This is actually the 4th diary in this series but the third one was a digression and was not really historical or directly dealing with evolution. So I'm calling this one part 3. Sorry for the long delay between installments.
The late 19th Century might well be called the lost half century of evolutionary biology. Evolution was now a respectable topic in biology, thanks to Darwin, but there were few major advances during this time. Partly this was due to a lack of knowledge of genetics, as alluded to in part 2 and discussed in more detail in the digression. But I think, equally importantly, there was a failure to really grasp the nature of Natural Selection. The idea really required a different kind of thinking than was typical of the time - populational thinking rather than essentialist/typological thinking. In the essentialist way of thought a group has an underlying set of ideal/typical characteristics. For example each breed of dog has a description of the ideal characteristics of the breed and any given individual of the breed can be judged by how close it is to the ideal for each trait.
The populational view is that organisms exist as populations of individuals that vary. The population can be characterized by mathematical values that summarize the characteristics of all the individuals (the mean, median, standard deviation, etc.). All of these characteristics change over time. For this among other reasons, although evolution became widely accepted by biologists quite quickly after the publication of the 'Origin', natural selection was widely criticized. It was not that hard to see that selection could change the mean (average) value of a characteristic of a population but harder to accept that this kind of quantitative change could lead to qualitative changes in organisms. Jump below the squiggle to get an example and an illustration.
In a laboratory or on a farm a scientist or breeder can mimic the action of natural selection. By breeding the biggest pigs on the farm in a few generations you can produce larger pigs in just a few generations. But they are still pigs and not obviously different from the parental stock except for the characteristic of size. This has remained a creationist critique of natural selection to this day - that it can cause minor changes in a population but can't actually cause an organism (species) to turn into a different type of organism. The problem here is creating boundaries that are human constructions. We have a concept of a cat that has definite boundaries. If we saw a 'cat' with a long snout, tiny ears, flattened teeth, and hooves we would say "that's not a cat". However the descendent population of some group of cats might well look like that in the future as the characteristics of the population change over time.
Differing Methods of Study and Differing Views of Evolution One thing that did happen in the late 1800s was the origin of different 'schools' of evolutionary thought. These are certainly not formal schools by any means and any given biologist might fall into more than one of these groups. But they do represent different methodological approaches that have, over the last 150 years, tended to foster certain attitudes towards evolution.
1) The Naturalists. A better term would be ecological evolutionists but the term ecology was not in common use in the 19th century. These are biologists who study evolution by examining the interaction between organisms and their environment. Alfred Russell Wallace whom we met last time is a classic example of this school from the 19th century. Naturalists have tended to support a strong role for natural selection in evolution.
2) The Geneticists. I'm using this term broadly to include any one who studied inheritance as geneticists as we would understand the term now didn't exist until after 1900. Geneticists vary considerably in the importance they place on natural selection and this has also varied over the last century.
3) Organismal Biologists. These are biologists who study the form and function of the bodies of plants and animals. This group would include paleontologists, anatomists, and developmental biologists. Biologists in this category tend (again a gross generalization) to think that natural selection is over-rated and that and that the bodies of organisms place constraints on evolutionary processes that may be more important on the large scale than the processes themselves.
4) Historical Biologists. These are scientists who study the history of life on earth. They are paleontologists and/or systematists, biologists who develop phylogenies (evolutionary trees). They have also tended to emphasize historical constraints on evolution.
The attitudes of biologists can be distilled down to a more functionalist view of evolution (groups 1 and 2 (to a certain extent)) and more structuralist view of evolution (3 and 4 (again to a certain extent)).
Structuralism and Functionalism in Biology My understanding is that functionalism and structuralism are well established concepts in other disciplines such as psychology. In biology they have been applied relatively infrequently. I think they are useful in understanding several major conflicts in evolutionary biology. The relationship of structure and function is one of the central tenets of biology. The structure of a cell or an organ is related to its function. For example, the elongate structure of a neuron and the branching structure of the dendrites are related to the function of transmitting signals through electrical and chemical media respectively. Structuralism and functionalism differ in their interpretation of the causal direction between form and function. Functionalists see function as driving structure. Structuralists see structure as defining possible functions.
Evolution by natural selection is a functionalist idea, probably the most important functionalist idea in all of biology. The interaction between organisms and their environment influences the direction of evolution. One major theme in the history of evolutionary biology is the tension between functionalism and structuralism over natural selection and adaptation. In the popular presentation of evolutionary biology from the 1970s through the turn of the century these two schools were represented by Richard Dawkins and Stephen Jay Gould. But let's not get too far ahead of ourselves. We will discuss more recent controversies over the role of natural selection in evolution in a later diary.
What is important to note is that natural selection is both a populational and functional idea. Many evolutionary biologists who didn't accept natural selection as an important force were arguing from a more structuralist and essentialist viewpoint. One manifestation of this was a rejection of evolution by gradual change. Even some of Darwin's strongest supporters didn't believe that organisms could slowly and gradually evolve from one form to another. Thomas Huxley (1825-1895), self described as 'Darwin's Bulldog', was a fierce proponent of evolution by natural selection. But he felt Darwin's model of gradual evolution was incorrect, that new species could not be formed by gradual change. Instead he proposed rapid jumps to get to new forms. As we shall see in a later diary this has been proposed quite a few times in different ways.
Another strong supporter of Darwin had the same criticism. Francis Galton (1822-1911) was Darwin's cousin, and, again, a strong advocate of evolution by natural selection. Galton also felt that gradual change couldn't lead to evolution. So we see an interesting dichotomy here. Galton and Huxley both saw a role for natural selection in maintaining a species as it is but felt that changes required something more.
Evolution and 'Natural History'
Among the most enthusiastic champions of evolution and natural selection were many young scientists with extensive experience of the natural world. Alfred Russell Wallace himself applied evolutionary ideas to the study of animal geographic distribution and founded the field that we now know as biogeography. His studies indicated that the distribution of animals could be understood in the context of the geological history of the areas in which they occurred. His travels in southeast Asia, documented in 'The Malay Archipelago' gave him an extensive knowledge of the distribution of animals across the islands of the region.
He noted that the distributions were not random and, despite the similarity in climate across the area, very different species were found in different areas. The Greater Sundas (Borneo, Sumatra, Java) had a fauna similar to mainland Asia while the islands further to the east had animals more similar to Australia and New Guinea. The island of Sulawesi (then called Celebes) had a unique fauna containing both Asian and Australian elements.
Wallace's original traveling and collecting partner, Henry Walter Bates (1825-1892) spent over a decade in the Amazon. He enthusiastically accepted Darwin and Wallace's ideas and used them to explain the curious pattern of resemblance between different species of butterflies. He proposed Batesian mimicry in which a palatable (i.e. tasty) mimic species evolves to resemble a toxic model species living in the same area. The model species typically have evolved distinctive bright coloration. Wallace documented how the same species would mimic different models in different parts of its range.
Another naturalist, Fritz Mueller (1821-1897) expanded on this to propose a different variant, Muellerian mimicry, in which two toxic or distasteful species are selected to more closely resemble one another.
The ideas of the naturalists incorporated both the historical aspects of Darwin's ideas and the populational aspects.
Evolution and Development
Ernst Haeckel (1834-1919) was a German biologist who is now most famous for his recapitulation theory (Ontogeny recapitulates phylogeny). This association is a bit unfortunate as it is only a single aspect of Haeckel's immense body of work as a natural historian, embryologist, and evolutionist.
Haeckel's idea was that the developmental stages of an embryo followed the same changes in form as the evolutionary history of the organism in question. A human embryo would therefore sequentially exhibit the features of a fish, an amphibian, and a reptile before becoming mammalian in appearance. This idea is not unique to Haeckel and his been applied to other systems such as the evolution of language. Unfortunately this does not appear to be generally true in biological development.
Darwin's own view of the relationship of evolution to development was built on the work of Estonian embryologist Karl Von Baer (1792-1876). Von Baer noted that the embryos of different species tended to be similar at early developmental stages or (phrasing it a different way) general characteristics shared by different species appear in early developmental stages and specific traits unique to a particular species tend to appear in later developmental stages. Darwin saw how this fit in which his view of evolution as a process of descent with modification coupled with branching and divergence.
Darwin's work after the 'Origin'
Darwin continued to publish throughout his life. His published work prior to the 'Origin' had been varied. Some was related to his voyage on the Beagle but others such as a monograph on Barnacles and another on the formation of islands through coral reefs were more varied.
After his evolutionary ideas were published Darwin put his research efforts into three major areas: gathering more evidence for the idea of evolution by natural selection, trying to understand inheritance better, and mustering more evidence for aspects of his work that were particularly controversial.
To better understand evolution and natural selection in general Darwin pursued work on plants, including topics such as the fertilization of orchids (the coevolution of plant and pollinator), the effects of self-fertilization, the mechanisms of climbing plants, and so on. This was the most prolific area for Darwin in terms of publication. Two of his closest friends Asa Gray (1810-1888) and Joseph Hooker (1817-1911) were the most prominent botanists of their day in the US and Britain respectively. Both were extremely strong supporters of Darwin's ideas and valuable colleagues in his botanical studies. These botanical studies were used to gather further support for the ideas he had proposed in the Origin.
Darwin also published a book entitled 'The Descent of Man and Selection in Relation to Sex' in 1871. This work addressed two topics he felt required further documentation. Darwin had left humans completely out of the Origin in a vain hope of avoiding controversy. The first part of this new book laid out the evidence for the evolution of human beings and our place in the evolutionary tree of life. The second part of dealt with his theory of sexual selection. Sexual selection had been dealt with in the Origin but he felt it required a more detailed study. Sexual selection is the force thought to be responsible for the evolution of many differences between males and females, particularly those not directly related to fertilization or care of offspring. Examples would include bird song, the antlers and horns of many mammals and insects, and the bright colors and displays found in many animals. Darwin proposed that these characteristics evolved through selection due to competition for mates and that this took two forms.
The first was male/male competition in which individuals of one sex directly competed with one another for access to members of the other sex. Traits such as antlers or greater size of one sex are examples of that are likely to have evolved through this mechanism. Today this is referred to as intrasexual selection to indicate that it is not only males that compete in this way.
The second was female choice in which members of one sex choose mates based on the characteristics of members of the other sex. Today this is known as intersexual selection.
Intrasexual selection (direct competition) has never been controversial as it can be easily observed in nature. Intersexual selection was not accepted by the scientific establishment in Darwin's time and didn't become a major area of research until the 1960s. Victorian scholars were unwilling to acknowledge that a) animals had an aesthetic sense (which isn't necessary in any event) or b) that females would actively choose mates. Sexual selection has been one of the most dynamic areas of evolutionary research and will be discussed in more detail later.
Inheritance and the Biometricians
As mentioned in part 2 the greatest challenge Darwin failed to meet was the problem of inheritance and variation. Francis Galton (1822-1911), a noted Victorian polymath, was inspired by the work of his cousin Charles Darwin to directly investigate variation and inheritance in humans. He did so by measuring characteristics (what today we would refer to as phenotypic traits) across generations and performing complex statistical analyses on his findings.
Late in the 19th century Galton's efforts were joined by mathematician Karl Pearson (1857 - 1936) and zoologist Raphael Weldon (1860-1906). They founded what would be known as the Biometrician method of studying inheritance. Using the statistical techniques of regression and correlation, the biometricians studies the relationship between the characteristics of parents and offspring. These studies were the mathematical beginnings of the field known today as quantitative genetics (the informal beginnings of quantitative genetics date to first efforts at selective breeding in agriculture).
The Biometricians had an inherently populational view of the world but, as we shall see in the next diary, ended up being hampered by fights with the early geneticists in which each side was more or less half right.
Evolution at the end of the 19th Century
There were quite a few other evolutionary biologists of note in this time period. In the US Edward Drinker Cope (1840-1897) was a prominent paleontologist and David Starr Jordan (1851-1931) was an ichthyologist and the president of Stanford University. In Britain Ray Lankester (1847-1929) was a prominent invertebrate biologist while George Romanes (1848-1894) was the founder of comparative animal behavior (and was also a native of Ontario, Canada in a shout out to my own roots). So there was a lot of solid work being done to expand biological knowledge with an evolutionary perspective but relatively little expansion of our basic knowledge of evolution itself.