treat humans as being any different from other organisms and they should be studied in the same way as everything else. Feelings can get quite high about this, particularly among the latter camp. Personally I don't find humans all that interesting from an evolutionary perspective and so, while I will discuss human evolution it won't be a major part of the series.
"Update' - I've fixed a number of typos and awkward sentences although I'm sure many remain.