EVOLUTION
TABLE OF CONTENTS
INTRODUCTION
2 DARWINIAN THEORY OF EVOLUTION
4 THE THEORY OF BIOLOGICAL EVOLUTION:
CONTRIBUTING ELEMENTS
7 WALLACE’S CONTRIBUTIONS
13 HARDY-WEINBERG
PRINCIPLE
15 COMPARISON: LAMARCK vs. DARWIN
16 DARWIN’S INFLUENCES
20 METHODS OF SCIENTIFIC DEDUCTION
23 LIMITS TO DARWIN’S THEORY
25 MORPHOLOGICAL & BIOLOGICAL CONCEPTS
27 BIO-EVOLUTION: POPULATION vs. INDIVIDUALS
29 MECHANISMS FOR GENETIC VARIATION
31 GENETIC VARIATION AND SPECIATION
35 DARWIN’S FINCHES
37 SPECIATION vs. CONVERGENT EVOLUTION
39 CONCEPT OF ADAPTATION
41 PUNCTUATED EQUILIBRIUM
43 VALUE/LIMITATIONS: THE THEORY OF BIOLOGICAL EVOLUTION
45 ALTERNATE EXPLANATIONS OF BEING
47 CONCLUSIONS
48 INTRODUCTION
Theories explaining biological evolution have been bandied about since the ancient Greeks, but it was not until the Enlighten of the 18th century that widespread acceptance and development of this theory emerged. In the mid-19th century English naturalist Charles Darwin – who has been called the “father of evolution” – conceived of the most comprehensive findings about organic evolution ever1. Today many of his principles still entail modern interpretation of evolution. I’ve assessed and interpreted the basis of Darwin’s theories on evolution, incorporating a number of other factors concerning evolutionary theory in the process. Criticism of Darwin’s conclusions abounds somewhat more than has been paid tribute to, however Darwin’s findings marked a revolution of thought and social upheaval unprecedented in Western consciousness challenging not only the scientific community, but the prominent religious institution as well. Another revolution in science of a lesser nature was also spawned by Darwin, namely the remarkable simplicity with which his major work The Origin of the Species was written – straightforward English, anyone capable of a logical argument could follow it – also unprecedented in the scientific community (compare this to Isaac Newton’s horribly complex work taking the scientific community years to
interpret2). Evolutionary and revolutionary in more than one sense of each word. Every theory mentioned in the following reading, in fact falls back to Darwinism. DARWINIAN THEORY OF BIOLOGICAL EVOLUTION Modern conception of species and the idea of organic evolution had been part of Western consciousness since the mid-17th century (a la John Ray)3, but wide-range acceptance of this idea, beyond the bounds of the scientific community, did not arise until Darwin published his findings in 19584. Darwin first developed his theory of biological evolution in 1938, following his five-year circumglobal voyage in the southern tropics (as a naturalist) on the H.M.S. Beagle, and perusal of one Thomas Malthus’ An Essay on the Principle of Population which proposed that environmental factors, such as famine and disease limited human population growth5. This had direct bearing on Darwin’s theory of natural selection, furnishing him with an enhanced conceptualization of the “survival of the fittest” – the competition among individuals of the same species for limited resources – the “missing piece” to his puzzle6. For fear of contradicting his father’s beliefs, Darwin did not publish his findings until he was virtually forced after Alfred Wallace sent him a short paper almost identical to his own extensive works on the theory of evolution. The two men presented a joint paper to the Linnaean Society in 1958 – Darwin published a much larger work (“a mere abstract of my material”) Origin of the Species a year later, a source of undue controversy and opposition (from pious Christians) 7, but remarkable development for evolutionary theory. Their findings basically stated that populations of organisms and individuals of a species were varied: some individuals were more capable of obtaining mates, food and other means of sustenance, consequently producing more offspring than less capable individuals. Their offspring would retain some of these characteristics, hence a disproportionate representation of successive individuals in future generations. Therefore future generations would tend have those characteristics of more accommodating individuals8. This is the basis of Darwin’s theory of natural selection: those individuals incapable of adapting to change are eliminated in future generations, “selected against”. Darwin observed that animals tended to produce more offspring than were necessary to replace themselves, leading to the logical conclusion that eventually the earth would no longer be able to support an expanding population. As a result of increasing population however, war, famine and pestilence also increase proportionately, generally maintaining comparatively stable population9. Twelve years later, Darwin published a two-volume work entitled The Descent of Man, applying his basic theory to like comparison between the evolutionary nature of man and animals and how this related to socio-political development man and his perception of life. “It is through the blind and aimless progress of natural selection that man has advance to his present level in love, memory, attention, curiosity, imitation, reason, etc. as well as progress in “knowledge morals and religion”10. Here is where originated the classic idea of the evolution of man from ape, specifically where he contended that Africa was the cradle of civilization. This work also met with opposition but because of the impact of his “revolutionary” initial work this opposition was comparatively muted11.A summary of the critical issues of Darwin’s theory might be abridged into six concise point as follows: 1Variation among individuals of a species does not indicatedeficient copies of an ideal prototype as suggested by the platonic notion of Eidos. The reverse is true: variation is integral to the evolutionary process.2The fundamental struggle in nature occurs within single species population to obtain food, interbreed, and resist predation.
The struggle between different species (ie. Fox vs. hare) is less consequential. 3 The only variations pertinent to evolution are those which are inherited. 4 Evolution is an ongoing process which must span many moons to become detectably apparent. 5 Complexity of a species may not necessarily increase withthe evolutionary process – it may not change at all, even decrease. 6 Predator and prey have no underlying purpose for maintenanceof any type of balance – natural selection is opportunistic and irregular12. THE THEORY OF BIOLOGICAL EVOLUTION: CONTRIBUTING ELEMENTS The scientific range of biological evolution is remarkably vast and can be used to explain numerous observations within the field of biology. Generally, observation of any physical, behavioural, or chemical change (adaptation) over time owing directly to considerable diversity of organisms can be attributed to biological evolution of species. It might also explain the location (distribution) of species throughout the planet. Naturalists can hypothesize that if organisms are evolving through time, then current species will differ considerably from their extinct ancestors. The theory of biological evolution brought about the idea for a record of the progressive changes an early, extinct species underwent. Through use of this fossil record paleontologists are able to classify species according to their similarity to ancestral predecessors, and thereby determine which species might be related to one another. Determination of the age of each fossil will concurrently indicate the rate of evolution, as well as precisely which ancestors preceded one another and consequently which characteristics are retained or selected against. Generally this holds true: probable ancestors do occur earlier in the fossil record, prokaryotes precede eukaryotes in the fossil record. There are however, significant “missing links” throughout the fossil record resulting from species that were, perhaps, never fossilized – nevertheless it is relatively compatible with the theory of evolution13. It can be postulated that organisms evolving from the same ancestor will tend to have similar structural characteristics. New species will have modified versions of pre-existing structures as per their respective habitats (environmental situations). Certainly these varying species will demonstrate clear differentiation in important structural functions, however an underlying similarity will be noted in all. In this case the similarity is said to be homologous, that is, structure origin is identical for all descended species, but very different in appearance. This can be exemplified in the pectoral appendages of terrestrial vertebrates: Initial impression would be that of disparate structure, however in all such vertebrates four distinct structural regions have been defined: the region nearest the body (humerus connecting to the pectoral girdle, the middle region (two bones, radius and ulna are present), a third region – the “hand” – of several bones (carpal and metacarpal, and region of digits or “fingers”. Current species might also exhibit similar organ functions, but are not descended from the same ancestor and therefore different in structure. Such organisms are said to be analogous and can be exemplified in tetrapods, many containing similar muscles but not necessarily originating from the same ancestor. These two anatomical likenesses cannot be explained without considerable understanding of the theory of organic evolution14. The embryology, or early development of species evolved from the same ancestor would also be expected to be congruent. Related species all share embryonic features. This has helped in determining reasons why development takes place indirectly, structures appearing in embryonic stage serve no purpose, and why they are absent in adults. All vertebrates develop a notchord, gill slits (greatly modified during the embryonic cycle) and a tail during early embryology, subsequently passing through stages in which they resemble larval amphioxus, then larval fishes. The notchord will only be retained as discs, while only the ear canal will remain of the gills in adults. Toothless Baleen whales will temporarily develop teeth and hair during early embryology leading to the conclusion that their ancestors had these anatomical intricacies. A similar pattern, exists in almost all animal organisms during the embryonic stage for numerous formations of common organs including the lungs and liver. Yet there is a virtually unlimited variation of anatomical properties among adult organisms. This variation can only be attributed to evolutionary theory15. Biological evolution theory insists that in the case of a common ancestor, all species should be similar on a molecular level. Despite the tremendous diversity in structure, behaviour and physiology of organisms, there is among them a considerable amount of molecular consistency. Many statements have already been made to ascertain this: All cells are comprised of the same elemental organic compounds, namely proteins, lipid and carbohydrates. All organic reactions involve the action of enzymes. Proteins are synthesized in all cells from 20 known amino acids. In all cells, carbohydrate molecules are derivatives of six-carbon sugars (and their polymers). Glycolysis is used by all cells to obtain energy through the breakdown of compounds. Metabolism for all cells as well as determination of definitude of proteins through intermediate compounds is governed by DNA. The structure for all vital lipids, proteins, some important co-enzymes and specialized molecules such as DNA, RNA and ATP are common to all organisms. All organisms are anatomically constructed through function of the genetic code. All of these biochemical similarities can be predicted by the theory of biological evolution but, of course some molecular differentiation can occur. What might appear as minor differentiation (perhaps the occurrence-frequency of a single enzyme) might throw species into entirely different orders of mammals (ie. Cite the chimpanzee and horse, the differentiation resulting from the presence of an extra 11 cytochrome c respiratory enzymes). Experts have therefore theorized that all life evolve from a single organism, the changes having occurred in each lineage, derived in concert from a common ancestor16. Breeders had long known the value of protective resemblance long before Darwin or any other biological evolution theorists made their mark. Nevertheless, evolutionary theory can predict and explain the process by which offspring of two somewhat different parents of the same species will inherit the traits of both – or rather how to insure that the offspring retains the beneficial traits by merging two of the same species with like physical characteristics. It was the work of Mendel that actually led to more educated explanations for the value in protective resemblance17. The Hardy-Weinburg theory specifically, employs Mendel’s theory to a degree to predict the frequency of occurrence of dominantly or recessively expressing offspring. Population genetics is almost sufficient in explaining the basis for protective resemblance. Here biological evolutionary theory might obtain its first application to genetic engineering18. Finally, one could suggest that species residing in a specific area might be placed into two ancestral groups: those species with origins outside of the area and those species evolving from ancestors already present in the area. Because the evolutionary process is so slow, spanning over considerable lengths of time, it can be predicted that similar species would be found within comparatively short distances of each other, due to the difficulty for most organisms to disperse across an ocean. These patterns of dispersion are rather complex, but it is generally maintained by biologists that closely related species occur in the same indefinite region. Species may also be isolated by geographic dispersion: they might colonize an island, and over the course of time evolve differently from their relatives on the mainland. Madagascar is one such example – in fact approximately 90 percent of the birds living there are endemic to that region. Thus as predicted, it follows that speciation is concurrent with the theory of biological evolution
19. WALLACE’S CONTRIBUTIONS
There is rarely a sentence written regarding Wallace that does not contain some allusion to Darwin. Indeed, perhaps the single most significant feat he performed was to compel Darwin to enter the public scene20. Wallace, another English naturalist had done extensive work in South America and Southeast Asia (particularly the Amazon and the Malay Archipelago) and, like Darwin, he had not conceived of the mechanism of evolution until he read (recalled, actually) the work of Thomas Malthus – the notion that “in every generation the inferior would be killed off and the superior would remain – that is the fittest would survive”. When the environment changed therefore, he determined “that all the changes necessary for the adaptation of the species … would be brought about; and as the great changes are always slow there would be ample time for the change to be effected by the survival of the best fitted in every generation”. He saw that his theory supplanted the views of Lamarck and the Vistages and annulled every important difficulty with these theories21. Two days later he sent Darwin (leading naturalist of the time) a four-thousand word outline of his ideas entitled “On the Law Which has regulated the Introduction”. This was more than merely cause for Darwin’s distress, for his work was so similar to Darwin’s own that in some cases it paralleled Darwin’s own phrasing, drawing on many of the same examples Darwin hit upon. Darwin was in despair over this, years of his own work seemed to go down the tube – but he felt he must publish Wallace’s work. Darwin was persuaded by friends to include extracts of his own findings when he submitted Wallace’s work On the Law Which Has Regulated the Introduction of New Species to the Linnaean Society in 1858, feeling doubly horrible because he felt this would be taking advantage of Wallace’s position. Wallace never once gave the slightest impression of resentment or disagreement, even to the point of publishing a work of his own entitled Darwinism. This was his single greatest contribution to the field: encouraging Darwin to publish his extensive research on the issues they’d both developed22. He later published Contributions to the Theory of Natural Selection, comprising the fundamental explanation and understanding of the theory of evolution through natural selection. He also greatly developed the notion of natural barriers which served as isolation mechanisms, keeping apart not only species but also whole families of animals – he drew up a line (“Wallace’s line”) where the fauna and flora of Southeast Asia were very distinct from those of
Australasia23.HARDY-WEINBERG PRINCIPLEPrior to full recognition of
Mendel’s work in the early 1900’s, development of quantitative models describing the changes of gene frequencies in population were not realized. Following this “rediscovery” of Mendel, four scientists independently, almost simultaneously contrived the Hardy-Weinberg principal (named after two of the four scientists) which initiated the science of population genetics: exploration of the statistical repercussions of the principle of inheritance as devised by Mendel. Read concisely the Hardy-Weinberg principle might be stated as follows: Alternate paradigms of genes in ample populations will not be modified proportionately as per successive generation, unless stimulated by mutation, selection, emigration, or immigration of individuals. The relative proportion of genotypes in the population will also be maintained after one generation, should these conditions be negated or mating is random24. Through application of the Hardy-Weinberg principle the precise conditions under which change does not occur in the frequencies of alleles at a locus in a given population (group of individuals able to interbreed and produce fertile offspring) can be formulated: the alleles of a locus will be at equilibrium. A species may occur in congruous correspondence with its population counterpart, or may consist of several diverse populations, physically isolated from one another25. In accordance with Mendelian principle, given two heterozygous alleles A and B, probability of the offspring retaining prominent traits of either parent (AA or BB) is 25 percent, probability of retaining half the traits of each parent (AB) is 50 percent. Thus allele frequencies in the offspring parallel those of the parents. Likewise, given one parent AB and another AA, allele frequencies would be 75 percent A and 25 percent B, while genotype frequencies would be 50 percent AA and 50 percent AB – the gametes generated by these offspring would also maintain the same ratio their parents initiated (given, of course a maximum of two alleles at each locus). In true-to-life application however, where numerous alleles may occur at any given locus numerous possible combinations of gene frequencies are generated. Assuming a population of 100 individuals = 1,
30 at genotype AA, 70 at genotype BB. Applying the proportionate theory, only 30% (0.30) of the gametes produced will retain the A allele, while 70% (0.70) the B allele. Assuming there is no preference for AA or BB individuals for mates, the probability of the (30% of total population) AA males mating with AA females is but 9% (0.3 x 0.3 = 0.09). Likewise the probability of a BB to BB match is 49%, the remainder between (30%) AA and (70%) BB individuals, totalling a 21% frequency. Frequency of alleles in a population in are commonly denoted p and q respectively, while the AB genotype is denoted 2pq. Using the relevant equation p + pq + q = 1, the same proportions would be obtained. It can therefore be noted that the frequencies of the alleles in the population are unchanged. If one were to apply this equation to the next generation, similarly the genotype frequencies will remain unchanged per each successive generation. Generally speaking, the Hardy-Weinberg principle will not favour one genotype over another producing frequencies expected through application of this law. The integral relevance for employment of the Hardy-Weinberg principle is its illustration of expected frequencies where populations are evolving. Deviation from these projected frequencies indicates evolution of the species may be occurring. Allele and genotype frequencies are typically modified per each successive generation and never in ideal Hardy-Weinberg equilibrium. These modifications may be the result of natural selection, but (particularly among small populations) may simply result from random circumstance. They might also arise form immigration of individuals form other populations where gene frequencies will be unique, or form individuals who do not randomly choose mates from their wide-ranged species26.COMPARISON:
LAMARCK vs. DARWINDespite the lack of respect lamarckian theory was dealt at the hands of the early evolution-revolutionaries, the enormous influence it had on numerous scientists, including Lyell, Darwin and the developers of the Hardy- Weinberg theory cannot be denied. Jean Lamarck, a French biologist postulated the theory of an inherent faculty of self-improvement by his teaching that new organs arise form new needs, that they develop in proportion to how often they are used and that these acquisitions are handed down from one generation to the next (conversely disuse of existing organs leads to their gradual disappearance). He also suggested that non-living matter was spontaneously created into the less complex organisms who would evolve over time into organisms of greater and greater complexity. He published his conclusions in 1802, then later (1909) released an expanded form entitled Philosophie zoologique. The English public was first exposed to his findings when Lyell popularized them with his usual flair for writing, but because the influential Lyell also openly criticized these findings they were never fully accepted27. Darwin’s own theories were based on those of older evolutionists and the principle of descent with modification, the principle of direct or indirect action of the environment on an individual organism, and a wavering belief in Lamarck’s doctrine that new characteristics acquired by the individual through use or disuse are transferred to its descendants. Darwin basically built around this theory, adding that variation occurs in the passage each progressive generation. Lamarck’s findings could be summarized by stating that it is the surrounding environment that has direct bearing on the evolution of species. Darwin instead contested that it was inter-species strife “the will to power” or the “survival of the fittest”28. Certainly Lamarck was looking to the condition of the sexes: the significantly evolved difference of musculature between male and females can probably be more easily explained by Lamarckian theory than Darwinian. There was actually quite a remarkable similarity between the conclusions of Darwin’s grandfather, Erasmus Darwin and Lamarck – Lamarck himself only mentioned Erasmus in a footnote, and with virtual contempt. The fact is neither Lamarck nor Darwin ever proposed a means by which species traits were passed on, although Lamarck is usually recalled as one of those hopelessly erroneous scientists of past it was merely the basis for his conclusions that were hopelessly out of depth – the conclusions were remarkably accurate29. DARWIN’S
INFLUENCES In 1831 a young Charles Darwin received the scientific opportunity of lifetime, when he was invited to take charge of the natural history side of a five year voyage on the H.M.S. Beagle, which was to sail around the world, particularly to survey the coast of South America. Darwin’s reference material consisted of works of Sir Charles Lyell, a British geologist (he developed a concept termed uniformitarianism which suggested that geological phenomena could be explained by prevailing observations of natural processes operating over a great spans of time – he has been accused synthesizing the works of others30) who was the author of geologic texts that were required reading throughout the 19th century including Principals of Geology, which along with his own findings (observing the a large land shift resulting from an earthquake), convinced him of geological uniformitarianism, hypothesizing for example, that earthquakes were responsible for the formation of mountains. Darwin faithfully maintained this method of interpreting facts – by seeking explanations of past events by observing occurrences in present time – throughout his life31. The lucid writing style of Lyell and straightforward conclusions influence all of his work. When unearthing remains of extinct animals in Argentina he noted that their remains more closely resembled those of contemporary South American mammals than any other animals in the world. He noted “that existing animals have a close relation in form with extinct species”, and deduced that this would be expected “if the contemporary species had evolved from South American ancestors” not however, if there existed an ideal biota for each environment. When he arrived on the Galapagos islands (islands having been formed at about the same time and characteristically similar), he was surprised to observe unique species to each respective island, and particularly tortoises which possessed sufficiently differentiated shells to tell them apart. From these observations he concluded that the tortoises could only have evolved on the islands32. Thomas Robert Malthus was an English economist and clergyman whose work An Essay on the Principal of Population led Darwin to a more complete understanding of density dependent factors and the “struggle in nature”. Malthus noted that there was potential for rapid increase in population through reproduction – but that food cannot increase as fast as population can, and therefore eventuality will allow less food per person, the less able dying out from starvation or sickness. Thus did Malthus identify population growth as an obstacle to human progress and pedalled abstinence and late marriage in his wake? For these conclusions he came under fire from the Enlightment movement which interpreted his works as opposing social reform33. Erasmus Darwin, grandfather of Darwin, was an unconventional, freethinking physician and poet who expressed his ardent preoccupation for the sciences through poetry. In the poem Zoonomia he initiated the idea that evolution of an organism results from environmental implementation. This coupled with a strong influence from the similar conclusions of Lamarck shaped Darwin’s perception on the environment’s inherent nature to mould and shape evolutionary form34.METHODS OF SCIENTIFIC DEDUCTIONEarly scientists, particularly those in the naturalist field derived most of their conclusions from observed, unproven empirical facts. Without the means of logically explaining scientific theory, the hypothesis was incurred – an educated guess to be proven through experimentation. Darwin developed his theory of natural selection with a viable hypothesis, but predicted his results merely by observing that which was available. Following Lyell’s teaching, using modern observations to determine what occurred in the past, Darwin developed theories that “only made sense” – logical from the point of view of the human mind (meaning it was based on immediate human perception) but decidedly illogical from a purely scientific angle. By perusing the works of Malthus did Darwin finally hit upon his theory of natural selection -not actually questioning these conclusions because they fit so neatly into his own puzzle? Early development of logical, analytic scientific theory did not occur until the advent of philosopher Rene Descartes in the mid-17th century (“I think therefore I am”35). Natural selection was shown to be sadly lacking where it could not account for how characteristics were passed down to new generations36. However, it did present enough evidence for rational thought to be applied to his theory. Thus scientists were able to develop fairly accurate conclusions with very limited means of divination. Opposition from oppressive Judeo-Christian church allowed little room science. Regardless, natural selection became the basis for all present forms of evolutionary theory.37 LIMITS TO DARWIN’S THEORY Darwinism, while comparatively rational and well documented nevertheless upheld the usual problem that can be found in many logical scientific conclusions – namely deliberate ignorance of facts which might modify or completely alter years the conclusions of years of research. Many biologists were less than convinced with an evolutionary hypothesis that could not explain the mechanism of inheritance. It was postulated by others that offspring will tend to have a blend of their two parents characteristics, the parents having a blend of characteristics from their ancestors, the ancestors having a blend of characteristics from their predecessors – allotting the final offspring impure, diminished desirable characteristics38. Thus did they believe a dilution of desirable traits evolved even more diluted desirable traits – these traits now decidedly muted? It was more than two decades after Darwin’s death that Mendelian theory of the gene finally came to light at the turn of the century39. Because of this initial scepticism with Darwin’s natural selection, when Mendel’s work became widely available biologists emphasized the importance of mutation over selection in evolution. Early Mendelian geneticists believe that continuous variation (such features as body size) hardly factored in the formation of new species – perhaps nothing to do with genetic control. Inferences on the gradual divergence of populations diminished in wake of notions of significant mutations40. This gave rise to neo-Darwinian theory in the 1930’s, what is called “modern synthesis” which encompasses paleontology, biogeography, and systematics and, of course, genetics. Geneticists have noted that acquired characteristics cannot, indeed be inherited, while observing that continuous variation is inherited through the effects of many genes and have therefore concluded that continuously distributed characteristics are also influenced by natural selection and evolve through time. Modern synthesis, in other words, differs little from Darwinian Theory, but also incorporates current understanding of inheritance. Modern synthesis maintains that random mutations introduce variation into population, natural selection inaugurating new genes in greater proportions. Despite revolutionary progress the discovery of the gene has made, neo-Darwinian theory is still based on the arbitrary assumption that the primary factor causing adaptive change in populations is natural selection41. MORPHOLOGICAL & BIOLOGICAL CONCEPTS Species have been traditionally described based on their morphological characteristics. This has proven to be somewhat premature to say the least: some organisms in extremely different forms are quite similar in their genetic make-up. Male and females in many species develop more than a few many characteristic physical differences, yet are indeed the same species (imagine that!). Likewise some organisms appear to be quite morphologically similar but are completely incompatible. There are many species of budworm moths, all of which are highly indistinguishable – most of which do not interbreed42. The idea of species is usually called the biological species concept, stressing the importance of interbreeding among individuals in a population as a general description. An entire population might be thought of as a single unit o
