[Housekeeping - third tute, essay forms and submission] We all think we have a pretty good handle on evolution - it is one of those things we absorb from childhood via stories, classes, TV, and so on. It might surprise you all to know that, unless you did it in a university course, you probably don’t know it nearly as well as you might think. Renowned philosopher of biology David Hull once opened a book review with “Evolution is so simple, nearly anyone can misunderstand it.” And this is true. So this lecture is to clear the air a bit. To begin with, do not rely upon popularisations, even if they are written by experts. Neither Stephen Jay Gould of blessed memory, nor Richard Dawkins, has the copyright on the entirety of evolutionary theory. Evolutionary biology, like any science, is complex and at the interface between facts and ideas it is often contradictory. Part of the philosophy of biology is to investigate how these disputes affect the ideas of the theoreticians. So, let’s look briefly at the background and history of evolution as an idea. If you are interested in following it further, I can recommend the recent new edition of Peter Bowler’s Evolution: The history of an idea. Just before we proceed, though, another caution. Relative terms like “higher, lower, fast, slow, smooth, abrupt, gradual, jerky” cause enormous trouble in evolutionary biology. There is no absolute sense in which evolution proceeds at standard rates - large, extra large, and ginormous, as the Condom manufacturers label them. There have been many attempts to define absolute rate measures, such as the darwin, but they are always insufficient. When you read someone accusing another of being a “gradualist” or proposing “evolution by jerks”, it almost always resolves down to a matter of semantics. To conceive how to resolve such disputes, ask if you could plot the differences in absolute time. If you can’t, then the metrics are probably subjective.
The use of the word, originally used to denote the unrolling of a book scroll in Latin, came to mean a reading, the expression of information that was already there. During the 178th and 18th centuries, there was a debate on how organisms grew to be what they were - one side, the epigeneticists said that a fetus developed because it was formed by external processes. The other side was called preformationism , the view that what developed was already in some manner in the fetus, and that development was an evolution - an expression of what was already in the fetus either actually, or potentially. As biologists tried to apply the ideas of Aristotle, they came up with the idea that there was a formal transformation - a static ladder or scale from simple to complex - which individual organisms entirely or partly underwent as they developed. This ladder, known now as the Great Chain of Being , was not a temporal thing, but it was very easy to make it a change over time, and both Erasmus Darwin, the grandfather of Charles, and Jean Baptiste de Lamarck shortly afterwards, made this very change from static to changing. The obvious thing was that life as a whole mirrored the development of a single organism. It made sense therefore to call it “evolution”. There is a term in the philosophy of the time for the expression of something that is innate and unexpressed - entelechy. This evolution was an entelechy - it aimed at a telos, the Greek word for a goal or purpose.
From this usage, we find that the term applies to any series of changes that are in some manner predetermined. For example, we talk about “stellar evolution” for the series of physical changes that stars undergo depending on their initial composition and size. People will also talk about “cosmic evolution”. A recent book with a lot to recommend it is Eric Chaisson’s 2001. Cosmic evolution: The rise of complexity in nature . However, the term often is used in a confused and vague way. Because the Great Chain of Being was accepted by everyone in the western tradition from Aristotle to Teilhard, a French Jesuit popular in the 1960s, it is almost the “default” view of popular culture. And so “evolution” on Star Trek or Babylon 5 means that there is a “next step” in human or alien evolution. This is nothing like real evolution. Humans have no “next step” - we may be the same as we are until we go extinct. We may not speciate, that is, split off a new species, at all; many species don’t. There are no Almost-Dodos, for example.
The image on the left is from Raymund Lull around 1515; that on the right shortly after. You can see the moral dimension of the Great Chain - for each ontological rank - bare being, life, sensitivity and virtuous reason, a truncated version of Aristotle’s scale, there is a corresponding moral rank - virtuous studiousness, luxuriant sensuousness, gluttonous vitality and catatonic minerality. The fellow on the right is literally stoned.
Bonnet on the left, Lamarck shortly after on the right. Bonnet reads from bottom to top as a transformation; Lamarck reads from top to bottom as a transmutation. Bonnet is an elaborate version of Aristotle’s scale, although he doesn’t know where to put sea lions and whales. Lamarck’s is more sophisticated again - a lineage of animals can end up an insect, a bird or platypus, a marine mammal or one of a number of other kinds of mammals. However, the further down the page you go, the more complex and sophisticated you get. Of course, humans are Unguiculate mammals.
Evolution as modern science understands it derives from, but is neither restricted to nor in complete agreement with, the views proposed by Charles Darwin. If you want to know that history, do the “Darwinism” course next year. Evolution begins by meaning change of a species into another species - Transmutation. Darwin referred to it as descent (as in being a descendent) with modification - the new versions of an older species have differences to the older species. Next, we must note that there are several mechanisms that cause evolution. They are the explanations of the fact of evolution. Some people are dissatisfied with these explanations - for example, a good many thinkers have rejected the sufficiency of natural selection as an explanation of new species. They are not eccentrics, either, but leading evolutionary biologists. But the fact that evolution (change of species over time) occurs is not challenged. Then we must note that there is an increase in the number of species as species split into two or more. Sometimes the original species both splits and changes in both branches. Most of the time, the ancestral species remains unchanged, though. Splitting causes new genomes, new habitats to be occupied, and more groups of organisms to exist. We should not forget, though, that species also go extinct. These days this is in the headlines - for a very long time it was denied that they even did - it took the Dodo to prove it. But even if we are aware they do, it’s easy to overlook that diversity can decrease as well as increase.
Darwinian evolution, although just possibly not Darwin’s idea of evolution, is not progressive. That is to say, it doesn’t prohibit long-term evolutionary trends, but it doesn’t require it either. For something to happen over very long periods - and remember our caution about terms like “long” - there would need to be a continually operating mechanism. Since natural selection is usually something that is local and short-lived, it is hard to see how it could account for trends like the evolution of horses or hominid brain size growth. Accounts that proposed there was another mechanism than selection are often called “orthogenetic” accounts, from the Greek for “straight origin”. Orthogenesis is a swear word in the language of evolutionary biologists, and its promoters are regarded with suspicion, in part because the early views were almost mystical. So Darwinian evolution is not directional. It is also not linear - changes do not tend to occur in a big way along a single unbranching line, which would be called anagenesis. So intermediate forms are not, despite the popular idea of evolution, segments on a single line; they are nodes or branches on a tree diagram:
This is Darwin’s diagram from the Origin . You can see that the change in form or morphology (the horizontal axis) is not very great in each speciation. But you can see that the red forms are intermediate between the left and right blue forms. Under the Lamarckian view, intermediate forms must occur between two points on the line.
Modern geology was born when the assumption was adopted that the things that caused geological formation in the past were the same as the things that caused it in the present. Hence, a uniformity of causes was assumed in order to understand the past. Darwin was a direct adherent of this view when he adopted the notion of transmutation. He took an older saying of Linnaeus’, used all the way back into the middle ages, Natura non facit saltus , which means that nature does not make leaps. Under the Great Chain, this merely meant that all points on the scale were completed occupied, but Darwin meant that the transition from one species to another was gradual. Huxley, although notionally a “Darwinian” disagreed with him, saying that he had unnecessarily loaded himself with this saying. Darwin did not, though, mean that this meant rates of change stayed uniform. By the Sixth Edition he was happily stating that species might be maintained unchanged for the majority of their duration. What he did disallow, it that a species might be formed in a single or extremely rapid event. And he was wrong, at least in organisms like flowering plants, spore broadcasting organisms like ferns or corals, and in other organisms that hybridised across species lines freely. A process known as “polyploidy”, or the multiplication of chromosomes so that they could match up when the parental spores were of a different chromosome number, allows “instant speciation”. One person who discovered this effect thought it meant that all speciation was instantaneous, and he called his idea “mutation theory”. Anyone who reads the X-Men knows of this idea - it died rapidly in science but, like the progressive view of the Great Chain in Lamarck, survived in bad science fiction and comics. Note that again we might the subjective or relative terms “rapid” and “slow”. What is slow to a population biologist is instantaneous to a geologist. We always have to bear this frame relativity in mind, particularly when we consider punctuated equilibrium theory of Gould and Eldredge.
For a large part of the 20th Century it was not clearly understood that there are two kinds of evolutionary change. One is the idea Darwin called “transmutation” - that forms changed (of course, he thought this was mostly adaptational), which we now call Anagenesis, from the Greek word for regeneration. You can see this in the change of species, or form, A to species, or form, B. There is no splitting, but the earlier species has changed entire. This is thought not to be a speciation event by a good many specialists. But there is also a multiplication of species, termed “cladogenesis” from the Greek for “birth of branches”. And it is this that is presented in most evolutionary tree diagrams. There has been a long period of argument about how to show the two kinds of evolution. Another way to make this distinction, but one that can mislead, is to call evolution at the level of species or above Macroevolution , and evolution within species microevolution . Why this misleads is that adaptation and other within-species processes can cross over into multiple species lineages, and there is some argument about whether or not what happens in macroevolution is due only to within-species processes of population genetics. What sorts of things adaptation causes is the access to a new ecological niche or lifestyle. When you consider that a lifestyle is not itself something that is restricted to related lineages - birds and bats can be predators - it is best to call what is achieved in this way grades. These are sometimes called “designs” or “solutions” or, in Dennett’s book, “Good Ideas”. But grades are more than just adaptations, and anagenesis is more than natural selection. We’ll go into this more later. The splitting of genealogical lineages in evolution forms what are now called “clades” (branches), although they have also been called tribes. Notice that in the diagram, the two species C and D do not differ all that much either from each other, and from their ancestor B. Species that are not morphologically, ecologically or behaviorally distinguishable are called “cryptic” species.
Last week I said that Natural Selection was not Evolution. This was a deliberate provocation. What might be more accurate would be to say that Anagenesis is not Cladogenesis. Fisher made this point best. There is more to evolution than natural selection, and more things happen in evolution than are due to natural selection. Darwin himself had at least six theories I have been able to delineate: Transmutation, Descent with Modification, or common descent, Natural Selection, plus:
Sexual Selection, or the idea that what some adaptations are adaptations to was prospective mates; the idea that evolution by common descent explains the geographical distribution of organisms - or, in short, that time explains space; and a failed theory of pangenesis, in which the body cells throw off “gemmules” that are formed into “pangenes” in the sex cells, so that aspects of organisms that work are more likely to be inherited. We can ignore this.
The so-called Modern Synthesis, now about 70 years old and so referred to as the Synthesis, was a synthesis of Mendelian genetics, which had been redescribed in 1900, and the so-called “neo-Darwinian” view of evolution which was developed in the 1880s and 1890s. For a very long time these two approaches were thought to be at odds with each other. The rapprochement was performed by Fisher, JBS Haldane, and EB Ford over a period of about ten years. Then in 1940, TH Huxley’s grandson, Julian Huxley, published his Evolution: The Modern Synthesis , and the current orthodoxy was established, along with several other books by Dobzhansky, Mayr and others. Other additions to the Darwinian view included the notion of genetic drift by Sewall Wright in 1932. This is important, so I’ll cover it at length on the next slide. Antoher non-selective mechanism is more recent - Mootoo Kimura proposed that most molecular evolution was neutral - substitutions could be made that had little or not phenotypic (trait) change. Ernst Mayr revived a theory of speciation called allopatric speciation, or speciation by the isolation and independent evolution of a population at the periphery of a species, which became the standard view until very recently. And there is punctuated equilibrium theory, which claims that evolution proceeds at variable rates, and that once a species has separated and evolved, it remains largely static thereafter.
Natural selection is one of a class of explanations that posit an outcome without a controller or guidance, which following Adam Smith’s theory of economics and market equilibrium is called a “hidden hand” explanation. Smith had assumed that markets would find a stable and overall beneficial state if everyone in it was attempting to look out for his own interests. Although we did not know until recently, Smith’s geologist friend Hutton extended this to living things. On the current understanding of selection, there are several requirements for it to act in a straightforward manner, and as things get more complicated, so too do the ways in which selection works. In a large population, where the chance of any two (different sexed) organisms mating are roughly equal, and there is a correlation between genes and traits that affect the likelihood of the progeny surviving to breed and raising their own progeny, those alleles which improve that likelihood will spread to either take over the population entirely, called “fixation”, or to some equilibrium fraction. In smaller populations, or where mating probabilities depend on the densities of the populations, and so forth, some non-intuitive results come out. More on that later. Many people talk as if the default explanation for any trait whatsoever is that it is due to selection, and that claims something is due to non-selective mechanisms must be strongly argued for. I’d like you all to think that over a bit. What should the default explanation be? Some terminology - not everything that is a selectionist explanation is a case of Darwinian evolution, for example in culture or economic, and not everything that is a Darwinian explanation is selectionist. By the way, I reserve the term “Darwin ism ” for a non-biological account of human society, or philosophy.
The idea of genetic drift is that for populations that are either subjected to very weak or no selection, or are undergoing higher mutation rates, or which are too small for selection to operate effectively, to swamp the “noise” of random effects, as it were, evolution can cause changes at random. This can be caused by inbreeding, for example, where rare alleles in the parent population happen to find themselves sampled in the isolated population, then the chances of them “meeting” each other in a body are increased, and so the overall frequencies of alleles can change, just through sampling error. Dobzhansky thought that this accounted for a lot of variation between species. In short, he did not think that selection caused species, and Mayr took this further proposing that indeed geographical isolation was always needed, excepting polyploidy, for new species.
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