Excursion Chapter 2: Genetics And Evolution
Outline of the Pandas Chapter
Darwin's View Of Inheritance
Darwin was always aware of the difficulties that blending inheritance would cause for natural selection but he was also aware of other evidence, prepotency or dominance and reversion, which presaged Mendel, along with the ubiquity of individual differences which animal and plant breeders used so effectively.
Darwin makes no mention of a mechanism of heredity in the Origin of Species. In fact he states in chapter one that the laws of inheritance are unknown (Darwin, 1968, p. 76). He did assume that there was a hereditary mechanism and individual differences, and since they were compatible with the operation of artificial selection as practiced by animal breeders, they must be compatible with the operation of natural selection (Darwin, p. 63, Modern Library Edition of the Origin). After reading in 1867 Jenkin's criticism that random variation would not be sufficient to replenish the variation lost through blending inheritance, Darwin gave added weight to use and disuse and Lamarckian effects of the environment as mechanisms producing variation and downplayed the importance of "single variations", what we would call mutations (see Denton, 1986, p. 63 fol.; Vorzimmer, 1970, chapter 5; Darwin, p. 71 Modern Library Edition of the Origin). In his 1868 work, The Variation of Animals and Plants under Domestication, Darwin introduced his hypothesis of pangenesis where every part of the body produced heredity particles or "gemmules" which circulated throughout the body, aggregating in the gametes to be transmitted to the next generation where, during development they produced a new individual. In some cases they might remain dormant for generations (Moore, 1957, pp. 148-159.)
Mendel's principles are presented quite accurately but their consequences for evolution are not! As we have seen above, Darwin did not have his own theory of blending inheritance (Pandas, bottom of column 2, p. 60) but one of particulate inheritance. Why should knowledge of Mendelian inheritance lead Darwin to Blythe's view of selection as exclusively a conservative force? It is blending inheritance that produces a phenomenon similar to conservative or stabilizing selection. Mendelian inheritance, on the other hand, preserves new traits (even Pandas says so, top of column 1, p. 61) that are generated by mutation, thus allowing natural selection to act upon favorable ones just as Darwin believed (Denton, 1986, p. 64). Pandas' argument here is an example of totally muddled logic! Mendel was an ardent Darwinian!
Micro- and macroevolution are two ends of a continuous scale of phenomena with speciation in the middle. Some authors consider speciation as microevolution and others as macroevolution. Microevolution can be studied directly in terms of genetics and natural selection working on populations. Macroevolution is usually studied from the fossil record and hence involves different methodologies. Almost all evolutionists think that macroevolution is based on microevolutionary processes and involves no new or unknown principles. The ideas of punctuated equilibrium and species selection are applied to the origin and proliferation of new taxa and developmental processes are studied to shed light on the evolution of major structural changes (Ayala, 1983; Eldredge, 1989, chapter 1; Gould, 1983; Hecht and Hoffman, 1986; Levinton, 1983; Maderson, 1982; Stanley, 1979, pp. 2 fol.; chapter 7).
The Discovery Of DNA
The nature of the protein-assembling information stored in structural genes is restricted to the amino acid sequences of polypeptides. Once the polypeptides are synthesized on ribosomes, they self-assemble into their characteristic three dimensional shapes and unite with other polypeptide chains to produce the secondary, tertiary and quaternary levels of structure associated with that particular protein. Other genes, called regulatory genes are involved in "turning on and off" the various structural genes at the proper times during development. Thus, the DNA does not carry a "detailed plan" or blueprint of each protein molecule, only the amino acid sequence, which when fabricated, will self-assemble into the protein molecule.
We have already discussed the appropriate application of Hume's principle of uniformity in the previous chapter. Unlike the blueprints of engineers, DNA contains a recipe (Dawkins, 1986, p. 295 fol.) for making an organism, some parts of which are duplicated many times, some parts of which contain much "nonsense" information—hardly what one would expect from an "intelligent" designer. Recent discoveries, for example, have shown that a gene consists of sections called exons and introns. After the formation of messenger RNA, the introns are excised out. The non-coding fraction may be large—as much as 95% in the case of the beta-like globin genes in humans (Avers, 1989, p. 141 fol.) In general, coding sequences make up only a small part of the total DNA and the great bulk is largely a neutral and randomly drifting sink made up of pseudogenes, silent redundancies, introns, repetitive sequences, retrovirus DNA and transposable elements! (Britten, 1986; Carson, 1989).
The Hardy-Weinberg Law
According to Pandas, many biologists in the late 19th and early 20th century held the misconception that dominant traits would become more common with the passage of time. This is untrue! Provine, in his history of evolution and genetics of that period makes no mention of such a belief (Provine, 1971). Hardy was prompted to write his paper by a comment made by the statistician Udny Yule. In the discussion following an address in 1908 at the Royal Society of Medicine on "Mendelian Heredity in Man" given by the geneticist R. C. Punnett, Yule asked why dominant characters were not becoming increasingly common in the population. Punnett knew Yule's apparent expectation was wrong, but didn't know why and asked Hardy for help (Hardy, 1908, referred to by Pandas footnote 1; Provine, 1971, p. 133; Punnett, 1950, p. 9). Thus Pandas has the argument backwards! It is not quite clear what Yule was thinking when he made that query; previously he, in 1902 and later the biometrician Karl Pearson in 1904 had derived the Hardy-Weinberg law for the special case where the initial allelic frequencies were 0.5. Had they generalized their results for any allelic frequencies, we might very well today be discussing the Pearson-Yule Law (Provine, 1971, pp. 81, 133).
The Hardy-Weinberg states that allelic frequencies of a particular gene locus will remain unchanged from generation to generation unless changed by mutation, natural selection, migration, or random drift. If one further supposes random mating, the laws predicts the genotype frequencies (Avers, 1989, pp. 253 fol.; Futuyma, 1986. p. 82 fol.). Discovering Hardy-Weinberg disequilibrium in genotype frequencies is often a sign that various organizing factors are at work (Futuyma, 1986, p. 99).
Intelligent design proponents assume that in the beginning (they can't even agree as to when that was—see Pandas, p. 92) all basic types of organisms (they can't identify what these basic types were—see Pandas, p. 78) were given a set of genetic instructions (by means totally beyond our comprehension)—a highly uninformative statement to say the least!
Mutations occur and reoccur at a characteristic rate for each gene. Back (reverse) mutations also occur at characteristic rates (Avers, 1989, p. 163). They are random in the sense that the chance that a specific mutation will occur is not affected by how useful that mutation would be (Futuyma, 1986, p. 76). Most mutations are recessive and in a sexually reproducing population, instances of a mutation will accumulate reaching a steady state when the number of reverse mutations equals the number of forward mutations. Thus a species population contains a large number of mutations, each existing at a low frequency in the population, which can be used as raw material by natural selection if and when the environment should change.
The analogy between DNA and words in a book is very weak. All individuals in a population carry a few mutant genes. Human individuals probably differ from each other at as many as 5 million sites and new genomic difference appear by the hundreds with every birth (Britten, 1986). Also, as we have remarked above, DNA contains a lot of nonsense material. If books were like DNA, not only would the words be separated by nonsense sequences but the words themselves would have nonsense sequences (introns) inside them. One would hardly expect an "intelligent author" to write such a book! One must always question arguments whose conclusions depend upon weak or unsatisfactory analogies! Whether a mutant gene is an improvement or not depends upon the environment in which the individual is living. Human populations with high frequencies of the sickle-cell gene only occur in areas where malaria is prevalent. Penicillin-resistant strains of bacteria only appeared after the use of penicillin became widespread. If malaria were eliminated or the use of penicillin discontinued, these genes would be selected against. This has already happened to some extent with regard to the sickle-cell gene in the black descendants of African slaves in the United States (Avers, 1989, p. 227). There are many examples of drosophilid mutants whose fitness depends upon the environment; there are also a number of research studies on drosophilids demonstrating that mutations may increase fitness of populations (Dobzhansky et al, 1977, pp. 64, 66).
Evolution is "descent with modification" and does not necessarily involve additional complexity. And of course, mutants can not be selected for some future utility, although, as was mentioned above, every population has a store of mutant variation, some of which might prove useful if the environment changes. Pandas' statement that "it has not been demonstrated that mutations are able to produce the helpful traits and structures needed for the development of a new species" illustrates the confusion and misunderstanding of evolution existing in the minds of its authors. Natural selection will select genotypes that have increased probability of surviving and reproducing. New species are produced when populations are isolated from one another and they can no longer interbreed. In drosophilids many example of "sibling species" are known. These are species that are morphologically identical, but yet cannot interbreed.
Although most mutations that are large enough to be conspicuous are harmful, most mutations are neutral in effect (Britten, 1986). The existence of back or reverse mutations are uncontestable examples of beneficial mutations and show that the "wild-type" genes could have originated by mutation. The commonly-used Ames test for detecting carcinogenic (mutagenic) substances depends on the occurrence of such beneficial reverse mutations (Maugh, 1978). Exhaustive tests have shown that 90% of the time the Ames test will give positive responses to a mutagenic chemical! There are well-documented experiments with microorganisms where new enzymes have arisen by mutation (Clarke, 1974; Hall, 1982; Mortlock, 1982). Many of the newest "Green-Revolution" crop-plant strains have been bred from radiation-induced mutants (Sigurbjornsson, 1971).
Natural selection is differential survival and reproduction, a perfectly operational definition that can be simulated in a computer model and investigated mathematically. We shall investigate proof of its "creative" powers later. For a pair of authors so enamored of information theory, the Pandas' authors seem incapable of understanding that the intelligent design argument (i.e. creationism!) has no information content. They offer nothing concerning what the designers are, how they work, etc. New organisms just appear out of nowhere!
Natural Selection And The Fixity Of Species
If the intelligent designers wanted quality control, why did they endow virtually all eukaryote organisms with sexual reproduction that is continually producing new gene combinations at the expense of old? There are various DNA copy-error correcting mechanisms in the cell. Why couldn't these wonderful designers produce one that would eliminate all mutations? There are genes known to affect the replication mechanism and elevate mutation rates. Such genes are rare in natural populations but have not been completely eliminated suggesting that mutation rates may have evolved to an optimum level (Futuyma, 1986, pp. 72-74, 259). In certain selection experiments with bacteria, such "mutator" genes may increase in frequency giving such asexual clones an advantage because their genetic variability stems directly from new mutations (Futuyma, 1986, pp. 259-260).
Natural selection is the result of the action of the environment upon the individuals of a species. If the species is already well adapted to the environment and the environment is stable, it will subject the population to stabilizing selection. If on the other hand, the environment is changing, it will exert directional selection upon the population. In the last century and a half, most environmental changes have been due to the activity of man and directional selection has been observed in regard to industrial melanism in many species of lepidopterans including the famous peppered moth, pesticide resistance in insects, drug resistance in microorganisms, etc. (Futuyma, 1986, p. 158).
Pandas ignores the many documented cases of directional selection, but of all the cases of stabilizing selection that they could have cited (see Mayr, 1963, p. 282 fol.) the "classic" paper by Bumpus (Bumpus, 1898) is a poor example which suffers from several basic and fatal logical and statistical flaws (Robson and Richards, 1936. pp. 209-211). Bumpus studied a sample of sparrows that had been "blown down" by a sudden storm. These particular birds most likely fell victim to the storm because they were in open, exposed places when the storm struck and not because some subtle differences in morphology made them more susceptible. Thus this sample of birds probably has little relevance to the question of stabilizing selection in nature. Also, it is very likely that they all would have died if they had not been rescued and cared for in the laboratory. Thus the meaning of any differences found between those that survived and those that died is hard to assess. It took sophisticated statistical techniques to find significant differences between a few of the measurements on this sample of birds which supported Bumpus' conclusions! (Harris, 1911; Calhoun, 1947; Johnston et al, 1972; Grant, 1972; O'Donald, 1973).
This celebrated case of directional selection is a phenomenon involving the appearance of melanic forms in hundreds of species of moths following the discoloration of tree trunks due to air pollution produced by the industrial revolution in Europe and North America. These moths are night flyers and rest on tree trunks during the day. They are cryptically colored to protect them from predation by birds (Kettlewell, 1959; Kettlewell, 1973; Bishop and Cook, 1975).
The black gene producing melanism in the peppered moth may have been already present at low frequencies in that species population, but this is not so of many other species of moths that became melanic due to industrial pollution. These had to wait for rare mutations to take place. There was a lag of 100 years and more before the first record of a black form in Brachionycha sphinx and Xylocampa areola. Melanism has not yet appeared (1973) in Graptolitha ornitopus in Britain although all the related species in North America have distinct dark forms. In Biston betularia (the peppered moth) there has also been selection for modifier genes to increase the dominance of the black gene and make the black forms blacker than they were in the 19th century (Kettlewell, 1973, p. 313 fol.). Other genetic changes improving the viability of the black forms have occurred. "When moths of the dark form were crossed with moths of the light form 50 years ago, the resulting broods were significantly deficient in the dark form. When the same cross is made today, the broods contain more of the dark form that one would expect." (Kettlewell, 1959). Also, the moths tend to select surfaces of the same color as themselves upon which to alight and rest. Thus industrial melanism is much more than a simple change from light to dark. Lovtrup (1987, p. 319) also holds this view.
Change And The Origin Of New Structures
Ambrose' argument (Ambrose, 1982, Pandas' footnotes 2, 3, 4, and 6, referring to pp. 120, 123, 143 and 140-141 respectively) against the formation of new structures is based on several wrong-headed assumptions. First of all, Ambrose assumes that the new genes or mutations are neutral, totally without selective value, until their combination occurs. Thus he eliminates the possibility that natural selection could aid in forming the combination! In his example with five gene mutations, he can only imagine the impossibility of their ever coming together by chance. Yet Haldane gives a similar example using 15 genes. If each of these is only slightly favored by natural selection, they will all increase in frequency in the population and eventually all 15 will regularly occur in the same organisms. Haldane's argument will be discussed in more detail below. Secondly, Ambrose assumes that these are totally new genes, in no way integrated into the developmental system and so for the combination to be expressed, new regulator genes, etc. must be introduced. But all known "new" genes derive from mutations or duplicates of already existing genes. Such will hardly be totally "neutral" in effect and will automatically be subject to the regulator gene of the original structural gene. Evolution of new enzymes and metabolic pathways in experimental populations of bacteria have been reported and they arise by modification of existing pathways (Mortlock, 1982; Hall, 1982). Our knowledge of the evolution of regulatory genes is summarized by MacIntyre (1982).
Thus Ambrose' argument, which starts from wrong-headed assumptions about the Darwinian mechanism of evolution, cannot help but come to incorrect and irrelevant conclusions. Dressing up his argument in concepts from information theory cannot make up for wrong starting assumptions!
Truly "new" structures can only arise with difficulty by the mechanism of mutation and natural selection because of all the concomitant systems, nervous, circulatory, skeletal, muscular, etc. that would have to arise. But we observe very few truly new or novel structures in nature (Futuyma, 1986, p. 410; Mayr, 1960; Mayr, 1963, pp. 602-621). Wings don't sprout out of the backs of flying animals as in the mythical Sphinx, Pegasus and angels. No. The wings of birds, bats and pterosaurs are modified forelimbs. They operate with the nerves, muscles, blood vessels, etc. that were already present. Thus we find this incredible conservatism of "design" that has no parallel in human intelligent design—nearly all "new" structures are modifications of old ones. This is what you would expect from natural selection. It acts as a tinkerer, rather than a designer (Jacob, 1977). This is the evolutionary explanation for the ubiquitous, all-pervasive homologies that characterize the myriads of forms in each of the phyla and which were recognized by the 18th and 19th century pre-evolutionary anatomists (see the discussion in Excursion chapter 5 which deals with homology.)
Darwin was referring to this phenomenon when he referred to ". . .nature prodigal in variety but niggard in invention. . ."(Darwin, 1968, p. 445). Thus the "design" of animals shows incredible conservatism. The limbs of all vertebrates have the same set of component bones and muscles, yet they perform diverse functions—running, flying, swimming, digging, etc.. An engineering school that demanded that engineers design auto wheels, airplane wings, boat propellers, and excavation buckets from the same set of basic parts would be considered crazy. Similarly, the same set of insect mouthparts are modified for chewing and grasping like pliers, piercing and sucking like hypodermic needles or sponging and lapping like a vacuum cleaner. In development, terrestrial vertebrates produce gill pouches, clefts and arches and modify them for other functions.
Even insect wings show little that is truly "new." The wings themselves derive from lateral extensions of the dorsal tergal sclerotized plates. The muscles that flap the wings are modified body wall muscles, while the muscles that steer the wings are modified leg muscles (Snodgrass, 1935, 1958)! The incipient "paranotal" lobes that eventually evolved into wings may originally have had a thermoregulatory function, that is, the slightest increase in body surface area would allow the insect to warm up faster in the morning when the sun's rays struck it (Lewin, 1985; Gould, 1985).
The rod and cone sensors of the retina of the vertebrate eye are modified cilia of cells that formerly lined the interior of the hollow nerve cord. In the tiny transparent invertebrate ancestors of the vertebrates, it didn't make much difference that the "eyespot" was in the center of the body but as the vertebrates grew larger and the eye became a stalked cup projecting from the nerve cord so that it could be close to the surface of the body, its basic geometry resulted in the retina being functionally backwards with the sensory cells at the back and the nerve fibers and ganglia connecting them to the brain between them and the lens (Berrill, 1955, chapter 20). For a more general discussion of the evolution of photoreceptors and eyes, see Eakin (1968) and Salvini-Plawen and Mayr (1977).
This biological conservatism even extends to genes. Genetic studies have shown that "new" genes often arise through duplication of existing genes and then change their functions through mutations. Evidence from protein structures shows that some genes can perform more than one function without any changes. For example the various crystallin proteins that form the lens of the eye double as enzymes! A crystallin from birds and reptiles is identical to an enzyme in the urea cycle. Another from turtles is the same as a enzyme involved in sugar breakdown. A third from ducks and crocodiles is the same as the lactate dehydrogenase enzyme (Marx, 1988).
But all "new" structures can't have come from pre-existing structures. Somewhere in the evolutionary history of a group, novel structures undoubtedly arose. But they were very simple and had their origin very early in evolution, such as the paired limbs of vertebrates (Futuyma, 1986, p. 410). Or the vertebrate eye—the complex vertebrate eye begun as a few light-sensitive cells in the spinal cord to the Tunicata, the presumed invertebrate ancestor of the vertebrates. The eye then evolved through a series of stages, each functional and useful to its bearers: a simple light-sensitive stage; a directional light-sensitive stage once pigment cells partially surround the organ; a simple lens formed from the epidermis concentrating the light on the sensory cells increased the eye's sensitivity in dim light; proliferation of the sensory cells allowed for detecting shadows and large dark shapes; refinement of this organ led to the present-day camera type eye (Berrill, 1955, chapter 20). This "changing function" hypothesis explains how natural selection can govern the origin and evolution of a new structure without macromutations. It also explains some of the remarkably peculiar design flaws of the vertebrate eye, such as the backwards-retina and the blind spot. No sane electronics engineer would ever dream of designing a television camera the same way the vertebrate eye is organized! The peculiar "design" features of many complex organs is powerful evidence that they were fashioned by natural selection and not by an intelligent Creator.
All these facts and many more support the idea that "new" organs and structures evolved by natural selection and each of the "stages" were functional, although the functions themselves changed during the course of evolution.
Natural Selection And The Adaptational Package: The Giraffe
According to Pandas, discoveries have been made that question the evolutionists' belief that the giraffe's long neck functions for browsing in trees. Fossil giraffes are found side by side with fossils of sheep which are grazers, female giraffes are shorter than males and giraffes in zoos are observed to eat grass. Pandas would have us believe that giraffes really use their long necks to reach the ground to eat grass and drink water! This inane idea is quoted almost word for word (but not cited) from Taylor (1983, p. 40.) Taylor was a science writer and his book contains a number of other unbelievably incorrect "facts."
Presumably the giraffe's long neck is necessitated by its long legs. But why do giraffes have long legs? Was it a whim of the intelligent designer? Certainly it is not so that they can run fast. Top running speed of a full-grown giraffe is only about 35 miles per hour. Almost all the antelope that share the African savannahs with the giraffe can run faster! So why do they have long legs? Furthermore, if the function of the long neck is to reach the ground, then the intelligent designer didn't do a very intelligent job of designing. The giraffe's neck is not quite long enough to reach the ground! In order to do so, a giraffe must assume an awkward posture with its forelegs spread out and/or bent. (The very young of many ungulates have the same problem. Their disproportionately long legs enable them to keep up with their mother if they must flee from danger.) Giraffes only need to reach the ground to drink. Interestingly enough, their drinking habits are quite variable. Some populations appear to drink regularly while others rarely drink (if at all) as long as green browse and shade are available.
Giraffes are browsers able to feed on leaves up to 5 meters above the ground. In areas of the African savannahs where giraffe herds feed, there is a noticeable "browse line" on the acacia trees at a height of about 5 meters. No other browsing animals can reach above 2 meters except for elephants and they are just as likely to knock down a tree to get to its high branches. A newborn giraffe is about 2 meters tall and although they may nurse for up to two years, they normally begin browsing at about one month of age. Thus giraffes, because of their height, have access to a food supply not available to other ungulate herbivores. Male giraffes fight by hitting each other with their necks to determine their position in the social dominance hierarchy and who shall have first chance to mate with the females. Thus larger size may give a selective advantage to males. The height difference between males and females may also lessen feeding competition between them and lead to more uniform browsing at the complete range of browsing heights (du Toit, 1990).
When trees are overbrowsed, giraffes may feed on smaller bushes. In one instance in Nairobi National Park, the feeding activities of giraffes reduced height of whistling thorn bushes from 120 cm high to 67 cm high and producing an "inverse browse line". Giraffes are obviously adapted for reaching high rather than low. They really are browsers and Pandas' arguments against the idea that the long neck (and the long legs) allow the giraffe to reach leaves high above the ground are logically and biologically untenable. (See Dagg and Foster, 1976 for everything you ever wanted to know about giraffes).
But what about the complicated adaptational package of the giraffe? When a giraffe lowers its head, would the increased pressure due to the weight of the blood in the neck burst the blood vessels? No. Any increased blood pressure is counterbalanced by a similar increase in pressure in the surrounding extracellular fluids bathing the body tissues (Warren, 1974, p. 99). This same mechanism prevents blood vessel distension and edema at the bottom of the long legs. The giraffe does face the need to pump blood to the head which may be 7 to 10 feet above the heart when the giraffe is standing erect. To accomplish this the giraffe's heart generates a high blood pressure averaging 260/160 mm Mercury compared to the 120/80 average for a resting man (Warren, 1974, p. 98). But what about the pressure sensors, the rete mirabile, etc.? All mammals have arterial pressure sensors in the neck, usually located in the carotid sinus; all have valved veins (even human have valves in the jugular veins (Schaeffer, 1953, pp. 751, 762.); most if not all ungulates have a rete mirabile (Dagg and Foster, 1976, p. 168); the shunt between the carotid and vertebral arteries also occurs in other ungulates, including the short-necked relative of the giraffe, the okapi, although it is not as large as in the giraffe (Lawrence and Rewell, 1948). These all function to assist in the circulation of the blood, not to prevent bursting blood vessels. In the evolution of the giraffe's "adaptational package", no "new" structures had to be "invented", only old ones already present were modified to adapt to the giraffe's high blood pressure. This is the hallmark of natural selection, which can acts to improve or modify the functioning of pre-existing structures, although they eventually may be modified so drastically that the results may look like new, unique products. Even the lengthening of the giraffe's neck involves no "new" structures. For example, it still has only seven cervical vertebrae, the same number as in humans and all other mammals—except a few tree sloths with six or nine (Romer, 1949, p. 159).
The design proponents insist that only a "consummate engineer" could have anticipated the engineering requirements of the giraffe. How is it then that this "consummate engineer" missed a requirement as simple as giving the giraffe a neck long enough to reach the ground without engaging in postural contortions? Also, giraffes, like other large ungulates, have to spend most of their waking hours feeding because they rely on inefficient bacterial fermentation to digest their food—the "consummate engineer" neglected to endow them with enzymes for breaking down cellulose and lignins!
The stone plants: One way to adapt to dry, desert-like conditions is to reduce transpiration and evaporation of water by reducing the surface/volume ratio of the body by acquiring a spheroidal shape—which also happens to be the shape of a pebble. Is this "design sophistication" that requires an intelligent designer? Hardly.
The Neo-Darwinian Mechanism Of Evolution
But can "the blind, chance forces of nature produce what distinguishes such intelligent human beings?" This is a loaded question because the neo-Darwinian mechanism of evolution is not a mechanism of blind, chance forces. It has four basic components:
- Mutation, which produces the raw material of evolution. Mutations occur at random with respect to the phenotypic effects they produce. Viable mutations, however, normally produce variations on the morphology that already exists.
- Reproduction generates a constant supply of new individuals.
- Genetic recombination (sexual and other mechanisms) is constantly shuffling genes, producing new genotypes, and uniting favorable mutations that might have arisen in different individuals.
- Natural selection, the effect of the environment on the survival and reproduction of individuals. Individuals that are better able to survive and reproduce will contribute proportionately more offspring to the next generation that individuals that are less successful.
The last three components mold successful gene combinations from favorable mutations to produce better adaptations. They act as a strongly non-random and creative mechanism. When all four of these elements are included in a mathematical model or computer simulation program of evolution, it works! (Dawkins, 1986; Dewdney, 1989 a and b). The typical creationist endeavor along these lines however, only considers one of these elements, namely mutation (Bliss, 1976, pp. 47-49; Morris, 1974, pp. 59-69). Ambrose' argument (along with that of Edens, 1967) falls in this same category. Thus from the biological point of view, their calculations assume that all the mutations of some new trait or new species, arise simultaneously or in sequence in a single individual. The occurrence, anywhere along the line, of a single deleterious mutation negates the entire endeavor. This, of course, is not evolution, but instantaneous creation by purely random processes. It's no wonder the creationist version of evolution doesn't work.
For evolution to occur the other three elements are necessary. Even the most finely crafted automobile won't go very far when three of its four wheels have been removed! In a population the occurrence of a lethal mutation affects only the individual possessing it. Reproduction by normal individuals replace those killed by lethal mutations and conserve what evolutionary progress has been made. Deleterious mutations may be passed on to offspring but are eventually weeded out by natural selection. On the other hand favorable mutations spread through the population under the action of natural selection. Finally, genetic recombination insures that favorable mutations arising in different individuals will eventually be combined in descendent generations.
All the major taxonomic groups of eukaryote organisms have sexual genetic recombination. This is in addition to whatever asexual reproduction a group may possess. Those instances where sex is absent are apparently secondary losses. A variety of phenomena (transduction, etc.) bring about genetic recombination in prokaryotes. Recombination results in genetic diversity and the developmental systems of eukaryotes have mechanisms (ex: induction) for coordinating the production of a functional individual from diverse genetic messages. These mechanisms can also accommodate mutations that do not produce too drastic a change.
Haldane gives an example to show the creative power of selection which is similar to the situation posed by Ambrose (Haldane, 1932, p. 95; Haldane, 1936, p.67; see also Keeton, 1972, p. 594). Suppose that in a particular population there are 15 rare mutant genes. Each occurs in only 1% of the individuals in the population. The probability that all 15 would occur together in one individual as the result of random sexual recombination is vanishingly small. But even if there is only moderate natural selection for each of the 15 genes, in only about 10,000 generations each will have increased from a frequency of 1% to a frequency of 99% of the population. At this time, 86% of the population can be expected to have all 15 of these genes and display a phenotype that was previously nonexistent in the population. Thus natural selection can take mutations and mold them into combinations of high adaptive value, just as the painter, who didn't make either his canvas or paints, creates combinations of shapes and colors on the canvas that are pleasing to our eyes.
With the availability of fast, powerful computers and computer simulation techniques, even engineers (the prototypical intelligent designers!) are using the creative powers of natural selection to aid them in their design efforts. The technique of "genetic algorithms", pioneered by computer scientist John H. Holland at the University of Michigan, simulates the mechanism of Darwinian evolution, involving mating, genetic recombination, reproduction, selection and mutation to design jet engines, integrated circuit chips, scheduling work in a busy machine shop, operating gas-pipeline pumping stations and recognizing patterns (Peterson, 1989).
Random variations on the specifications for a device are encoded on "chromosomes" of "individuals". The computer evaluates the properties of each individual and the "fitter" one are allowed to mate; "genetic recombination" of chromosomes and even "crossing-over" takes place and a new generation of individuals is produced. Thus each new generation is created from the best pieces of the previous one. This approach efficiently and rapidly zeros in on the best design or solution to a problem. "Mutation" operations are often introduced to keep the process from getting stuck at a suboptimal answer because of a poor choice of starting configurations.
David E. Goldberg, an engineer at the University of Alabama and author of the book, Genetic Algorithms in Search, Optimization, and Machine Learning (1989, Addison-Wesley) reports that genetic algorithms work in lots of different problems but there is still room for improvement by incorporating artificial analogues of other evolutionary and genetic mechanisms. Creationists always claim that evolution can't possibly work. Yet here are engineers (no less!) using EVOLUTION TO DESIGN things. With all the evidence for evolution, could this is the non-supernatural mechanism used by the intelligent designers?
Natural Selection And Gene Combinations
The quotation from Ambrose again shows that author's ignorance of the nature of the environment-organism interaction which is natural selection. The environment does not ask "simple yes or no questions" of the organism, it "selects" those that are better adapted to survive and reproduce in that environment. The English sparrow example discussed below shows the power and subtlety of natural selection. Pandas claims that natural selection works with already-existing genes. But those variant genes, at one time in the past, arose by mutation. (Presumably Pandas would claim that they were all supernaturally created.) Because any particular mutation is recurring at some characteristic rate, we cannot rule out the possibility that North American English sparrows possess mutations acquired since the species was introduced into the New World and that one or more of these are incorporated into the gene combinations selected. Next to nothing is known about the genetics of English sparrows. There is only indirect evidence that the geographic differences in body size and shape have a genetic origin (Johnston, 1973; 1975). Pandas' example of multiple gene loci producing a quantitative gradational character such as skin color may or may not represent the genetic basis for the observed differences in the sparrows. Again, changing from one species to another involves the evolution of interspecies sterility which is appropriately discussed in connection with Chapter 3.
Advantageous Gene Combinations
The House Sparrow
House sparrows were introduced into North America several times between 1850 and 1881 from sources in central England along with some birds from Germany. Once they became established, many transfers of sparrows (80 by 1880) were made from the original sites of introduction to other parts of the country (mainly eastern states but also San Francisco and Salt Lake City) where they were fed and protected. In this fashion, along with the rapid expansion of their range by the birds themselves, they came to occupy most of the continental United States by 1900 (Summers-Smith, 1963, p. 175 fol.; Calhoun, 1947; Johnston and Selander, 1971).
Although earlier workers (Lack, 1940; Calhoun, 1947) investigated geographic variation in these birds, the first extensive investigations were carried out and reported by Johnston and Selander in a series of papers beginning in 1964. They found conspicuous differentiation in color and size. The color differences conform with Gloger's ecogeographic rule, which related color to regional variations in temperature and humidity. Specimens from northern and Pacific coastal localities and the Valley of Mexico were dark; those from the arid southwest were pale; those from the midwest were intermediate in shade. These color differences are not subtle average differences, but marked and consistent (Johnston and Selander, 1964; Halyard, 1965).
Measurement of a number of skeletal characters show a gross size factor with strong negative regression relationships with winter temperature, consistent with Bergmann's ecogeographic rule (larger forms are found in colder climates). They also exhibit a pattern of relative size between body core and limbs, consistent with Allen's ecogeographic rule (forms with smaller extremities—ears, limbs—are found in colder climates). Both of these rules are explained on the basis of the role of body surface/volume ratio in heat loss and retention (Johnston and Selander, 1971). Similar selection pressures resulted in the differences between the Eskimo and African mentioned by Pandas.
Variation within any local population is similar throughout the range in North America and similar to that in England and Germany. Variation between populations is less than that occurring throughout Europe, even though the North American forms cover a much wider range of climate. This is also true for the range in values exhibited by each character. This is probably due to the limited number of generations upon which selection has had the opportunity to act along with the relatively little interpopulation variability in the English and German birds making up the source stock of American sparrows. On the other hand, the character ranges of the North American males exhibit equal or exceed those for the English-German samples. North American females match or exceed the English-German samples in only five of the sixteen characters studied (Johnston and Selander, 1971). Few genetic data are available for the house sparrow, but electrophoretic studies show that the sparrows exhibit relatively little polymorphism (Johnston and Klitz, 1977).
The sparrows are an excellent illustration of the power of natural selection to create new combinations of characters subtly adapted to new environmental conditions. This is directional selection changing the species, not conservative, stabilizing selection.
Natural Selection And Genetic Diversity
At the end of this section, Pandas tries to convince us that directional selection does not occur, that it is exclusively a conservative and not a creative force. But the sparrow example shows otherwise. Natural selection has created new gene combinations that are better adapted to the extremes of climate found in North America. The species has been preserved by being changed. Pandas admits this on the preceding page where the authors say that natural selection provides a means for a species to establish new niches and to adapt to changing environments. And it does this by changing the species. The changed species is not considered a new species unless the changed populations become reproductively isolated from each other or from the unchanged parent populations.
Genetic diversity is beneficial to a species, allowing it to evolve to changing environments. The ultimate source of such variation is mutation—that may be why "mutator" genes (see above) are not completely eliminated from populations and mutation rates may have evolved to optimum values that are not zero. Also, one should remember that new mutations are the main source of variation in microorganisms such as bacteria, which are haploid forms, without a reservoir of recessive genes subject to sexual recombination—although there are several kinds of nonsexual recombination mechanisms (Mayr, 1963, p. 181).
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(from Frank Sonleitner's critique of Of Pandas and People)