In this Darwin year many popular accounts of 'Darwinism' have appeared, but these seldom make a clear distinction among the different components of Darwin's theory of evolution. Many popularisations are simplified to the point of caricature, and presented in an absurdly uncritical way. I yield to few in my admiration for Darwin, but I do not think his memory is best served by oversimplifying his ideas or pretending that he was always right.

So I will attempt to identify the key propositions of 'Darwinism', with an assessment of their current standing. This will probably run to five or six posts, with propositions grouped under the headings:

The Pattern of Evolution
The Mechanisms of Evolution
Heredity
Speciation
Gradualism
Levels of Selection





THE PATTERN OF EVOLUTION


[Note: unless otherwise stated, all page references are to Charles Darwin: The Origin of Species: a Variorum Text, edited by Morse Peckham, 1959, reprinted 2006.]


1. DESCENT WITH MODIFICATION

The single most important proposition of Darwinism is that present-day organisms have descended, by natural processes of reproduction, from organisms that were substantially different. In Darwin's own preferred terminology, they are the product of 'descent with modification'.

Assessment: This is now accepted by all scientists. It is a proposition common to all theories of evolution. Of course Charles Darwin was not the first person to advocate it, and in the first edition of the Origin he took it for granted that any educated reader would be aware of this. For example, he refers to the evolutionist best-seller Vestiges of the Natural History of Creation (1844) on the assumption that it would be familiar to his readers. Darwin was however the first to secure the general acceptance of evolution by the scientific community. This occurred remarkably quickly in the 1860s. Even scientists like Richard Owen, who disagreed strongly with Darwin on other points, quickly accepted the fact of 'descent with modification'. Only a few holdouts like Louis Agassiz continued to defend 'special creation'. Quite why the basic fact of evolution was so quickly accepted is rather a puzzle in itself, on which I will make a few suggestions in due course.

2. EVOLUTION IS INTERMITTANT

In Darwin's theory there is no inevitability about evolution: organisms only change if there is a selective advantage in doing so. In stable habitats some species may stay virtually unchanged almost indefinitely, like the brachiopod Lingula. (522) Lack of evolutionary change is especially common among very simple organisms. Even complex organisms may go for long periods without change if they are well-adapted to an ecological niche (222-6: mostly added in the 3rd edition.) Few organisms are changing much at any given point in time, and for any species the periods of change may be short in comparison with the periods of non-change. The clearest statement of this point is made in the 4th and later editions: 'the periods during which species have undergone modification, though long as measured by years, have probably been short in comparison with the periods during which they have retained the same form' (727), but there are comparable statements in the first edition, e.g. 'we have reason to believe that only a few species are undergoing change at any one period' (724, see also 202).

Assessment: The existence of evolutionary 'stasis' is now generally accepted, though there would be argument about its prevalence and the reasons for it; for example, S J. Gould's vague notions about 'developmental constraints'. In Darwin's time it was an important innovation in evolutionary theory, as previous evolutionists such as Lamarck and Robert Chambers (author of the Vestiges ) had assumed either that evolutionary change was continuous, or that organisms inevitably changed from time to time according to an inbuilt 'program'. Similar ideas were still prevalent in post-Darwinian biology, with many believing in inbuilt tendencies to progress or increased complexity.


3. THE EVOLUTIONARY TREE

According to Darwin, the general pattern of evolution resembles a branching tree, in which successful species give rise to more than one new species, diversifying to exploit different ecological opportunities. Once species have diverged, they do not merge again, which would produce the pattern of a network rather a tree. Tracing the branching pattern backwards from existing species, we usually find that they are more or less similar to each other depending on how far back we find their last common ancestor. This results in a hierarchical classification, traditionally represented by phyla, classes, orders, etc, in which the offshoots of each 'branch' are grouped together. (210-20, 648-60) The simplicity with which the 'branching tree' concept explained the traditional patterns of classification and comparative anatomy helps to explain the rapid acceptance of the general doctrine of evolution after 1859. Darwin adopted the 'tree' concept in his unpublished writings from the 1830s onwards. In 1855 Alfred Russel Wallace published a similar concept of a branching classification, but without an overtly evolutionary explanation. (At this point Wallace was already a covert evolutionist, but had not yet thought of the principle of natural selection.) Pre-Darwinian evolutionists had not generally seen evolution as 'tree-like'. Lamarck, for example, believed that new organisms of the simplest kind were continuously arising by spontaneous generation, and then evolved along a few main pathways from simple to complex (though Lamarck does allow for a limited amount of 'branching'). In the Lamarckian conception, similar species are not necessarily related to each other by common ancestry at all; for example, organisms of the type we call 'vertebrates' do not necessarily have common ancestors, because they may have separately evolved from different ancestors to the vertebrate stage of complexity. In Darwin's theory this is not possible. From the 3rd edition onwards Darwin did recognise the possibility of 'convergence of character', when species of different ancestry live under similar conditions (267-70), but thought it very unlikely ('incredible') that species from widely different ancestry could ever converge so far as to be indistinguishable in all respects.

Assessment: The branching concept of classification is generally accepted. It is most rigorously expressed in cladistic taxonomy. The main qualification to be made to the 'tree' concept in the light of modern knowledge is that 'branches' do sometimes join together. This can happen by symbiosis, as is thought to have happened in the origins of the eukaryotic cell; by hybridism between species (especially among plants); and by 'lateral genetic transfer' of genes by means of viruses, etc. These processes mean that there is an element of 'network' as well as 'branching' in evolution. This was the basis for a New Scientist cover story a few months ago with the sensationalist headline 'Darwin was wrong'. Most biologists regard it as a minor complication rather than a major upset to the conventional Darwinian account.


4. EXTINCTION

According to Darwin, the great majority of past species have become extinct without descendants: 'the manner in which all organic beings are grouped, shows that the greater number of species of each genus, and all the species of many genera, have left no descendants, but have become utterly extinct' (758).

Assessment: This is now taken for granted, to the extent that its importance is often overlooked. By the time of the Origin of Species, most biologists accepted that widespread extinction had taken place throughout geological history, but this had not been adequately incorporated into evolutionary theories. Previous evolutionists such as Lamarck, Etienne Geoffroy Saint-Hilaire, and Robert Chambers were very reluctant to accept extinction, and preferred to believe that species usually evolved into something else. Extinction is a natural corollary of an evolutionary theory based on natural selection and competition, whereas more optimistic theories, based on inherent tendencies to progress, etc, have difficulty explaining or accepting it.


5. ONE OR A FEW ORIGINAL FORMS

A branching pattern of evolution implies that existing species descend from a smaller number of ancestors, but it does not strictly imply that the number was originally very small. Based on comparative anatomy and other evidence, Darwin concludes that 'the theory of descent with modification embraces all the members of the same class' (by which he means a major group such as vertebrates or arthropods) (752). He continues, 'I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number. Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype.' After discussing his reasons for this, such as the chemical similarities between plants and animals, he concludes: 'Therefore I should infer from analogy that probably all the organic beings which have ever lived on the earth have descended from some one primordial form, into which life was first breathed' (753). G. H. Lewes later pointed out that this was too strong a conclusion, and in the 5th edition Darwin qualified it to say 'No doubt it is possible, as Mr G. H. Lewes has urged, that at the first commencement of life many different forms were evolved; but if so, we may conclude that only a very few have left modified descendants' (753).

Assessment: Darwin's conclusions have been vindicated by modern findings, and especially the fact that all organisms have essentially the same genetic code. There is however still a great deal of uncertainty about the origins and relationships of the metazoan phyla (Darwin's 'great classes'.) If anything, the problem has become more difficult since Darwin's time, when it was generally believed (e.g. by Haeckel) that there were only six or seven distinct phyla. Now over thirty phyla are generally recognised, and the relationships between them (e.g. between vertebrates and echinoderms), are far from clear.

6. THE ORIGIN OF LIFE

The Origin of Species provides no explanation of the origin of life itself. Natural selection presupposes the existence of reproducing organisms, and therefore cannot explain their first emergence. In the first edition of the Origin Darwin referred to a 'primordial form, into which life was first breathed'. In the second and later editions he added the words 'by the Creator'. He later regretted 'truckling to public opinion' in using such terminology, and said in a letter of 1863 'I really meant "appeared" by some wholly unknown process. It is mere rubbish, thinking at present of the origin of life; one might as well think of the origin of matter' (Life and Letters of Charles Darwin, ed. Francis Darwin, vol.3, p.18) He later privately speculated about the circumstances in which life might have emerged (in his famous 'warm little pond' letter of 1871), but I don't think he ever published any comments on the subject. In the 3rd edition of the Origin he did say 'Science in her present state does not countenance the belief that living creatures are now ever produced from inorganic matter' (223). In fact, this was more controversial than he recognised. In 1859 the French scientist Francois Pouchet claimed to have produced spontaneous generation under controlled conditions, and Charles Bastian later made similar claims in England. These claims were welcomed by some evolutionists, including A. R. Wallace and Ernst Haeckel, but Darwin was deeply sceptical - rightly, as it turned out, when the experiments of Pouchet and Bastian were convincingly discredited by Pasteur and John Tyndall.

Assessment: We still don't understand the origin of life. Vastly more is known about the chemical building blocks of life than in Darwin's time, so the subject is now open to serious scientific research, but we are probably still a long way from a solution. But such predictions are hazardous, and for all I know someone may already be cooking up the answer. Recent results on 'RNA world' are promising.


To summarise on the Pattern of evolution, and disregarding all questions of the mechanisms or causes of evolution, Darwin was largely successful in his conclusions. This was partly because he had sound judgement, but also to a large extent because he was able to draw on fifty years or so of progress in comparative anatomy, embryology, and taxonomy by such masters as Cuvier, Von Baer, Agassiz, Owen, and Milne-Edwards. This gave him a huge advantage over earlier evolutionists like Lamarck and his own grandfather Erasmus Darwin.

My next post, when I get round to it, will deal with the Darwinian mechanisms of evolution - natural selection, and about half a dozen others.