What Are the Causes of Natural Selection?

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The forces of evolution are too important to ignore. It is plain and simple to see that all life on earth and indeed in the universe is controlled by the forces of evolution. We are all affected by the laws of natural selection acting on the population of life. There is a wide range of evidence to support both the gradual and the rapid evolution of life. Evidence for evolution has been found in fossils, molecular biology, and archaeology. The forces of evolution include Natural Selection, Survival of the fittest, and gene pool mutation.

There is much evidence of natural selection acting on genetic variation within species to produce a highly diversified range of complex diseases. Evidence for the role of genes in disease has been found in all taxonomic groups, and these include plants, animals, and humans. The observation that some genetic diseases are highly common in human populations and occur without specific genetic mechanisms supporting their occurrence is referred to as ‘genetic epidemiology’. This evidence for the role of natural selection in causing complex diseases and their subsequent spread across the human genome supports the hypothesis of evolution of the genome.

Evidence for gene-of-deficiency-iciency is accumulating. It is possible to analyze a gene for genetic drift if it is outside the expected sequence, or if the sequence can be repeated without changing the genetic junction. When sequences are repetitive, it can mean two things: either the sequence has evolved because it is no longer functional (for example, amino acid coding starts to a different arrangement from what it is supposed to be) or that the sequence is non-functional, but the population size is small and so it is favored by natural selection. The second case occurs when the sequence has become functional but the population is small; the sequence may be non-recessive and therefore, may have arisen through gene duplication or gene flow.

One example is the analysis of sugar cane, which has high levels of saturating glycogen. Glycogen is easily maintained by positive selection and apparently did not accumulate genetic drift over thousands of years. In contrast, wild sugar cane appears to have undergone positive selection since its domestication. The reason for this is still unknown. However, the observations on diabetes and obesity strongly imply that there is an evolutionary link between obesity and diabetes.

Positive Selection and Population Size Genes are important in understanding how Pleistocene populations colonized the earth, leading to the rise of agriculture. The key factor driving population size increases is selection with time. If all people are closely related, then the founder effect applies, and each generation the genetic variation leading to that change is fixed. So, if the genetic variants leading to obesity are common in human populations, then the founder effect also applies to obesity.

Positive Selection Genes often lead to positive selective pressure but sometimes lead to negative selection, also known as drift. A major example is seen in the work of Richard Lenski, who showed that humans diverged significantly from their ancestral lineages about four hundred thousand years ago, even though they are genetically very similar. After that divergence, however, the variation leading to obesity appears to have been very slight, thus, the selective pressure on the human body was minimal. It did, however, lead to a slight increase in the genetic variation leading to obesity. This observation, combined with the results from the Lenski study, led critics to conclude that positive selection had passed through the generations, resulting in a slight enrichment of the human gene pool.

The critics went on to postulate that these results demonstrate the principle of chance, rather than of evolution, and that it is more realistic to expect a random number generator to produce a completely random result, rather than the previously mentioned enrichment of the genetic variety seen in the case of the founder effects. However, recent studies have cast doubt on the claim. These studies indicate that although chance may play a role, the founder effect is actually an occurrence caused by natural genetic drift, which occurs with time and with increasing frequency. Furthermore, molecular clock studies have indicated that the drift does not occur randomly, but is instead thought to be caused by the weakening of the genetic code over time. Therefore, critics argue, the results of the founder effect can be explained by a combination of natural and non-natural forces of evolution.

Although the critics have provided several different explanations for the origin of genetic variation, the most accepted explanation is the differentiation of genealogical history into smaller genealogies that, over time, became separated. The more distant branches of a family became isolated, the more genetic differences between them became. This process has been shown to occur both between human populations and within different species of animals. When this process began, it probably began with small islands within the Polynesian Archipelago, which eventually separated into separate Melanesian and East Asian populations. Over the course of history and along the way, genetic differences among human populations and between animal species have increased significantly.

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