Ariston said:
My point was not of course that any of the writers that are quoted find the fossil record problematic for evolution. My question is simply, why not?
As I said earlier, this is a great question, and deserves a full and serious answer.
So here is the beginning of a full and serious answer to this question.
Indeed, they dont find the record overly problematic, but rather highly supportive of evolution and more and more so as more of it has been discovered.
You are right to focus on gradualism. Why was gradualism important to Darwin? For Darwin, gradualism was the only means by which natural selection could produce the complex features William Paley had discussed in his influential work Natural Theology: or Evidences of the Existence and Attributes of the Deity. You will know that Paleys thesis is that the close match between organic form and its function especially in relation to habitat and way of life shows that organic forms were designed by the Creator and that each species, being so exquisitely fitted to its place in the natural world both geographically and ecologically must have been designed in and for that place. If Paley is right, species change is simply impossible. Darwin was so impressed by Paley that he practically memorized this book. But as his own thought developed, he realized he had to reject Paleys vision. And since Paleys vision dominated both science and theology in his day, he had to provide some strong evidence to support his own case.
His response is that change had to occur in small incrementson the same level as the small variations among individuals that even a Paleyite agreed existed. This is best seen in his famous comment on organs of extreme perfection and complication. Of these he says: If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. (This sentence, btw, is often cited in a quote-mining fashion without the rest of the paragraph or the paragraph following, so it will be good to cite these here as well:
But I can find out no such case. No doubt many organs exist of which we do not know the transitional grades, more especially if we look to much-isolated species, round which, according to my theory, there has been much extinction. Or again, if we look to an organ common to all the members of a large class, for in this latter case the organ must have been first formed at an extremely remote period, since which all the many members of the class have been developed; and in order to discover the early transitional grades through which the organ has passed, we should have to look to very ancient ancestral forms, long since become extinct.
We should be extremely cautious in concluding that an organ could not have been formed by transitional gradations of some kind. Numerous cases could be given amongst the lower animals of the same organ performing at the same time wholly distinct functions; thus the alimentary canal respires, digests, and excretes in the larva of the dragon-fly and in the fish Cobites. In the Hydra, the animal may be turned inside out, and the exterior surface will then digest and the stomach respire. In such cases natural selection might easily specialise, if any advantage were thus gained, a part or organ, which had performed two functions, for one function alone, and thus wholly change its nature by insensible steps. Two distinct organs sometimes perform simultaneously the same function in the same individual; to give one instance, there are fish with gills or branchiae that breathe the air dissolved in the water, at the same time that they breathe free air in their swimbladders, this latter organ having a ductus pneumaticus for its supply, and being divided by highly vascular partitions. In these cases, one of the two organs might with ease be modified and perfected so as to perform all the work by itself, being aided during the process of modification by the other organ; and then this other organ might be modified for some other and quite distinct purpose, or be quite obliterated.
The illustration of the swimbladder in fishes is a good one, because it shows us clearly the highly important fact that an organ originally constructed for one purpose, namely flotation, may be converted into one for a wholly different purpose, namely respiration.)
There is a lot to discuss in that segment alone and perhaps we will get back to it. But clearly, if organs and other features of species are changed in small increments, it will take a good deal of time, many generations to get from (in the case of an eye) a small light-sensitive nerve to a functional camera eye complete with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration,
But how much time? And how many of these small incremental changes will be evident in the fossil record? In the first part of chapter 6 and all through chapter 9, Darwin gives several reasons why we will not see every small incremental change in the fossil record.
First, since fossils can only be embedded in sediment, most fossils will be formed at places where sediment is being deposited in sufficient quantities, over sufficient time to allow for enough pressure to compact the sediment into rock. The situation in which this occurs is primarily the continental shelves which are constantly added to by sediment washing into the sea and in the depths of the ocean. However, before these fossils can be discovered, the same rocks must emerge above water, either through the land being lifted up or the sea level lowering. And then it is subject to erosionin fact, unless erosion exposes the interior of the rock, it will be most difficult to find the embedded fossils. So for any place in which fossils form, there must be a change from ocean to terrestrial conditions; from depositional to erosional conditions. Darwin here compares the fossil record to a grand museum, but one for which collections are made only at intervals remote in time from each other. One might likeie compare it to an encyclopedia of which only a few disconnected volumes remain and between them only rare pages from other volumes. Further the time period in which any fossil formation and preservation is occurring will also vary from place to place.
Second, in a continuous area, transitional forms between two emerging species will be found not scattered among the main bodies of both other forms, but on the common boundary. Darwin uses the example of a species expanding its habitat through three neighbouring areas, a mountainous area, a plain and the narrow range of foothills between them. That third population, being a boundary population, will be less numerous than the others, and less dispersed. So on those grounds alone, are less likely to have a sample preserved in the fossil record. Further, being pressed by its neighbours on both sides, it is likely to go extinct quicker and have a shorter history. So it has less dispersal in time as well as space. Another reason to make fossilization and discovery of a fossil less likely. This situation is at the heart of Goulds theory of punctuated equilibrium. Interesting to see that even without using that phrase, Darwin actually thought of it first.
A third practical reason we have so few species compared with the number that must have existed is that so little of the earths surface has been explored for fossils. Of course this was even more true in Darwins day, but even after 150 years, with all the exploration done in that time, probably less than 1% of the earth has been opened up by paleonotologists. It is, quite simply, an enormously expensive and time-consuming project to conduct a paleontological dig. How much must have been spent by Shubin et al in the five-year project which unearthed Tiktaalik?
Fourth: Soft-bodied species rarely leave organic remains. Most fossils relating to them are trace fossils (e.g. tracks and burrows) which tell us little about the organism itself. Today, we can add that huge numbers of species are microscopic and leave few if any remains. Take that statistic of 8.7 million living species. Well over half of them are microscopic, including some that are multicellular. Consider nematode worms. A single gram of soil contains hundreds of them, but you wont see them without a microscope. There are over 15,000 known species of nematode and an estimate of more than 200, 000 undiscovered species. But although some fossil nematodes are known, they are few and far between and to say the fossil record of this group of animals is spotty is an understatement. When we consider unicellular organisms, the record is even spottier unless they are the type that make mineral outer coverings for themselves (e.g. diatoms, radiolaria, testate amoeba).
Fifth, terrestrial conditions are much less conducive to fossilization than marine conditions, so we have relatively fewer species of terrestrial animals such as bird, land reptiles and mammals than of marine animals with shells such a clams, oysters, foraminiferans, etc.
That is scratching the surface; you can read more in Origin yourself: here is the link to chapter 9
The Origin of Species: Chapter 9
Scientists have, of course, uncovered many many more fossils than those known in Darwins time, but even so, the idea that the fossil record would begin to preserve a sample of every species or even every genus is highly improbable.
So we have to come at the fossil record a different way. Given that it is spotty, that many species were never preserved, that others have not been discovered and possibly never will be, what does the theory of evolution tell us it is reasonable to expect of the fossil record.
The fossil record is still a good test of macro-evolutionbut to see why we need to do a little exploration of scientific hypothesis formation and hypothesis testing. If you are interested, I will post something on that.