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Evolution: As Theory and Fact
Evolution, Darwinism, Mendelian Inheritance, The Modern Evolutionary Synthesis, phylogeny, convergent evolution, genetic drift, Origin of Species, heredity, mutations, differential reproduction, and genetic recombination are all fascinating aspects of the questions of evolution. Charles Darwin, Sir Charles Lyell, Alfred Russel Wallace, Joseph Dalton Hooker, and many other notable scientists devoted their lives to these concepts. The theories and evidence of evolution and its processes surround us. Evolution contains lots of evidence and mechanisms, descriptions of processes, and individual opinions. To me, these very mechanisms provide a foundation for the explanations of life. Evolution is a theory that explains the origins and the diversity of life on Earth.
The evidence that evolution exists is overwhelming. Exhaustively, there are ten major biological studies that have strongly indicated the presence of a force (evolution) dictating the history of life and diversity. These ten contingencies include evidence from genetics, paleontology, comparative anatomy, geographical distribution, comparative physiology and biochemistry, complex iteration (from computer studies), interspecies fertility and modifications, antibiotic and pesticide resistance, and speciation. The evidence from genetics and heredity is the best and most basic evidence of evolution itself. Comparative sequence analysis analyzes the relationship between DNA sequences of different species, resulting in similar genetic sequences being realized between the supposedly “different” species. This confirms Darwin's original hypothesis of common descent from a common ancestor. Logically, if certain species share a common ancestor, then the posterity will have inherited certain gene sequences of the DNA common to the ancestor.
The strongest evidence of evolution is provided by phylogenetic reconstruction. These reconstructions, usually done with slowly-evolving protein sequences, have been used to reconstruct most of the history of modern organisms. Certain reconstructions have even assisted in the development(s) of the history of extinct organisms such as mammoth, Neanderthals, T. Rex, and many other species. The reconstructed phylogenies clarify the relationship(s) inferred through previous biochemical studies, resulting in reconstructions of genetic sequences from very few ancient proteins or recent extracted RNA sequence(s). Developmental biology additionally shows that common morphology is the outcome of cumulative genetic principles. For example, our familiar vertebrate body “plan” is controlled by a set of genes, a single source for a wide variety of animals.
Paleontology is the study of past life based on fossil records and their relations to different geological time periods. Fossils are rather rare, (resulting in the “patchy” appearance of the fossil record itself) simply because in order for an organism to be preserved, (considering they usually decompose rapidly or are eaten/slowly dismembered by scavengers) the remains must be quickly buried and “deprived” of oxygen in order to prevent decomposition. Then skeletal structures must either be covered with sediment, mud, sand, ice, resin, tar, or anaerobic (generally acidic) peat in order to either leave a hardened imprint, such as the famous Kentuckian leaves to the Paleolithic-era Homo habilis footprints, or a petrification (which occurs when the porous skeletal structures are filled with mineral-rich mud, eventually turning to stone or other hardened mineral). Because fossils do form, they are essential to understanding the complete evolutionary history of life on Earth.
The combination of our recorded fossils is known as the fossil record. Because of the fossil record, it is possible to find out how a particular group of organisms evolved by arranging its fossil records in a chronological order (This can be done because fossils are mainly found in sedimentary rock, which means that there are layers of silt and/or mud on top of each other resulting in “time layers” known as strata. Each layer contains a geological section from a certain segment of time. The lowest strata will contain the layer with the oldest fossils from the most ancient time segment, the most recent fossils will be found in the “higher” strata. By using these stratigraphic levels, it has been observed many times that the oldest segments contain fewer types of fossilized organisms and simpler body structures, the opposite also being true (younger fossils contain more and more complex structures). Without relying on the strata layers, a more accurate conclusion may be drawn (in terms of the age of the fossil(s)) when utilizing radiometric dating. This dating uses the known proportions of radioactive/stable elements in a given rock in order to find the half-life and therefore the age. Due to this form of dating, paleontologists have discovered many things including: many species appear at early stratigraphic levels and disappear at later levels (signifying extinction and therefore a change in an external factor such as continental drift, climate change, etc.), and that there are some species on our Earth that are “living fossils”, such as the horseshoe crab and the nautilus that have not changed for millions of years.
Additional evidence includes evidence from interspecies fertility and modifications. A good example of this is the polar bear. The polar bear can mate with a brown bear and produce fertile offspring, yet it has adapted to the Arctic that the brown bear could not survive in. The polar bear has a different diet, ears, mouth, teeth, stomach, fur, coat, neck, eyes, ears, feet, claws, endurance, etc. than the brown bear. Therefore, according to the theory of common descent, these two separate species must have diverged from a common ancestor. There is also evidence from speciation. An example of this is the hawthorn fly, or Rhagoletis pomonella, which at one time only fed on hawthorn trees, but now different populations feed on different things. Many North American populations now feed only on apples. This is an example of speciation and evolution in progress. The development and spread of antibiotic resistant bacteria, like the spread of pesticide resistant forms of plants and insects, is evidence for the evolution of species and change within a species. Also, DDT resistance in various forms of Anopheles mosquitoes also directs evidence of the existence of evolution where generations proceed quickly. Another form of evidence comes from studies of complex iteration. Computer science now allows mathematical analysis of complex genetic systems by predicting the probability of changes.
Other evidence includes evidence from geographical distribution. For example, while Africa, South America, and Australia mainly occupy the same latitudes, they have species that are completely unique such as Australia's platypus and echidna that exist nowhere else in the world, Africa's short-tailed monkeys, elephants, lions, and giraffes that do not exist in South America or Australia, and South America's long-tailed monkeys, cougars, jaguars, and llamas that do not natively exist elsewhere. However, identical plant specimens and dinosaur fossils have been found on each of these three continents. They could not have evolved separately but identically. Therefore, the continents have drifted since the Mesozoic period when the continents were connected, possibly by land bridges. When separated by continental drift, the isolated species diverged from each other, though even today some species resemble each other, supporting the theory of common descent. Additionally, there is evidence from comparative anatomy. For example, under similar environmental conditions, different structures in different populations of organisms may undergo adaption/mutations that serve similar functions, called convergent evolution. A good example of this is the eyes of vertebrates and cephalopod molluscs. Though different phylum, they all have the same feature. Vestigial structures and embryonic development also provide evidence for evolution, considering the whales' vestigial hind limbs and pelvic girdles and the ungulates extra toes that do not even reach the ground, they must have once had a purpose or they wouldn't have ever evolved them. Therefore, this is proof that whales evolved from land-dwellers while ungulates evolved from browsers and more rodent-like creatures. Additionally, since most embryonic structures develop very similarly to other organisms, they must have diverged from a common ancestor at some point. Additional comparative anatomy evidence includes the comparisons of pentadactyl limbs among tetrapods. When compared, it has been found that they all have a similar structure and placement pattern (Please see Figure 1.). Finally, when organisms are compared, the species that contain the most similar phenotypes are diverged from a common ancestor more recently than more exotically compared organisms.
The final evidence of evolution is from comparative physiology and biochemistry. This evidence is based on the fact that all known existent organisms are organized by: genetic information encoded as nucleic acid, transcribed into RNA, then translated into proteins by ribosomes. The “Genetic Code” (the translation key between DNA and amino acids) is the same for almost every organism, which concludes that DNA in bacteria still is code for the same amino acid in a giraffe cell, and ATP is the energy source for all organisms. This proves that in Pre-Cambrian time, all life originated from a common ancestor. Molecular variance patterns additionally provide templates for the examination of evolutionary changes within cytochrome c, DNA sequencing, additional proteins, etc.
The process of evolution occurs through a set of mechanisms. One such mechanism is heredity. Heredity is the basis of all evolutionary processes. Evolution occurs in organisms through changes in heritable traits-the particular characteristics of an organism. Heritable traits (such as eye color in humans) are continued between generations by DNA, a molecule which encodes genetic and protein information for the use of the cell. Remember that the direct phenotype (appearance of the organism) is determined by a mixture of the inherited genotype and the resulting interaction with the environment surrounding the organism (This means that certain external features or conditions, such as a suntan, will result only from an organism's interaction with the environment, not the combination of inherited traits. However, an organism's reaction with UV rays may differ as a genetic result.).
Variations, mutations, sexual recombination, and population genetics are also vital mechanisms of evolution. Variations and mutations within an organism involve the genotype and the cause-and-effect relationship results in the specific interaction with the environment, about 95% of the time. For example, the fact that Equus caballus have such a variety of coat and hair colors results from mutations or variations within the four-code bases within the DNA (and the enzymes that repair them or destroy them)...the genotype. The frequency of an allele within a set population changes, depending on the environment and either survives or disappears. Variations may also originate from exchange of genes between species.
Mutations portray the randomness of evolution. Mutations are changes in the DNA base sequences. They may be caused by radiation, mutagenic chemicals, transposons, and even viruses or errors in DNA replication or meiosis. These changes may result in no effect at all, variations in the products of the gene(s), or disable the gene from functioning at all. However, organisms have evolved DNA repair enzymes to repair mutations. Other factors that may lead to variations include sexual recombination and population genetics. Sexual organisms may have offspring that contain random mixtures of their chromosomes through independent assortment. This does not alter allele frequencies, but it does change which alleles are associated with each other. Population genetics is a generation-to-generation change in the frequency of alleles within a population because of the phenotypes and variations within a population gene pool varying because of certain interactions with the environment. For example, the white peppered moth and the black moth existed within population gene pools, yet the allele frequency of the white peppered moth declined due to external pollution and eventually recombination of the alleles. However, as stated by the Hardy-Weinberg principle, it is possible for no evolution to occur within a population when external forces are disregarded, known as the equilibrium of evolution.).
Natural selection is the process by which genetic mutations that enhance reproduction become, and remain, more common in successive generations of a population. This concept is commonly known as “survival of the fittest”, for it is based on three facts: heritable variations exist within populations of organisms, organisms then produce more offspring than can survive, and finally, these offspring vary in their ability to survive and reproduce. These conditions result in competition between organisms for survival and reproduction. As a result, organisms with favorable traits that give them an advantage over their competitors pass advantageous traits on, while unfavorable traits result in death before reproduction for certain organisms, or lack of production.
Two other more abstract mechanisms of evolution include genetic drift and gene flow. Genetic drift is a change in allele frequency (similar to sexual recombination) from one generation of a population to the next, resulting in the random probability that an organism will survive/reproduce. Genetic drift is based on the mathematical fact that alleles are subject to sampling error, resulting in fluctuations in allele frequencies. This process disappears when an allele either disappears or replaces “ancient” alleles entirely. Even with a lack of external forces, genetic drift (similar to speciation) may result in two originally divergent populations becoming separate populations and species.
Gene flow is occasionally used in order for genetic drift and natural selection to occur. Gene flow is the exchange of genes between populations, which are usually of the same species. Gene flow may also disturb the allele frequencies within a population and introduce more genes into the population's gene pool, resulting in increased genetic variations. Gene flow may occur due to migration or emigration, and this may lead to the slowing of the process of speciation between two diverging population by spreading genetic differences among the populations. Depending on how far two species have diverged, if may still be possible for them to produce offspring through hybridization (resulting in hybrids), such is the case with donkeys and horses producing mules that are sometimes to produce an intermediate species or an entirely new phenotype. Horizontal gene transfer may also occur through gene flow in smaller forms of life such as bacteria and archaea and occasionally viruses. This is a process where genes are exchanged through species not by means of the production of offspring. This process contributes to the spread of certain antibiotic resistance and the acquisition of chloroplasts and mitochondria.
My opinion of evolution is best explained through this quote: “ Today, many religious denominations accept that biological evolution has produced the diversity of living things over billions of years of Earth's history. Many have issued statements observing that evolution and the tenets of their faiths are compatible. Scientists and theologians have written eloquently about their awe and wonder at the history of the universe and of life on this planet, explaining that they see no conflict between their faith in God and the evidence for evolution. Religious denominations that do not accept the occurrence of evolution tend to be those that believe strictly in literal interpretations of religious texts.”
-Science, Evolution, and Creationism, National Academy of Sciences.
Simply stated, I am educated and observant enough of our modern times to realize/believe that evolution is beyond a theory, moreover, a fact of biological laws. Yet I am neutral on the subject [of evolution] when it may come to public debate or discussion, however light-hearted it was originally intended to be. Rather, I have found that not participating in such discussions at all seems to be a worthwhile choice. From observation, I have determined that the touchy subject of evolution is rather unique in the fact that anyone undecided or ignorant of the subject in any aspect tends to be either forced or verbally yanked between two loyal patriots, usually resulting in the poor victim being subject to brutal interrogation of the two defendants (both of whom have now mutually agreed on at least one thing: you must be persuaded or be subject to verbal torture).
To settle my own internal dispute, I have educated myself on the theory now and have come to a conclusion: the theory itself is correct, while the variables of the “external” factors are yet to be determined (such as a supreme being, the uncontrollable environment, etc.). Why and how did I come to this consensus? I have had the opportunity to analyze a sequence of the human genome and a sequence of a primate's genome. The two chosen sections were remarkably similar (I recognized this only after a certain self-described professional described the electronic image and demystified the purpose of scattered rectangular-like objects across the screen). Due to the theory of common descent and the comparative sequencing analysis capabilities, the “truth” was undeniable: we are descended from common ancestors. My trust in the fact that the evolutionary theory is correct is stable.
The general assumption by the average citizen is that evolution is a fact or fiction, a theory or tale. Many Darwinists or casual conversationalists seem to assume that the theory of evolution has answered all of our abstruse questions of how life began, and perhaps how Homo sapiens arrived at this moment in time. Despite this popular myth, the theory itself does not explain our universe entirely. However, the evolutionary theory has also provided the foundations for many scientific discoveries. Even now, amidst the first decade of the twenty-first century, modern biology and life science studies are based on the ideas/concepts of evolution (the theory), the modern synthesis, and other aspects of heredity, genetics, and selection. Evolution, for now, is still ostensibly a theory, which is constantly progressing, developing modern foundations, and providing a unifying explanation for the diversity of life on Earth.