Sunday, September 20, 2009

What Darwin Said: Part 5 - Gradualism (A)   posted by DavidB @ 9/20/2009 05:43:00 AM

This is the fifth in a series of posts about Charles Darwin's view of evolution. Previous posts were:

1: The Pattern of Evolution.
2: Mechanisms of Evolution.
3: Heredity.
4: Speciation

The present part deals with the subject of gradualism. Gradualism is contrasted with views of evolution as a sudden, discontinuous or even instantaneous process. At various times, from Darwin's lifetime onwards, discontinuous processes of evolution have often been advocated, most recently by the proponents of 'punctuated equilibrium'.

What is gradualism?

It is necessary to distinguish between

(A) gradualism with respect to the rate of evolutionary change, and
(B) gradualism with respect to the size of the variations adopted by natural selection.

A gradualist in sense (A) maintains that evolution is always or usually slow, while a gradualist in sense (B) maintains that successful variations (mutations, in modern terminology) are always or usually relatively small in effect. Gradualism in senses (A) and (B) often go together, but in principle they can be separated. It would be possible that evolution, when it occurs, usually occurs rapidly (on a geological time scale), but that the mutations responsible are individually quite small. For example, small increases in size might cumulatively double the size of an organism within a few thousand years, which would be 'sudden' on a geological timescale. Conversely, it would be logically possible for a large phenotypic change, such as a reduction from four to two legs, to occur as the result of a single mutation, but to take a very long time to spread through a species. In practice this seems unlikely. Large mutations are seldom advantageous, but when they are, the selective advantage is also likely to be large, and the spread of the mutation will be rapid.

These two senses of 'gradualism' depend on terms such as 'slow', 'rapid', 'large', and 'small', which are themselves vague. They can be made more definite by specifying what is meant by 'large', 'rapid', etc. For example, it might be suggested that an individual mutation affecting a quantitative trait is 'large' if it produces a trait more than three standard deviations larger or smaller than the current mean size. Or the rate of evolution might be considered 'rapid' if more than 90% of evolutionary change occurs within only 10% of the available time. Stephen Jay Gould proposed that speciation be considered 'punctuational' if it takes place in less than 2% of the duration of the species.

I had intended to cover both types of gradualism in this post, but for convenience I will now defer consideration of type (B) to another post.

Darwin's views

So far as the rate of evolutionary change is concerned, Darwin says repeatedly that it is slow, e.g.: 'we have every reason to believe the process of manufacturing new species to be a slow one' [Origin, Variorum text, 140], 'that natural selection will always act with extreme slowness, I fully admit' [201. In the 6th edition he changed 'will always act' to 'generally acts']. This does not imply that the rate of evolution is constant, and Darwin said several times that it is irregular and intermittent (see Part 1 of this series).

'Slow' is a vague term, but Darwin believed that evolution in natural populations is unlikely to be detectable within a human lifetime. Evolution by deliberate artificial selection could produce noticeable changes more quickly, but Darwin thought this was unlikely for species in nature. A closer analogy would be with 'unconscious' artificial selection, by which 'various breeds have been sensibly changed in the course of two or three centuries' [486], but species in nature 'probably change much more slowly' than this [486], implying a timescale of thousands rather than hundreds of years. One secondary source does however hint at a possibility of more rapid change: E. B. Ford, in his Ecological Genetics [4th edn., p,393], says 'Major Leonard Darwin told me of a conversation with his father, the great Charles Darwin, who expressed his belief that by choosing the right material it might be possible actually to detect evolutionary changes taking place at the present time. For this purpose he said that long-continued investigations and careful records would be needed, extending over a period which he estimated at perhaps fifty years in species reproducing annually'.

Darwin apparently believed that substantial evolutionary change, sufficient to form a new species, was usually slow even by geological standards: 'although each formation may mark a very long lapse of years, each probably is short compared with the period requisite to change one species into another' [495. The word 'probably' replaced 'perhaps' from the 4th edn. onwards]. It is not entirely clear what Darwin means by a 'formation'. The term is generally used to refer to the smallest units of the stratigraphic column having their own distinctive rock type and fossil fauna, such as the Chalk of the Cretaceous period. With modern dating methods most such units would be estimated as having a duration of over a million years. In Darwin's time there was much debate about the absolute length of geological time, with physicists arguing (wrongly) that the age of the Earth, and therefore the entire geological record, could not be greater than 100 million years. Even with this timescale, formations would usually have a duration of at least 100 thousand years.

Darwin recognised that his doctrine of slow evolutionary change, in which the emergence of a new species would usually take longer than the duration of a geological formation, faced serious difficulties. It appeared to conflict with a literal reading of the geological record. Darwin identified three main difficulties:

1. New species were not observed to evolve gradually from old ones: they usually appeared quite suddenly in the record

2. Whole groups of related species sometimes appeared simultaneously in the fossil record

3. The fossil record itself began rather suddenly at the base of the Paleozoic era, and showed considerable diversity from the outset.

These phenomena appeared inconsistent with Darwin's gradualism. Darwin's answer was that the fossil record could not be taken at face value. In his chapter on 'The Imperfection of the Geological Record' he examines the processes by which fossils are formed, preserved, and then discovered, and argues that the fossil record gives a very incomplete picture of past life. Sediments can only be deposited over long periods in the same area if the land (or more usually the sea bed) is gradually subsiding at a rate just matched by the supply of new sediment, and the circumstances in which this can happen are rare. The geological record in any one place is therefore usually intermittent, and shows occasional periods of sedimentation interrupted by long gaps during which either there is no deposition, or in which deposits are subsequently eroded. Even where there is no conspicuous discontinuity, as shown by differences in the inclination of bedding planes, there may be long gaps in the record. This is not just a theoretical prediction, but can be demonstrated by cases where formations known to exist in some localities are missing from a seemingly continuous sequence elsewhere.

The sudden appearance of new species in a formation therefore does not imply that they have been suddenly evolved (or created). The alternatives, consistent with gradualism, are that evolution has taken place in the same locality during an unrecorded period, or that the species has evolved elsewhere and then migrated into the area, producing a seemingly instantaneous change in the fossil record. Darwin recognises both of these possibilities [496, 499].

Modern views and controversies

It would now generally be accepted that the fossil record in any one locality is very incomplete. As the geologist Derek Ager put it, 'there is more gap than record'. Whether this entirely explains the apparent patterns of evolution as observed in the record is more controversial. The doctrine of punctuated equilibrium (PE) maintains that the apparent pattern is true to the reality, and that evolutionary change (in any given species) is usually concentrated in relatively short periods of time, too short to be often preserved in the fossil record. The corollary of this is that for most of the time there is 'stasis', i.e. an absence of significant change. However, in the 'fine print' of PE it is admitted that there may be gradual changes in size. The doctrine of stasis is also weakened to the claim that there is no 'directional' change, leaving the possibility that there is fluctuating change rather than strict stasis. It is also admitted that there may be changes in pigmentation, behaviour, and other traits which are not detectable in the fossil record. Taken together, these qualifications substantially dilute the doctrine of 'stasis'.

Most evolutionary biologists accept that punctuation and stasis are possible, but maintain they are consistent with orthodox neo-Darwinism. They point out that the notion of 'phyletic gradualism' denounced by PE - a belief in steady and continuous evolutionary change in a definite direction - is a straw man, and that ever since G. G. Simpson's work in the 1940s evolutionists have believed in wide variations in the rate of evolution, including bradytely (near-stasis) and 'quantum evolution' (rapid bursts of change, often associated with an adaptive radiation).

There remains the question how far the core doctrine of PE is empirically supported. Advocates of PE have compiled meta-analyses which purport to show that the majority of evolutionary change conforms to the PE pattern. However, any meta-analysis can only be as good as the primary data it is based on. In view of the notorious disagreements among paleontologists, such as the division between 'lumpers' and 'splitters', one may be sceptical about any analysis which simply takes paleontological data (such as lists of species and genera) on trust. As recently as 1989 (some 20 years after PE was first advocated) an eminent paleontologist could still say that 'many practising palaeontologists do not record adequate stratigraphic data because they do not think that such information is useful'. [Christopher Paul in Evolution and the Fossil Record, p. 102] There appear to be few studies which directly show punctuation 'events' occurring in the fossil record, and it is part of the PE doctrine that such events are unlikely to be recorded. (In this respect PE agrees with Darwin in appealing to the incompleteness of the fossil record.) The sudden appearance of a new species in a formation is not in itself proof of punctuational evolution, since the species may have evolved slowly elsewhere and then migrated. One of the few cases claimed as showing punctuation in action (P. G. Williamson's study of Lake Turkana molluscs) has recently been re-assessed as a case of migration: see here.

There is wider agreement that 'stasis' is often observed, as can be shown by striking cases of near-identical fossils separated by millions of years in time. Whether stasis is a predominant mode is less clear. The majority of recorded fossil species are known only from a single specimen, so there is no means of assessing their rate of evolutionary change. In studies of living species, there is little evidence of stasis in a strict sense. Almost every population that has been studied in detail has shown some evolutionary change going on. Most species that are geographically widely distributed also show geographical variation, which implies recent evolutionary change. Some evolutionary biologists, including John Maynard Smith and George C. Williams, have been disturbed by the apparent conflict between the evidence of change among living species and the evidence of stasis in the fossil record. One possible resolution is to suppose that most changes are short-term fluctuations which are soon reversed, and that species have an underlying stability due to ecological factors. Only if the ecosystem breaks down, as in a mass extinction, will there be a burst of more substantial and lasting change. Another possible resolution of the puzzle is that different types of species in fact show different evolutionary patterns. Most studies of evolution among living species are of land animals, whereas most paleontologists study marine invertebrates. It is possible that evolution among marine invertebrates is in general slower than among land animals. A case in point is the marine inverterbrates on the opposite sides of the Panama isthmus. The emergence of the isthmus is dated to around 3.5 million years ago. Since then the populations have been isolated from each other and have generally evolved slight morphological differences, to the extent that they are classified as closely related 'geminate' species. If however they were only known as fossilised remains, and it were not known that the populations had been geographically separated, they would probably be lumped together and regarded as a case of 'stasis', with no significant change and no speciation over a period of at least 3 million years. If such slow rates of change are the rule among marine invertebrates, Darwin could be right in arguing that evolution may be too slow, rather than too fast, to be often observable in the fossil record. A final consideration is that paleontologists are interested especially in those parts of animals which are convenient for classification, and these are often features that can be counted, such as the number of teeth in the hinge of a bivalve shell, rather than continuous quantitative variables which require careful measurement. But there are theoretical reasons for supposing that these countable or 'meristic' traits are subject to stabilising selection; see the section on meristic traits in Fisher's GTNS. If so, they would tend to change only rarely, but when they do, the change would be rapid as the species moves from one stable state to another.

The Cambrian Explosion

The sudden appearance of a diverse range of fossil life at the base of the Paleozoic (now often known as the Cambrian Explosion) was recognised by Darwin as a special problem [512-6]. He considered that if his theory was true, there must have been a long period of evolution before the earliest known fossils, probably longer than the period of the known fossil record itself. Why then were there no fossils from this early period? (Darwin did mention fossils from the late pre-Cambrian Longmynd formation, and in later editions of the Origin he mentioned the so-called Eozoon in the pre-Cambrian of Canada. Eozoon is now regarded as inorganic, while the interpretation of the Longmynd fossils remains uncertain - see here ) Darwin speculated on possible explanations, such as very long term changes in the position of land and sea, which might have obliterated the pre-Cambrian record, but he concluded 'The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained'.

Subsequent discoveries have partially resolved Darwin's problem. Microscopic fossils have now been discovered far back in the early pre-Cambrian, but larger organisms are not found until quite late in the pre-Cambrian (the Ediacaran fauna), and hard-bodied animals not until the base of the Cambrian. The appearance of hard-bodied animals was not quite as sudden and simultaneous as believed in Darwin's time, as small shells, and trace fossils attributed to arthropods, have been found in the early Cambrian before the appearance of the trilobites, molluscs, echinoderms, and other forms known in Darwin's time. But the appearance of hard bodies is itself a relatively sudden event which requires some explanation, and a number of speculative hypotheses have been proposed. It also remains unclear whether the division of animals into the existing major phyla (arthropods, echinoderms, etc) was simultaneous with the Cambrian Explosion, or whether there was a long prior period of 'cryptic' evolution of phyla among small soft-bodied forms. There is some molecular evidence to support the 'cryptic' theory, and it is not inherently absurd. Most existing phyla of small soft-bodied animals, such as Rotifera, have no fossil record to speak of, despite presumably having existed in vast numbers since at least the Cambrian (see the table on p.186 of James W. Valentine, On the Origin of Phyla, 2004). The issue of timing remains unresolved (Valentine, p.195).

Overall, it is not clear whether Darwin overestimated the gradualism of the rate of evolution. Some of the issues raised by Darwin, such as the nature of the Cambrian Explosion, remain active and controversial areas of research. Darwin was undoubtedly right - and frank - in highlighting 'the most obvious and gravest objection which can be urged against my theory' - the lack of direct evidence in the fossil record for the gradual transitions which the theory postulated. Subsequent paleontological research has provided some examples of the kind of transitions required, but these remain a small minority of all species in the fossil record. This has led some paleontologists and biologists to think that Darwin also put too much emphasis on gradualism in sense (B): that is, his insistence that the variations favoured by natural selection were always very small. If larger variations - 'saltations' or 'macromutations' - are important in the evolutionary process, the problem of the scarcity of gradual transitions in the fossil record would largely disappear. I will consider gradualism in sense (B) in my next post.