Anthropology: Natural and Artificial Selection

Anthropology Natural and artificial selection (Question 1) Natural selection is the evolutionary process through which organisms best suited in the environment naturally compete, survive, and pass their genes to the subsequent generation, whereas those that are not well adapted in the environment become eliminated due to inability to compete. The environment is composed of biological and physical forces that influence the survival of living organisms. Biological factors include diseases, competition, and predatation among others while physical factors include famine, floods, fire outbreaks, and others. Natural selection is of different forms, including stabilizing, directional and disruptive selection.

Stabilizing selection occurs when the selection favors characters that are considered average.

Directional selection is due to forces that favor one extreme of the trait, while disruptive selection occurs when the average trait is chosen while both extreme traits are not affected.

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According to Darwin’s theory of natural selection, the many species in existence were not made separately but have developed from primitive forms. Organisms vary from one another and they reproduce at a geometric rate hence not all organisms live to maturity. Organisms less adapted to the environment become extinct as they die before reproducing. This is because nature selects characteristics that favor survival and reproduction. To survive for long enough to reproduce, individual organisms must find resources available in their environment and utilize them appropriately, at the same time avoiding limiting factors that tend to affect them.

Organisms with the ability to respond to these environmental influences become the most successful hence transmitting their genes to the next generation (Cumbay & Cumbay, 2011).

Example of natural selection is in the variants of peppered moth. Before industrial revolution, the peppered moth existed in two color variants, light grey and dark. The predominant color type was light gray while the dark colored moths were very rare. The population of dark colored moths was low because they were easily visible against grey background, hence were easy prey to their predators. The light grey moths blended well with their environment which was covered with lichens. During the industrialization period, soot from industries made the environment dirt and killed the light grey lichens exposing the dark tree barks. The change in environment made the light grey moths become increasingly visible to their predators and conferred a survival advantage to the dark colored moths as the predators could not see them. Consequently, the population of the dark colored moths increased as they were able to live long and reproduce hence they became more prominent around cities during this period.

With advocacy for cleaner environments, there is reduced air pollution leading to proliferation of lichens and as a result, the light colored moths have increased (Radick, 2003).

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Artificial section is as a result of interference of man through domestication and other activities that favor certain traits. Man chooses breeds of organisms with certain traits such as body size or color. The offspring of subsequent generation differ though the variations may not be detectable. The favored traits are selected for further breeding and with time, the small differences accumulate causing a visible change in the organism (Cumbay & Cumbay, 2011).

An example of artificial selection is in dog breeding whereby favoring certain traits such as appearance and temperament, a great variety of dogs are available in contrast to the original ancestral forms of Canis domesticus.

Comparisons between Natural and Artificial Selection Both natural and artificial forms of selection cause changes in living things over time.

However, in natural selection, the environment is the driving force that causes change while in artificial selection, humankind acts as the agent of evolution.

Human Evolution (Question 2) According to the human evolution theory, the present day man originated from primate ancestors. The theory involves the patterns of evolution and the process of evolution. Man and ape trace a common apelike ancestor that existed millions of years ago. Through the interplay of environmental and genetic factors, the human being emerged as a species while the present day apes followed a different evolutionary pathway. Thus through gradual genetic changes, man evolved from primitive ancestors. The human evolution theory is based on Darwin’s theory of evolution which emphasizes on descent with modification, where living organisms developed from non-living organisms. Complex creatures developed from simple ancestors as a result of random genetic mutations that occur and those mutations which are advantageous are preserved

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and enhance the survival of the organism. With time, the beneficial mutations accumulate and cause development of a very different organism (Moore, Vodopich & Cotner, 2011).

Evidences of evolution of man Paleo-anthropologists evidence supporting the evolution of man from earlier forms includes anatomical evidence (homology), fossil (comparative anatomy) and molecular evidence.

Homology Homology theory derives its evidence based on anatomical or genetic similarities between different species of organisms. The assumption in this argument is that resemblance equals a degree of relationship, and therefore if two species have some resemblance to each other, then they have a close evolutionary relationship. Anatomical evidence for human evolution is thus based on the close similarities that human beings have with the African apes.

Man bears close resemblance to the apes except for unique characteristics such as proportion of arms and legs, body hair, skin pigmentation, opposable thumbs, and the size of central nervous system (Rechtman, 2004). Man and apes have close similarities in the structure of their skeletons, and this resemblance correlates to the homologous genetic sequences within each species.

Man, bat and penguin are thought to have evolved from a common ancestry because of the anatomical structure of their forelimbs. The skeletal structure of their forelimb is common, having the humerus, radius, ulna and carpals. They also develop from the same structures in the embryo.

Comparative anatomy Fossil records are used to deduce evolutionary relationship between different species.

Fossil evidence links man to primates because of the many fossils that have been excavated

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having human and animal characteristics. Fossil records linking human being to Australopithecus africanus suggest a common ancestry. Fossil records have depicted gradual strengthening of the spine, expansion of brain capacity, modification of facial features, and decreased mastication muscles showing a strong relationship to higher primates (Rechtman, 2004).

Molecular Evidence Molecular evidence attributes a common ancestry between man and apes due to identical molecules such as hemoglobin that are shared between man and apes. The shared hereditary material between man and other organisms, common genetic code, and similar expression of proteins shows a common evolutionary history (Rechtman, 2004). Cytochrome c, which is needed for aerobic respiration is found in all aerobic organism of different species, indicating that they descended from the same ancestor.

Trends of Hominid Evolution (Question 5) The major trends of hominid evolution are bipedalism, increase in brain size and complexity (encephalization), use of tools and caring capacity, and dentition.

Encephalization of the Brain This relates to increase in brain size with time and the size of the brain as compared to the body mass. In hominins, the ratio of the brain size to body mass remains largely unchanged.

While gradual change, expansion of brain size was noticed in the evolutionary lineage; it is in the Homo where significant change occurred. During encephalization, great rewiring of the brain took place while the use of materials such as stone tools developed. After this period of cultural advances, the hominins left Africa. The brains of early hominins and that of non-human primates bear a resemblance since both are symmetrical. The brain became lateralized when Homo emerged hence becoming asymmetrical (Wilson, 1992). By studying endocasts, Paleo-

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anthropologists have been able to establish that three regions of the brain began to change in the Homo. These regions are the cerebellum, the limbic system, and the cerebral cortex. The cerebellum controls learned motor activities, while the limbic system is concerned with processes such as motivation, emotion, and social communication. The cerebral cortex controls sensory activities. These modifications in the primitive ancestors may have contributed to cultural adaptations to the environmental forces.

The Homo population developed large brains due to possible interplay of different variables. These variables included group size, complex subsistence patterns, and the nutritional content obtained from meat. Development of big brains helped the early individuals to monitor things such as dominance hierarchies, friendship, and others since brain size and size of social groups show a positive trend in the primates. The big brain was also necessary in monitoring of large subsistence territories while increased consumption of meat helped in the provision of metabolic energy required for maintenance of brain functions (Kent, 2000).

Bipedalism One of the major uniqueness of humans from other primates is the bipedal movement.

The earliest known primates who were bipedal lived more than 3 million years ago. These species inhabited forests before coming into open grasslands. Waking on two feet affects body parts such as the rib cage, pelvis, neck and limbs. The process of bipedal evolution involved modification of these body parts. It is through evolution that the upright bipedal movement has developed as a habitual form of transportation. The earliest hominid fossils show a significant pattern of bipedalism. This shows that bipedalism could be a hominid adaptation. Humans have bipedalism involving a striding gait with fluid flow sequences of action including the swing phase and the stance phase. The anatomy of the lower primates is an adaptation of tree climbing

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and terrestrial while human anatomy is purely terrestrial. The human posture therefore relates to the ancient primate evolutionary history. Humans have evolved through vertical clinging and leaping to quadrupedalism, brachiation, to the current erect body. The best scientific explanation for bipedalism is that humans evolved because of the change in the distribution of food resources. Because of the scarcity of these resources, exploitation of the environment led to development of an efficient means of movement, hence bipedalism (Wilson, 1992).

Dentition Hominid dentition is correlated to their feeding habits and the innovation and use of tools. The reduction in size of the anterior teeth of Homo is related to development of tools, which that performed the function initially done by the incisors. The anterior teeth therefore became neutral as selection pressure for large incisors was decreased by use of tools (Wilson, 1992).

Interaction of Trends of Evolution Bipedal movement resulted into many anatomical changes such as skeletal structure, limb, pelvis, and cranium. Thus, the ability to walk upright led to other evolutionary trends like innovation of tools and enlargement of the brain (Wilson, 1992).

Forces of Evolution (Question 7) The four forces of evolution are mutation, natural selection, gene flow and genetic drift.

Natural selection In natural selection, individuals best adapted to the environment have increased potential of surviving and reproducing. Those individuals with desirable traits sire more offspring as compared to individuals not well suited. Natural selection is based on the principle of survival of the fittest; hence organisms best suited to their environment are likely to survive (Vinicius,

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2010). Natural selection acts through individuals and determines which individuals are best suited to reproduce successively. Hence, it influences the genetic characteristics of organisms with time as it regulates the gene frequency in a population by determining individuals fit to reproduce.

Mutations Mutations are random and can be beneficial, neutral, or harmful to an organism.

Mutations that are passed to subsequent generation result into evolution, therefore mutations on body cells do not influence evolution. Mutations are changes in the genetic sequences. Mutations affect offspring if they occur on germ cells and if they have an influence on the DNA. DNA is made up of nucleic acids as the building blocks. For evolution to occur, mutations must involve germ cells. DNA changes in the cells that produce the next generation is the basis of evolutionary theory. Through mutations, new genes are continuously added to the gene pool.

Evolutionary changes arise from cumulative effect of many mutations having small effects, conferring an organism effect, which may be beneficial, harmful, or neutral (Vinicius, 2010).

Genetic Drift Genetic drift refers to the drastic alteration on the frequency of the alleles within the gene pool of a population. Genetic drift can cause variation of the genetic makeup of organisms in a population. When it occurs, genetic characteristics can be lost from individuals, or it can cause certain traits be widespread across the population even when the traits have no survival or reproductive advantage to the organisms. Since genetic drift occurs randomly, it only occurs in small populations that are isolated and thus the genetic makeup of these populations is small to be affected greatly. For large populations alleles exist in many individuals and are transmitted to subsequent generations unless it has undesirable effects.

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Genetic drift occurs because of a small random change in the gene pool of a population sample. The change can be due to disasters, which kill a large portion of the population, drastically decreasing the size of the population, consequently causing genetic variation. For each generation, some organisms by chance can leave behind more offspring than other individuals can. The genes of the next generation will be drawn from the lucky individual who may not necessarily be the healthier one. This thus results into genetic drift. Sometimes genetic drift can cause populations to lose genetic variation. With less genetic variation, natural selection cannot work efficiently (Vinicius, 2010).

Gene Flow Gene flow is as a result of migration of individuals causing genetic variation. Gene flow can lead to evolution as genes are transferred between different populations. The loss or addition of individuals alters the gene pool frequencies even in the absence of evolutionary mechanisms.

Gene flow can introduce genes to a population causing greater genetic variation within a population, or cause genetically distant populations have resemblance (Vinicius, 2010).

Sometimes gene flow occurs without migration as in travelers.

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References Cumbay, M. & Cumbay, T., (2011). Biology essentials for dummies. Indianapolis, IN: John Wiley & Sons.

Kent, M., (2000). Advanced biology. Oxford: Oxford University.

Moore, R. C., Vodopich, S. D. & Cotner, H. S., (2011). Arguing for evolution: An encyclopedia for understanding science. Santa Barbara, CA: ABC-CLIO. Radick, G., (2003). The Cambridge companion to Darwin. Cambridge: Cambridge University Press.

Rechtman, M., (2004). ClifStudySolver biology. Hoboken, NJ: John Wiley & Sons.

Vinicius, L., (2010). Modular evolutionary: How natural selection produces biological complexity. Cambridge: Cambridge University Press.

Wilson, O. E., (1992). The diversity of life. Harvard: Harvard University Press.