How do we know that evolution is really happening?
The short answer is: surprisingly well, despite a rising tide of criticism in recent years. The theory had enormous difficulty winning acceptance from the scientific community, but by the middle of this century its updated version - called the ''modern synthesis'' - claimed the allegiance of the vast majority of professional biologists.
That consensus has broken down over the last decade or two with fierce professional debates over the precise mechanisms and rates of evolution. But virtually all participants in these battles wrap themselves in the mantle of Darwinism. They say they are battling over subsidiary issues, not challenging the grand structure of Darwinian evolution. The theory has been strong enough and flexible enough to incorporate virtually all the major 20th century advances in molecular biology and genetics with only temporary shudders.
As the British scientific journal, Nature, commented in an editorial last year: ''The way in which the theory of evolution has been able to survive such a long succession of discoveries is striking evidence of its overwhelming consistency. No theory of such a grand scope in the physical sciences has done as well in the past century. Even so, modern evolutionary biologists still cannot give a firm answer to the question that Darwin set out to explore more than a century ago -namely, the precise mechanisms by which the different species, the tigers, the orchids, and the bumblebees of this world, were formed.
Evolution and Religion Research Package
The theory has been quite effective in describing how organisms within a given population develop new forms and characteristics over time, but it has failed so far to explain persuasively how species and larger taxonomic groupings evolve. Provine, a Cornell University historian of evolutionary science, said at a symposium last month. Darwin's theory had two major elements. The first, not wholly original with him, was that all the diverse forms of life on earth have evolved over enormous periods of time from a common ancestry.
The other, the crux of Darwinism, is that the chief guiding force of this evolution has been ''natural selection,'' popularly known as ''survival of the fittest. Darwin believed that small variations often arise in organisms and that, if these variations allow the organisms to survive and reproduce more successfully than competitors, then the fitter form of the organism will gradually replace the old form. Darwin did not contend that natural selection was the only guiding force of evolution, but some of his later adherents became more dogmatic on this point.
A third element of the theory, not considered essential by many Darwinians even in Darwin's day, is that evolution has proceeded at a slow, steady, gradual pace. Darwin's first point - that all life has evolved from common ancestry -is no longer seriously doubted by leading biologists. The findings of modern genetics, which show all organisms governed by the same genetic code, have powerfully reinforced the likelihood of common descent.
Transformism e. What the immaculate pigeon teaches the burdened mind.
Norton, New York. A lucid example of the use of an orthogenetic mechanism in the study of pigeon variation in the early part of this century. On reserve in Bobst Library. Ontogeny and Phylogeny. Take special note of the account of the origin of the modern meaning of "evolution" from preformationist ideas about developmental mechanisms. Darwin's evidence for evolution.
Furthermore, he says, the nested hierarchy of varieties, species, genera, families, orders, classes and so on makes perfect sense when explained as the result of natural selection as shown in the diagram, whereas it would make no sense at all if species were special creations unrelated to other closely similar species. Darwin closes this long chapter with a summary of all the key points. He has shown how all these establish the competence of natural selection to explain the history and diversity of life on Earth which he finally illustrates with the ancient metaphor of the Tree of Life:.
As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications. This chapter serves two functions. Firstly, it bolsters Darwin's case that species are not fundamentally different from varieties.
Secondly, it continues Darwin's theme of establishing natural selection's credentials as the most likely cause of evolution. Darwin has to pull together a vast array of disparate observations about the way organisms vary, how they live, grow and reproduce, and with how offspring inherit their traits. In this chapter he shows that variation suggests underlying similarities between allied species but gives no support to the view of species as independent creations.
Chapter five opens with a survey of how animals and plants vary.
Darwin and His Theory of Evolution
His first sub-section is devoted to 'use and disuse' and explores how a change in the habits of a species can cause a character to grow, or wither. He is particularly interested in the loss of wings in insects and birds on islands which no longer benefit from flying, and the loss of sight in cave dwelling animals. Whatever the cause may be of each slight difference in the offspring from their parents—and a cause for each must exist—it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to survive.
Long before having arrived at this part of my work, a crowd of difficulties will have occurred to the reader.
Some of them are so grave that to this day I can never reflect on them without being staggered; but, to the best of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal to my theory. Darwin starts with the lack of transitional forms preserved in the fossil record. Natural selection automatically forces the less successful varieties to go extinct so that they may not last long.
And, as a geologist, he shows that the fossil record is extremely incomplete, so that one must not expect to find as fossils more than a tiny fraction of the forms which have ever lived. Geography, Darwin says, is constantly changing. Climate fluctuates and barriers to migration keep coming and going. Thus, over time, descendants from a common ancestor become adapted to different ways of life.
Darwin's Evidence for Evolution
Hence the marsupial animals of Australia are clearly related to the extinct fossil marsupials of that country. Darwin then moves on to how an organism can change from one way of life to another, for example to be able to fly through the air. How could such varied creatures as insects, fish, reptiles, birds and mammals have managed to acquire this power?
His strategy is to show that once again the problem is illusory and that there are gradations all around us between non-flying and flying forms. Darwin's next section is on the apparent 'difficulty' of organs of extreme perfection and he chooses to illustrate his solution with the human eye.
This example had been Paley's paradigm for the 'proof' of God's design in his Natural theology Paley said that the eye was so extremely perfect that it can only have been designed by an intelligent designer. Darwin asks on page , however, if we have 'any right to assume that the Creator works by intellectual powers like those of man?
He shows that there is in nature an abundance of intermediary stages between complex eyes and simple light sensitive tissues, as for example in many molluscs. Darwin's next example is the fish's swim bladder which he cites as an example of an organ which has multiple functions, chiefly buoyancy and breathing, and formed a step on the transition from gills to lungs.
In all these cases, however, Darwin is confident that the intermediate steps can indeed exist by the modification of existing organs.
He uses this fact to turn special creation on its head by arguing that if every organism were designed for perfection there would be no reason for such intermediates. In fact, he says, the very existence of intermediates is strong evidence for evolution. The final group of difficulties are 'organs of little apparent importance'.
Darwin cautions his readers not to assume their lack of importance; a fly-swatting tail can mean the difference between life and death to a mammal plagued by blow flies on the African plains, and if not now, an organ may have been important in the past. What right have we to think we understand everything that is going on in nature? We might assume for example that woodpeckers had to be green if we had never known that there are black and white woodpeckers.
In answering these difficulties Darwin provides a catalogue of amazing examples to show that natural selection will always enhance and never erode the net reproductive success of a species. If it makes a species more beautiful to us humans, that is just a by-product of its usefulness to the species. He introduces the concept of what we might call relative, or 'good enough' adaptation, whereby natural selection creates adaptations which are less than perfect but still better than what went before, as proof against special creation which should produce perfect adaptation.
The endemic species of New Zealand, for example, show 'good enough' adaptations in isolation but are not good enough to defend them against introduced species evolved in larger countries where the competition has been tougher. The bee's sting, Darwin says, evolved from ovipositors for boring into wood and has been 'co-opted' for defence, but this is not a perfect adaptation as it often kills the bees.
The stinging function will, however, be retained by natural selection because it has a net benefit to the bee's extended family. Darwin's chapter summary returns to the 'two great laws' which were widely believed to underlie all living things. The 'conditions of existence' were believed to determine an animals structure. For example an aquatic creature like a whale has fins for swimming. In his closing and arguably most profound paragraph in the entire Origin , Darwin - for the first time in the history of biology - sweeps these two great laws together.