Biological and Cultural Factors in Human Evolution

October 24, 2017 | Author: balu426 | Category: Natural Selection, Evolution, Mutation, Genetics, Vertebral Column
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An analytical essay....


Biological and Cultural factors in human evolution: Homozinization Process: Homozinization process is the evolutionary transformation of hominoids into hominids. It is a process that occurred in the hominid line since the divergence of last common hominoid ancestor shared with any living ape. The term is applied to include all those aspects of structural and behavior changes that occurred in the hominid line finally leading to evolution of man. Erect Posture and Bipedal locomotion: The most obvious thing about the human beings that differentiate them from all other members of the animal kingdom is their upright posture and their associated habit of walking( technically known as bipedal locomotion). Many of the distinctively human morphological traits are directly attributable to these facts of human life. The shape of vertebral column in mammals delicately adjusted to support the animal’s center of gravity. Characteristics of human vertebral column that are specific to an upright posture include: enlarging of lower vertebrae to absorb the forces of compression, relative constancy in the size of the spines protruding from each vertebra, resulting from a lack of weight bearing stress points along the spine; increase in size and number of spines in sacrum to take up the transmission of weight through the pelvis and legs; and finally a sharp backward curving of the spine in the lumber region (lower back) providing a solid platform to transfer the weight of the body onto the pelvis and giving the human spinal column its distinctive S-shape. Even the skull shows adaptation to erect posture, with the shifting forward of the foramen magnum closer to the centre of base of the skull. Remodeling face and Teeth: When an infant is born, it passes out of the mother’s womb through the birth canal through the vagina and into the realities of social life. The birth canal is surrounded by skeletal bones that forms lower front of the mothers pelvis. This bone sets the limit on the size that a new born infant can be at the moment of its birth; if it (or any part of it ) is too large it will get stuck, with the result that both infant and the mother are quite likely to die – which means they will not contribute their genes to the ongoing evolutionary process. It was noted that a major trend of human evolutionary development was a dramatic increase in brain size since the Australopithecines. Since the size of the head as a whole should not keep getting larger beyond the limits allowed by the process of birth, there was a strong adaptive pressure to shorten the snout and reduce the size of the face to make room for cranial expansion in the course of human evolution. Specific modifications in face in the evolution of modern man includes: the brow ridges diminishes; the forehead approaches being vertical; the chin is prominent; the face is flat, with a nose that protrudes,. The foramen magnum is now exactly at the centre of the skull base, the head balancing nicely on the

vertebral column. The nasal cavity and palate are shortened and arched, the tongue thickened and shortened. Expansion and development of Brain: There is a dramatic increase of the brain case as we approach specimens of homo sapiens. In general the overall size of our ancestors increased at each evolutionary stage. But the brain grew larger, proportionately did the body. The most likely reason – of an increase in both the number and kinds of connections between brain cells. It is an increase in the kinds of connections between brain cells that is apparently responsible for the emergence of new kind s of mental operation such as thinking and using language, operations that are fundamental to human existence. Increase in size and complexity of the outside surface of the brian called Cerebral cortex. This expansion of the cortex is the most recent evolutionary development of the brain, and this is primarily associated with thinking and language use. Culture and Hominization Process: Each stage of hominid organic evolution seems to have been accompanied by the major advances in cultural evolution. Because stone tools are relatively indestructible, much of theearly cultural evolution is represented by the evolution of tool industries. In order for a tool to be useful, one must have it with him when the moment arises to put it to work. This takes making tool in advance- planning; walking on two legs to free ones hands to carry the tool; and commitment to using that tool. Those parts of human brain most needed for manipulating tools are much evolved. The hand itself marvelously evolved. It combines powerful curled- fingered grip with which heavy objects can be moved, with the delicate manipulations possible when small objects are held between the fingers and the thumb. One of the most important features of the culture is language, which also profoundly influenced and was influenced by human evolution. Language and Evolution: Many important aspects of man’s brain seem to have evolved as speech specializations; there are three areas of brain highly evolved in humans and appear to be crucial for human linguistic ability. They are Broca’s area Wernicke’s area and angular Gyrus. And it can be reasonably supposed that verbal communication was so adaptive for the man’s ancestors, that strong selective pressures molded these changes. Certainly, language is a principle cornerstone of human existence.

Theories of Organic Evolution: Lamarckism (Theory of Inheritance of acquired characteristics): The theory of inheritance of acquired characteristics states that modifications which the organisms acquires in adaptation to the environments which it meets during its life time are automatically handed down to its descendents and so become part of heredity. French naturalist in his publication “Philosophical Zoologique” explained his theory. Although his views on evolutionary mechanisms are outdated now, he was the first evolutionist to conclude that evolution is a general fact covering all forms of life. Lamarckian theory summarized in four propositions: 1. Living organisms and their component parts tend continually to increase in size. 2. Production of new organ in an organism result from the new need and from the new movement which this need starts and maintains. 3. If an organ is used constantly, it will tend to become highly developed, where as disuse results in degeneration. 4. Modifications produced by the above principles during the life time of an individual will be inherited by its offspring. To explain his theory Lamarck used examples like long neck of Giraffe, limblesness of snakes, webbed feet of duck etc. he visualized the evolution of giraffe as follows: an original deer like animal finding the supply of grass and herbs inadequate, started to feed on the leaves of trees. It need greater height to reach the higher leaves and in the process of reaching, its neck became longer and longer. In the course of generations the long neck became more accentuated and modern giraffe was the result.

Theory of Natural Selection: In the year 1859, Darwin published a book “The Origin of species by Natural Selection”. It contained overwhelming evidence of the fact that evolution has and does occur, and presented a logical theory regarding the major mechanism of evolution and natural selection. Darwin’s Evolutionary Propositions: The change in species by the survival of an organism type exhibiting a natural variation that gives it an adaptive advantage in an environment, thus leading to new environmental equilibrium, is evolution by natural selection. This, Natural selection is a continuous process of trial and error on a gigantic scale for all living matter is involved. The Universal Occurrence of Variation: Variation is the characteristic of every group of animals and plants and there are many ways in which organisms may differ. (Darwin and Wallace did not understand the cause of variation and assumed it was one of the innate properties of the living things) An excessive rate of Multiplication: Every species in the absence of the environmental check tends to increase in geometrical manner. For example, a single pair of English sparrows would be ancestors of over 275 billion individuals in 10 years if they could reproduce at their natural rate without any check. Thus, more organisms of each kind are born than can possibly obtain food and survive. Struggle for existence: since more individuals are born than those that can survive, there is an intre specific and inter specific or environmental struggle for survival- a competition for food, mates anad space. Survival of the Fittest: the consequent elimination of the unfit and survival of only those that is satisfactorily adapted. Some variations exhibited by living things make it easier for them to survive while others have handicaps that bring about the elimination of their possessors. This idea of “The survival of the Fittest” is the core of theory of natural selection. Inheritance of Variations: The surviving individuals will give to the next generation their characters and in this way, the successful variations are transmitted to the succeeding generations. The less fit will tend to be eliminated before they have reproduced.

Synthetic Theory of Evolution: Biologists working on models of evolutionary change in the early 1930’s came to realize that genetic and selective processes are not opposing themes but that a comprehensive explanation of organic evolution requires both. Small and new changes in the genetic material in fact the fuel for natural selection. The two major foundations of the biological sciences had thus been brought together in what Julian Haxley termed “The Modern Synthesis”. From the perspective of modern synthesis, evolution is a two stage process: 1. Production and redistribution of variation 2. Natural selection acts on variation. From the modern genetics perspective, Evolution is defined as “ The change in allele frequency from one generation to the next.” Factors that produce and Redistribute variations: Mutations: an actual alteration of genetic material is called Mutation. A mutation is a change in the base sequence of DNA. For such changes to have evolutionary significance they must occur in sex cells, which are passed between generations. If a mutation does not occurs in sex cells either the egg or the sperm, they will not be passed to the next generation and no evolutionary change can result. It would be rare to see evolution occurring by mutation alone. Mutation rates for any given trait are quite low, and thus, their effects would be rarely seen in small populations. However, when mutation is coupled with natural selection, evolutionary changes are quite possible. Mutation is the basic creative force in evolution in fact is the only way to produce” New” variations. Migration: Migration is movement of genes from one population to another. If all individuals in a population do not choose their mates from within the group, significant changes in the allele frequencies could occur. If a change in the allelic frequency does take place, evolution will have occurred, this time by migration. Migration works in both ways – in- migration and Out- migration. Genetic Drift: The random factor in evolution is called Genetic Drift, and is due primarily to sampling phenomenon. Since the evolution occurs in populations, it is directly tied not only to the nature of initial allele frequencies of population, but to the size of the group as well. Evolutionary change due to genetic drift is directly and inversely related to population size. To put simply, the smaller the population the larger the effect of the drift. Recombination: since in any sexually reproducing species both parents contribute genes to the offspring, the genetic information inevitably reshuffled in every generation. Such recombination doesnot itself change allele frequencies. However, it does produce a whole array of genetic combinations, which natural selection can then act upon. Infact the reshuffling of chromosomes during meosis can reproduce literally trillions of gene combinations, making every human being genetically unique. Recombination is the midpoint in the process of evolution.

Natural Selection Acts on Variation: The evolutionary facts – mutation, migration, genetic drift and recombination – interact to produce variation and distribute genes within population. Given that there is variation among individuals within a population some of these variations may influence reproductive success. If, as a result of genetic variation, some individuals contribute more offspring to succeeding generations, this is natural selection. In fact, natural selection is defined as “Differential net reproductive success”. A result of natural selection is a change in allele frequency relative to specific environmental factors. If environment changes the selection pressures change as well. Such a functional shift in allele frequencies is what we mean by adaptation.

Modes of Selection: Stabilizing selection When selective pressures select against the two extremes of a trait, the population experiences stabilizing selection. For example, plant height might be acted on by stabilizing selection. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease.

Directional selection: In directional selection, one extreme of the trait distribution experiences selection against it. The result is that the population's trait distribution shifts toward the other extreme. In the case of such selection, the mean of the population graph shifts. Using the familiar example of giraffe necks, there was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favor individuals with long necks.

Diversifying Selection: In Diversifying selection, selection pressures act against individuals in the middle of the trait distribution. The result is a bimodal, or two-peaked, curve in which the two extremes of the curve create their own smaller curves. For example, imagine a plant of extremely variable height that is pollinated by three different pollinators, one that was attracted to short plants, another that preferred plants of medium height and a third that visited only the tallest plants. If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against and the population would tend toward both short and tall, but not medium height plants. Such a population, in which multiple distinct forms or morphs exist is said to be polymorphic.

Isolation and Speciation: According to Mayr, the way new species are first produces involve some form of isolation. If a single species of some organism composed of several populations distributed over a whole geographical area. Gene Exchange (migration) will be limited if a geographic barrier, such as a river, ocean, or mountain range separates these populations. The extremely important form of isolation mechanism is Geographical isolation. If population (A) is separated from another population (B) of same species by a mountain range, individual animals of A will not be able to mate the animals from B. As time passes genetic differences will accumulate in both populations. If roup size is small we can predict that drift will cause genetic frequency changes in both population A and B. Drift – Random in nature Different environments – natural selection Finally, both populations may no longer interbreed fertilely. New species evolved. The pre requisite for speciation is Reproductive Isolation.

Basic Concepts and terms in evolutionary Biology: Convergence: Convergence refers to the development of similar characteristics or adaptations in animals that differ in direct ancestry. Convergence ordinarily applied to one or a few characteristics rather than overall makeup. Ex: The Humming bird and the humming moth, have converged in their flying habits as a result of their common search for nectar in flowers as a source of food. Parallelism: An evolutionary development similar to Convergence, but in related forms is called Parallelism. Parallelism implies a similarity in biological makeup of the ancestral forms, where as convergence doesn’t. If the common ancestor of two organisms is not very ancient, and if evolution in the descendant lines followed more or less the same course, the term parallelism is used. The term usually applies to two species of organisms that were similar in origin, and that remained similar as they evolved like having some of the same changes occurring in both of them even they have separated and evolved into two different species. The old world and new world monkeys provide an excellent example of parallelism between groups living today, since they appear to have evolved in parallel from a prosimian ancestor that lived 35 million years ago. The reason for parallelism and convergence are same. The organisms, in order to survive in similar environment, must develop similar biological structures. Parallelism, like convergence, is a matter of adaptation under the control of natural selection. Homology and Analogy: In evolutionary biology the term,    

Homology means similarity in Origin (implying a common ancestor) Homoplasy means similarity in appearance but not in origin Analogy means similarity in function but not in Origin. Serial homology is the similarity of structures between one part of an animal with another part of the same animal.

Adaptive Radiation: In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly into a multitude of new forms, particularly when a change in the environment makes new resources available and opens environmental niches. The evolutionary spread and differentiation of the descendents of one type of animal, of whatever level of classification, is Adaptive Radiation. Unlike the parallelism and convergence, which refer to the ways two species of organisms remain similar or become more similar, adaptive radiation refers to the way a particular species evolves into progressively dissimilar organisms. The descendents of a single species sometimes evolve to take advantage of many different environments and opportunities, and rapid changes in external environment may cause new forms of animals to develop from a single ancestral form. Dollo’s Law of irreversibility: Dollo's law of irreversibility is a hypothesis proposed by Louis Dollo which states that evolution is not reversible. This hypothesis was first stated by Dollo in this way: "An organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors." According to this hypothesis a structure or organ that has been lost or discarded through the process of evolution will not reappear in exactly the same form in that line of organisms. A change of the natural environment in exact step wise reverse order is exceedingly rare. Hence, an occasion for reversed natural selection involving a number of factors in a given order would be just as rare. Furthermore, if a previous allele has been entirely eliminated from the gene pool, a reversal of evolution would also require precisely the correct reverse mutations just as the environmental conditions also in reverse proper order. Mosaic Evolution – Evolution in parts and Wholes: According to this principle evolution of species tends to be inconstant and asymmetrical. That is, it may be rapid at one time and slow at another. In rare cases it may even virtually stop altogether. At one time, evolution may affect limbs and at another it may affect jaws. With a change in food supply or some other alterations in the environment, running or biting ability, for example, would become more important or less important in natural selection. This variability in tempo of evolution of different anatomical structures in the same line makes it unwise to draw sweeping conclusions concerning the relationship of two fossil forms on the basis of single characteristics. Instead, it is necessary to follow the evolution of whole functional systems. The evolutionary trends in different organ systems like loco motor system, Nervous system, Respiratory system, and so on, respond separately to the environmental conditions. Example: Hominid evolution: the early evolution of bipedalism in Australopithecines, and its modification of the pelvic girdle took place well before there was any significant change in the skull, or brain size.

Cope’s Rule: According to this principle, living organisms have a tendency to increase in their size during the course of organic evolution. This tendency has been found in vertebrates, invertebrates, and plants. To some extebt this principle is very similar to Lamarckian proposition that every organism has a tendency to continually increase in size. Though a majority of present species are probably the largest of their respective class, this principle is not universal and has exceptions in both plant and animal kingdoms. Principle of Competitive exclusion – Gause’s Rule: Principle of competitive exclusion is a proposition which states that two species competing for the same resources cannot coexist if other ecological factors are constant. When one species has even the slightest advantage or edge over another, then the one with the advantage will dominate in the long term. One of the two competitors will always overcome the other, leading to either the extinction of this competitor or an evolutionary or behavioral shift towards a different ecological niche. The principle has been paraphrased into the maxim "complete competitors cannot coexist".

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