2.2 Darwin, Linnaeus and Simpson
Activity 1
Timing: 0 hours 15 minutes
In the first clip, Dr. Colin Patterson introduces and explains Darwin’s ‘tree of life’, image, shown below (Figure 3). This was the only image included in his book, Origin of Species.
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Dr. Colin Patterson
Darwin's diagram is bound in with his chapter on natural selection, and he used nine pages of that chapter to explain it. He gave the diagram another three pages in the chapter on geological succession, and then another five pages in the chapter on classification.
Of course we don't have time to work right through Darwin's seventeen pages, but the format of the diagram is probably familiar to you. It has a vertical time-scale of fourteen periods, which Darwin says might each represent a thousand generations, or ten thousand generations, or a million, or even a hundred million. The capital letters A to L, at the bottom of the diagram, represent, "the species of a genus large in its own country," in Darwin's words.
If you take a species capital A as an example, the diverging and branching dotted lines represent its offspring, and the lower case letters and superscript numbers, going from a1 to a14, m1 to m14, and so on, represent well-marked varieties, with distance along the horizontal axis indicating amount of divergence. So, if each time period represents a thousand generations, then after 14,000 generations, at the top of the diagram, species A has produced eight descendant species, numbered a14 to m14. And Darwin says, "Thus, as I believe, species are multiplied and genera are formed".
Among those eight species, Darwin says that the three on the left, a14, q14 and p14, "will be nearly related from having recently branched off from a10, whereas b14 and f14 will be more distinct from those three, and o14, e14 and m14, the three on the right, will be nearly related to each other, but having diverged at the first commencement of the process of modification, will be widely different from the other five species, and may constitute a distinct genus," end of quote.
Darwin goes on to say that the six species descended from species I at the top right of the diagram, will have to be ranked in a different subfamily from the species descended from A. And then he says that he sees no reason to limit this kind of descent with modification to species and genera alone. If the amount of change in each time period was greater, we might end up with two different orders, represented by the descendants of species A and I.
In his chapter on classification, Darwin uses the diagram to show how his theory of descent with modification predicts and explains what he calls, quote, "the grand fact in natural history of the subordination of group under group, which, from its familiarity, does not always sufficiently strike us". And then he goes on, "propinquity of descent - the only known cause of the similarity of organic beings - is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications," end quote. And then Darwin goes on, "that the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking".
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The second clip begins with some background into the system of hierarchy formalised by Linnaeus in the eigthteenth century. Dr. Patterson then asks the question ‘what does “relationship” mean in systematics?’ He looks for answers in the works of three eminent systematists, beginning here with George Gaylord Simpson.
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Dr. Colin Patterson
I shall quote no more from Darwin, but I want to emphasise the fact that he saw classification as one of the most important pieces of evidence bearing on his theory. Since antiquity, naturalists have found that animals and plants fall into a hierarchy, a system of groups and subgroups. And this system was formalised by Linnaeus in the 18th century, into classes containing orders, orders containing families, families containing genera and so on.
Linnaeus, and most other systematists before Darwin, saw this natural hierarchy as an expression of an abstract natural order, the creator's plan. But Darwin saw it in material, or concrete, terms, as the inevitable result of descent with modification, and as something predicted by and so explained by his theory. That's why he said that all true classification is genealogical.
So now, having worked through Darwin's diagram, and his comments on it, we might be ready to answer some questions. The first, and the most basic, is this, “What does „relationship‟ mean in systematics?” I shall outline three different answers, each of them given by one of the three most influential and authoritative systematists.
The first answer's by George Gaylord Simpson, who lived from 1902 to 1984. Here's what he said in his 1961 book, "Principles of Animal Taxonomy": "Is a man more closely related to his father, son, or brother? The degree of genetical relationship to father and son is invariably the same, 0.5 on proportion of shared chromosomes. Genetical relationship to a brother is variable, from 1.0 to 0.0 in terms of chromosomes, although the probability of those extremes is exceedingly low, but the mean value is the same as for father and son, 0.5. Unfortunately, relationships among taxa do not have such fixed a priori expectations. The same two kinds of relationships nevertheless exist among successive taxa in an ancestral-descendant lineage, and among contemporaneous taxa of more or less distinct common origin. The former relationships are called vertical, and the latter horizontal," end of quote.
Now we can easily picture Simpson's two kinds of relationship by looking back at Darwin's diagram. If you find the point in the left-hand lineage, the A line, where it splits at a3 and gives rise to a4 and d4, you can see that the relation between a3 and a4 is the „father-son‟ kind, Simpson's vertical relationship, and the relation between a4 and d4 is the „brother‟ kind, Simpson's horizontal relationship.
Simpson goes on to say that one kind of relationship is obviously just as objective as the other, that classification by either vertical or horizontal relationships alone is absolutely impossible, and that the art of taxonomy is in using your taste and ingenuity to effect a compromise between the two kinds of relationship. That's Simpson's answer to the question "what is relationship", and it leads him to see classification as an art, a matter of taste and ingenuity.
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Darwin, C. R. (1859) On the Origin of Species ©
From Darwin, C. R. (1859) On the Origin of Species ..., facing p.117
Figure 3: From Darwin, C. R. (1859) On the Origin of Species ..., facing p.117. Hypothetical evolutionary lineages derived from 11 ancestral species (A-L), are shown over successive time intervals (I-XIV)