Discussion question

From Biological Science by William Keeton

“DARWIN'S THEORY consists of to major parts: the concept of evolutionary change, for which Darwin presented a great deal of evidence, and the quite independent concept of natural selection as the agent of that change.

The Concept of Evolutionary Change

Until only two hundred years ago, it seemed self-evident that the world and the animals that fill it have not changed: robins look like robins, and mic like mice year after year, generation after generation, at least, within the short period of written history. This common sense view is very like our untutored impression that the earth stands still and is circled by the sun, moon, planets, and stars: it accords well with day-to-day experience, and until evidence to the contrary appeared, it provided a satisfying picture of the living world. The idea o an unchanging world also corresponded to a literal reading of the powerfully poetic opening of the Book of Genesis, in which god is said to have created each species independently, simultaneously, and relatively recently—a little over six thousand years ago by reckonings based on Scripture.

But problems with the commonly held scriptural theory of creation arose from many sources: scientists attempted first, quite naturally, to discount the evidence and then, when that proved impossible, to construct a new explanation. Let's look at the evidence for evolution that confronted Darwin and his contemporaries.

The most dramatic findings came from geology. In the eighteenth century a picture of a changing earth had begun to emerge. Extinct volcanoes and their lava flows had been discovered; most geological strata were found to represent sedimentary deposits, laid down layer upon layer a millimeter at a time in columns three thousand meters or more deep; the gradual erosive action of wind and water were seen to have leveled entire mountains and carved out valleys; unknown forces had caused mountains to rise where ocean floors had once been. This latter fact in particular was impressed upon Darwin when he discovered fossilized seashells high in the Andes. Each of these phenomena implied continuous change during vast periods of time.

Another problem for the static view of life was presented by the fossils themselves. Many represented plants and animals wholly unknown in Europe, and though theologians had argued that the organisms these fossils represented were alive in the New World, increasingly intensive exploration of the Americas indicated that the hundreds of species of dinosaurs, for instance, were really extinct. In addition, many previously unknown and often bizarre animals inhabited the Americas.... As the realization grew that the number of animal species for which evidence was accumulating ran at least into the hundreds of thousands, Noah's ark began to seem very small indeed. In fact, it appeared that the extinct species greatly outnumbered the living, and that the new constellations of species had come and gone several times in the past. Moreover, the lowest, oldest rocks contained only the most primitive fossils—seashells, for instance---and these were followed in order by the more modern forms: fish appeared later, for example; reptiles still later; then birds and mammals. The hypothesis of a young earth populated almost overnight by a single bout of creation began to seem very unlikely.

Jean Baptiste de Lamarck (1744-1829)....was the first to offer the major alternative explanation of the fossil record: evolution. Lamarck had arranged fossils of various marine molluscs in order of increasing age: he saw clearly that certain species had slowly changed into others, and concluded that this process of slow change had continued right to the present day. As Lamarck put it in 1809. 'it is no longer possible to doubt that nature has done, everything little by little and successfully,' over a nearly infinite period of time. In Lamarck's view, the living world had begun with simple organisms in the sea, which eventually moved onto land, and evolution had culminated with the appearance of our species, the inevitable result of the gradual trend toward change and 'increasing perfection.'

2.

Lamarck was basically on the right track, though, as we shall see, his mechanism for evolutionary change was incorrect. Burt he was ignored for the very understandable reason that he could not offer sufficient evidence for the fact of evolution. Darwin, only fifty years later, was in a far better position: there was much more evidence of the sort Lamarck had pointed to, and Darwin, a respected geologist, was well acquainted with it. Furthermore, he had the ability to spot important data in the midst of apparent chaos. He could find powerful support for the idea of evolution where Lamarck and others—if they looked at all—saw only irrelevancies.

One of the most important lines of evidence put forward by Darwin was the existence of morphological resemblances among living species (the findings of what we call today comparative anatomy). If, for example, we observe the forelimbs of a variety of different mammals, we see essentially the same bones arranged in the same order (Fig.1.17). The basic bone structure of a human arm, a cat's front leg, and a seal's flipper is the same: the same bones are present even in a bird's wing. True, the size and shape of the individual bones vary from species to species, and some bones may be missing entirely in one species or another, but the basic construction is unmistakably the same. To Darwin the resemblance suggested that each of these species dad descended for a common ancestor from which each had inherited the basic plan of its forelimb, modified to suit its present function. The observation that structure with important functions in some species appear in vestigial form in others further convinced Darwin of the reality of evolutionary change. Why other wise would pigs, which walk on only two toes per foot, have two other toes that dangle uselessly well above the ground? Why would certain snakes, such as the boa constructor, and many species of aquatic mammals, such as whales, have pelvic bones and small, internal hind-limbs bones....? [Fig. 1.18] Why would flightless birds, such as penguins, ostriches, kiwis, and the cormorants of the Galapagos Islands still have rudimentary wings—or feathers, for that matter...? Why would so many subterranean and cave dwelling species have useless eyes buried under their skin?

Embryology—the study of how living things develop from eggs or seeds to their adult forms—also provided powerfully suggestive evidence. Darwin pointed out that in marine crustaceans as different as barnacles and lobsters the young larvae are virtually identical, implying a common descent. Telltale traces of their genealogy are obvious in vertebrates as well. Human embryos, for instance, have gill pouches and well-developed tails that disappear before the time of birth....It seemed clear to Darwin that such inappropriate structures are inherited vestiges of structures that functioned in ancestral forms, and that may still function in other species descended from the same ancestors.

Another particularly convincing line of evidence offered by Darwin was the well-known ability of breeders to produce dynamic changes in both plants and animals. How could anyone contemplating the historical evidence of the alteration of domesticated species doubt that vast changes are possible, given sufficient time? Great Danes, sheep dogs, Irish setters, Yorkshire terriers, poodles, bulldogs, and dachschunds, for instance, are all members of the same species, bred from tamed wolves to look like almost anything breeders have fancied. Similarly, cabbage, brussels sprouts, cauliflower, broccoli, kohlrabi, rutabaga, curly greens, and savoy have all been bred from the same species, the wild form of which looks nothing like its domesticated progeny. So, too, the many varieties of chickens, cattle, horses, flowers, grins, and so on have been bred over the years. Who could compare the colors and shapes of the wild rose or jonquil to the many colors and shapes of the far larger domesticated roses or daffodils, with the new varieties bred each year, and doubt that a species has the capacity to change enormously even in a hundred years? In everything Darwin looked at—fossils, anatomy, embryology, and breeding—he saw the same message: species can and do change...

3.

The Concept of Natural Selection

But what mechanism accounts for the changes? Lamarck's now-discredited hypothesis was one of the first attempts at a plausible explanation. Lamarck was impressed by how well suited each animal was to its particular position in the web of life, even though the environment had changed enormously again and again over countless millions of years. To account for this ability to adapt, he imagined that God had given each species a tendency toward perfection which allowed for small alterations in morphology, physiology, and behavior to accommodate changes in the environment, and that these alterations could be inherited by the offspring.

Belief in Lamarck's idea of a natural tendency toward perfection and the inheritance of acquired characteristics required no more faith in his day than did belief in other invisible everyday forces, such as gravity and magnetism. His hypothesis was a perfectly logical extension of the prevalent Western view that God had set things going by creating nature and nature's laws, and had then left things for the most part to run themselves. But where in the vestigial legs of whales or the dangling toes of pigs was there evidence of perfection? Instead the clear mark of compromise was everywhere. Plants and animal were well adapted to their places in the environment, but they were by no means perfect.

Darwin proposed a different mechanism—natural selection—requiring no internal tendency other than the one toward variation so obvious in nature. Darwin had conceived the idea of natural selection two years after his return from his voyage to the Americas on the Beagle, but was only goaded into publishing, twenty years later, by the receipt in 1858 of A. R. Wallace's manuscript proposing essentially the same theory. (We normally associate Darwin's name with the theory because of the impressive evidence he presented—he had been collecting it for two decades—and because of the thorough exploration of his thorough exploration of the theory's many ramifications.)

In essence, Darwin put together two ideas. The first was that numerous variations exist within species, and that variations are largely heritable. Immersed as he was in the Victorian preoccupation with plant and animal breeding, Darwin knew that while cuttings produce plants identical to the parent, sexual reproduction produces individual offspring that differs both from their parents and from each other. Variation is a fact of life: breeders, as we know, are able to select for desirable traits and create new, morphologically distinct lines of plants and animals.

Darwin's second inspiration came on 28 September 1838, when he reread the Essay on the Principles of Population by the economist Thomas Robert Malthus....Malthus pointed out that humans produce far more offspring than can possibly survive; population growth always outruns any increase in the food supply and is held in check largely by war, disease, and famine, with vast numbers of people perpetually on the edge of starvation. Both Darwin and Wallace were struck at once by the consequences of applying the gloomy Malthusian logic to plants and animals: like humans. The creatures of each overpopulated generation must compete for the limited resources of their environment, and some—indeed most-- must die. Each female frog, for example, produces thousands of eggs per year, and a fern produces tens of millions of spores, yet neither population is growing noticeably. Any organism with naturally occurring heritable variations that increase its chances in this life-or-death contest will be more likely than others to survive long enough to have offspring, some of which will inherit these variations. They in turn will have an above-average chance to survive the struggle, and so will form an increasingly large part of the population. As a result of the 'selection,' the population as a whole will become better adapted, and the never-ending struggle for existence will then turn on the possession of still better adaptations. To distinguish this process from the sort of directed selection practiced by agriculturalists, Darwin called it natural selection.

Nowhere is Darwin's attitude toward nature and his link with the tradition of natural religion and the idea of a God of Secondary Causes clearer than in the final [summary paragraph] of later editions of

4.

The Origin of Species: 'It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, and with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complete a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the conditions of life, and from use and disuse: a Ratio of Increase so high as tl lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one: and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and wonderful, have been and are being evolved.'

The contrast between artificial and natural selection that served Darwin so well provides on instructive summary of the evolutionary process. In both, far may offspring are born than will reproduce; in both, differential reproduction, or selection, occurs, causing some inherited characteristics to become more frequent and prominent in the population ad others to become less so as the generations pass. But in the breeding of domesticated plants and animals, selection results from the deliberate choice by the breeder of which individuals to propagate. In nature, it takes place simply because individuals with different sets of inherited characteristics have unequal chances of surviving and reproducing. Notice, by the way, that selection does not change individuals. An individual cannot evolve. The changes are in the makeup of populations.

Artificial and natural selection also differ significantly in the degree of selection, and its effect on the rate of change. Breeders can practice rigorous selection, eliminating all unwanted individuals in every generation and allowing only a few of the most desirable to reproduce. They an thus bring about very rapid change....Natural selection, which involves a large measure of chance, is usually much less rigorous: some poorly adapted individuals in each generation will be lucky enough to survive and reproduce, while some well-adapted members of the population will not, Hence evolutionary change is usually rather slow; major changes may take thousands or even millions of years, depending on the degree of selection pressure imposed by the environment and by other species.

Darwin's evidence for evolutionary change and the common descent of at least the major groups of organisms was widely accepted in his time, but the idea of natural selection by small steps remained controversial until the 1930's. Some biologists had difficulty seeing how an elaborate and specialized structure like an eye, for instance, could evolve, since the first rudimentary but necessary steps might lack obvious survival value......an expanding understanding of the nature and organization of genes and their role in development has now made it clear that natural selection does explain most evolutionary change.

In summary, then, Darwin's explanation of evolution in terms of natural selection depends upon five basic assumptions:

1. Many more individuals are born in each generation than will survive and reproduce.

2. There is variation among individuals; they are not identical in all their characteristics.

3. Individuals with certain characteristics have a better chance of surviving and reproducing than individuals with other characteristics.

4. Some of the characteristics resulting in differential survival and reproduction are heritable.

5. Vast spans of time have been available for change.

All the known evidence supports the validity of these five assumptions.