© 2007 Jones and Bartlett Publishers, Inc. NOT FOR SALE OR DISTRIBUTION 35 3 Arguments and Evidence for Evolution Chapter Summary The publication of Charles Darwin’s On the Origin of Species in 1859 aroused a variety of objections against his theory of evolution by natural selection. The prevalence of the idea that an individual’s traits were a blend of those of the parents made it diffi cult to see how traits could be selected for; successive gen- erations of mating with nonadapted individuals would dilute any trait. Darwin’s insistence that evolutionary forces act on small and continuous variations led to criticism that selection could not recognize such slight variations and could, therefore, not lead to the formation of new species. An additional problem was the appearance of new traits, which Darwin thought might form by preadaptation or the conversion of an existing structure to a new use. While selection might account for the evolution of one species into another, it remained diffi cult to understand how many species might arise from a single one. A branching pattern of evolution comes about through isolation of groups from one another, something that Darwin did not emphasize, but which others such as Gulick and Wagner did. Finally, given that geologists had yet to establish Earth’s great antiquity, there did not seem to be suffi cient time for evolution to occur. Nevertheless, Darwin’s theory had the advantages of relying on explicable mechanisms, being supported by a massive amount of data, and successfully explaining many features of natural systems. Intermediate forms observed by taxonomists could be explained using Darwin’s theory. Evolutionary theory could also explain why organisms with similar char- acteristics occur geographically close to each other, while groups separated by geographical barriers have fewer characteristics in common. Structures found in different organisms but having an underlying similarity of plan could be explained by their relationship through a common ancestor (homology). Vestigial structures 35 00665_CH03_035_061.indd 35 7/24/07 4:49:18 PM © 2007 Jones and Bartlett Publishers, Inc. NOT FOR SALE OR DISTRIBUTION 36 PART 1 THE HISTORICAL FRAMEWORK such as the rudiments of limb bones in snakes and most complete and continual fossil record of evo- whales would be the remnants of organs that were lution of a group. The horse evolved from a small, present, fully formed, in ancestors. Indeed, whale four-toed browsing animal to a large, single-toed embryos produce hind limb buds that regress early creature with continuously growing teeth, adapted in development. The similarities among vertebrate to chewing tough grasses. Finally, Darwin recognized embryos during early developmental stages also that humans, in seeking to perpetuate desirable traits suggested a common evolutionary past. in domestic plants and animals, could select (over a Fossil evidence was particularly important. Some relatively short period of evolutionary time) radical fossil forms intermediate between extant organisms alterations in organisms, albeit artificially rather than (such as Archaeopteryx, intermediate between rep- naturally. The heavy accumulation of data provided tiles and birds) were found. But horses provided the support for Darwin’s theory for evolution. While the brief papers on natural selection by Darwin and Wallace, published in the Journal of the Linnaean Society in Scientific Objections 1858, evoked little response from the scientific and the non- Objections came rapidly and in various forms, not only in scientific communities, publication ofThe Origin of Species attacks from the established churches but also as objections a year later had profound effects. The first printing sold out to specific aspects of his theory from Darwin’s scientific within hours of its release, and its contents initiated the first colleagues. Some of these are discussed below, along with global public debates concerning science and the place and responses from Darwin, or, more usually, from one or more role of science in society. of his contemporary supporters, such as Huxley. Many found Darwin’s detailed and elegant exposition of his theory, supported by 20 years of thought and documenta- Blending Inheritance tion, impossible to overlook. Many recognized evolution as the Because the prevailing concept of inheritance was that cause for the diversity of species known to exist. Some accepted, maternal and paternal contributions blend in their offspring, to various degrees, that natural selection was an important a number of critics objected that new adaptations would if not primary mechanism for evolution. Thomas Huxley be successively diluted with each generation of interbreed- (1825–1895), who later became Darwin’s main public defender, ing. According to this argument, natural selection would be wrote, “. How extremely stupid of me not to have thought of incapable of maintaining a trait for more than a few genera- that!” This oft-quoted phrase gives the impression that Huxley tions. Darwin made various replies, among them the five immediately captured the essence and significance of Darwin’s outlined below. theory. However, the first part of the sentence (which is not usually quoted) is, “My reflection when I first made myself 1. A beneficial trait could maintain itself if the individuals master of the central idea of the Origin was, how incredibly carrying the trait were isolated from the rest of the pop- stupid . .” He had to work to comprehend (“master”) Darwin’s ulation. Darwin pointed to the familiar practice among ideas, which were revolutionary and counter to all prevailing animal breeders of isolating newly appearing “sports” thought, even to one as versed in the sciences as Huxley.1 and their offspring. This mechanism was commonly used to develop new stocks of domesticated animals 1 Huxley did not accept Darwin’s theory uncritically, finding natural selection difficult to accept and disagreeing with Darwin on the reality of species. or plants. Darwin placed major emphasis on artificial Thomas Huxley was a nineteenth-century polymath. One of the most eloquent selection as engineered by man as evidence supporting statements of science and the scientific method was contained in a sermon his conception of natural selection. Huxley delivered on 6 January 1866. “I cannot but think that the foundations 2. Some traits are prepotent (dominant) and appear undi- of all natural knowledge were laid when the reason of man first came face to face with the facts of Nature; when the savage first learned that the fingers luted in later generations. of one hand are fewer than those of both; that it is shorter to cross a stream 3. An adaptive trait does not appear only once in a popu- than to head it; that a stone stops where it is unless it be moved, and that it lation. Rather, such traits must arise fairly often; wit- drops from the hand which lets it go; that light and heat come and go with ness the large amount of variation in most populations. the sun; that sticks burn away in a fire; that plants and animals grow and die; that if he struck his fellow savage a blow he would make him angry, and Because variation is common, went the argument, it perhaps get a blow in return, while if he offered him a fruit he would please would not dilute out as easily as it would if it was rare. him, and perhaps receive a fish in exchange. When men had acquired this Moreover, Darwin believed, some forms that carry a much knowledge, the outlines, rude though they were, of mathematics, of particular variation pass on to future generations the physics, of chemistry, of biology, of moral, economical, and political science, were sketched” (cited from T. H. Huxley, 1910, pp. 48–49). tendency for the same variation to arise again. 00665_CH03_035_061.indd 36 7/24/07 4:49:19 PM © 2007 Jones and Bartlett Publishers, Inc. NOT FOR SALE OR DISTRIBUTION CHAPTER 3 Arguments and evidence for evolution 37 4. Natural selection both enhances the reproductive suc- be a cause and effect relationship? Although differences in cess of favorable variants and diminishes the reproduc- size may stem from genetic differences of small effect, selec- tive success of unfavorable ones. Thus, the frequency of tion may act on accumulations of such differences in indi- favorable variations increases when unfavorable ones viduals of various sizes (see Chapters 10 and 22). die out, reducing the likelihood of diluting out favorable A further aspect of this same problem was the ques- variations. tion of determining the initial adaptive level that a trait or 5. As explained in Chapter 2, Darwin developed the con- organ would have to reach to be selected. If the trait already cept of pangenesis by gemmules to help explain the existed before selection acted on it, perhaps some quantita- inheritance of traits — which he believed were affected tive expression of the trait would suffice for further evolu- by use and disuse — and to provide constancy for the tion; a larger eye might function better than a smaller eye. determining agents of inheritance. There were presum- But if the trait did not exist, or only barely existed, how ably many gemmules for each particular trait, and their could selection act on it? Many critics felt that the earliest numbers could vary during passage from one genera- incipient stages of complex organs such as the eye or brain tion to another. Gemmules could be lost but were not would have no function at all and could hardly be selected. changed by “blending.” How could one possibly imagine an appropriate adaptive function in only one cell of an eye, brain or leg? An evolutionary answer to this question of the origin of Variation new traits came from the concept of preadaptation, a principle In The Origin, Darwin explicitly confined evolution by natural of which Darwin was aware.2 According to preadaptation, a selection to small, continuous variations and (in the earlier new organ need not arise de novo but may be present in an editions of his book) excluded larger variations as not being organism but being used for a purpose other than that for which useful.