Evolutionary Mechanisms Of Natural Selection example essay topic

Charles Darwin had two great themes in 'On the Origin of Species' which accounted for the similarities and adaptations characteristic of living organisms. To account for the adaptations of organisms and those innumerable features that equip them for survival and reproduction, Darwin (and Wallace) independently came up with the central theory of evolutionary process: natural selection. Natural selection gives insight in to why organisms are the way that they are. Adaptations are phenotypic variants that result in the highest fitness among a specified set of variants in a given environment. In reference to humans, there are many traits that have been selected and adapted for throughout their evolutionary history giving them the characteristics that they have today. In this paper I will discuss some parts of the human body, which have been found to be selected for by the evolutionary mechanisms of natural selection, adaptation and mutation.

Natural selection, adaptation, and mutations are three components of the evolutionary process, each one having either positive or negative effects on the other. What exactly is natural selection? There are many variations of the definition but most agree that it must include the following concepts: some attribute or trait must vary among biological entities, and there must be a consistent relationship, within a defined context, between the trait and one or more components of reproductive success, where 'reproductive success' includes both survival and the reproductive processes themselves (Futuyma, 1998). Charles Darwin's theory of natural selection states 'if variations useful to any organic being ever occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance, these will tend to produce offspring similarly characterized. ' Regardless of how it is defined, natural selection and its action can have tremendous effects on the members of a population. It is natural selection that causes adaptation, and these adaptations evolve to ensure the survival of a species.

Evolution, in a very broad sense, is defined as descent with modification and often with diversification (Futuyma, 1998). Darwin's idea of evolution was that it occurred by descent with modification, from common ancestors. Some mechanisms involved in evolution are mutation, recombination, gene flow, isolation, random genetic drift, and natural selection. These ideas together constitute our idea of the evolutionary theory.

When you factor in all of these elements of evolution you can view evolution as a 'progressive development through time from simple to gradually more complex and more diverse organisms' (Ayala, 1997). Darwin devoted one single sentence in 'The Origin of Species' to the controversial topic of human evolution: 'Light will be thrown on the origin of man and his history' (Futuyma, 1998). Although his evasion did not work, human evolution is still a controversial topic today. Since this area is so controversial there have been many studies conducted in the area of human evolution. These evolutionary studies have provided us with abundant, well-documented evidence on the relationship of humans to other species, on the steps by which some human characteristics evolved, and on genetic variation within and among human populations.

Some theorists believe that natural selection of random changes in nucleic acid sequences and genetic information are the primary agents of evolution. In a study done by J.L. Mountain and L.L. Cavalli-Sforza, it was shown that natural selection may have played a major role in the evolution of certain polymorphisms in humans. It was found that the frequencies of ancestral alleles of humans strengthen the conclusion that the earliest major separation of modern humans was between Africans and non-African. The differences among ancestral allele frequencies in human populations suggest that natural selection may have played a key role in the evolution of a subset of these polymorphisms. The average time since mutation of the alleles producing the current set of polymorphisms is estimated at 700,000 years.

Genetic recombination is another mechanism of evolution. A study done by Jamie Fitzgerald et al. discusses the genetic aspects of human evolution and natural selection. This study brings to light the idea that genetic exchange can occur between genes that are not located in close proximity to each other. This possibility should be taken in to account in comparative analyses of these kinds of genes. We can only speculate as to the mechanism by which the exchange occurs to be one of natural selection. Human retroviruses have been of major concern and discussion in the last decade with the eruption of the AIDS epidemic.

In a study done by Paul W. Ewald, he talks about mutation rates and the ways in which we can use natural selection as a tool to control the AIDS epidemic and prevent similar epidemics from arising in the future. Mutations always carry the potential for untuning finely tuned biochemical machinery. For this reason it is important for us to understand the evolution of high mutation rates. Because understanding the evolution of high mutation rates can help us identify slowly replicating, mutation-averse viruses that might evolve into more dangerous mutation prone viruses (Ewald, 1994). Certainly AIDS is a serious retrovirus and even though it may have existed in the human body for thousands of years, all it took was one random mutation for the virus to become lethal. the perception of other viral cells mutating, as AIDS is frightening.

The entire race could possibly develop high mortality rates and eventually become extinct if the virus were to keep on replicating itself. The morphological characteristics of humans have become highly sophisticated throughout our evolutionary history. In one study, there are two different views on how the evolution of the post cranial skeleton occurred in humans. The Neo-Darwinian perspective hold that evolution in the post cranial skeleton was largely mosaic and that temporal change in specific traits informs us about behavioral shifts of genetic evolution affecting isolated anatomical regions. The second view is the epi geneticist paradigm, which see change in specific post cranial traits as correlated responses to change in overall body form. By this view, integration of functional systems both constrains and directs evolution of various traits, and morphological contrasts informs us about overall change in body form related to change in such things as overall growth patterns, climatic adaptation, and technological dependency (Churchill, 1996).

Humans will never (at least in our lifetime) be immortal beings. Therefore we all can expect to die roughly 85 years after we are born. Natural selection can play a role in determining mortality. A recent study by Laurence D. Mueller and Michael R. Rose found that there is a slight hint to the action of natural selection on the mortality rate of humans. There has been an exponential increase in recent years in adult mortality, which can be explained in terms of the decline in the force of natural selection acting on age specific mortality. For example, middle aged individuals that are surviving better than expected, on the evolutionary theory can be explained as the beneficiaries of the action of natural selection at earlier ages.

This work provided an explanation for why populations should obey a law of mortality. The human diet many times consists of whatever happens to be available in the fastest amount of time. Sometimes the types of 'quick' foods can contain large amounts of ingredients that are not beneficial to the diet of humans. One of the main ingredients is fat. With the amount of fat consumed by the human body, , it will be interesting to see how the body adapts biochemically (if it does) to the amount of fat consumed in the human diet. Eventually this type of diet will either be selected for or selected against by the force of natural selection, which will select for those with the type of diet that will be most beneficial to reproductive success of the species.

Our primate ancestors had a diet very high in carbohydrates and what happened was that the brain and reproductive tissues evolved a specific requirement for glucose as a source of fuel. But the Ice Ages dominated the last two million years of human evolution brought about a low-carbohydrate, high protein diet. Certain metabolic adaptations were therefore necessary to accommodate the low glucose intake. Many indicate that the adaptive response to the low carbohydrate intake is insulin resistance (Miller, Colagiuri, 1994). A study done by J.C. Brand Miller and S. Colagirui proposes that insulin resistance offered a survival and reproductive advantage during the Ice Ages. This study also proposed that a carnivorous diet would have disadvantaged reproduction in insulin-sensitive individuals and positively selected for individuals with insulin resistance.

Another factor mentioned was the various environmental factors that contributed to the low carbohydrate intake such as the types of foods that were available. The forces of natural selection on the evolutionary processes in humans can have a broad range of effects on the characteristics of humans. There are no concrete answers to some of the questions proposed in this paper, but time will soon tell exactly how the evolutionary mechanisms have effected the human race. The human species will continue to evolve as long as the mechanisms of evolution and environmental factors act upon us.