Evolution

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 paralogous 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