BIO254:NaturalSelection

Definition
Natural Selection is the process through which individuals with beneficial traits are more likely to survive and reproduce. In the next generation, those traits, if they have heritable components, will be more common. Through generations of accumulation, the traits will spread in the population, which can result in speciation and adaptation. On the other hand, individuals with injurious variations will have less chances to survive or reproduce, and their traits will be eliminated from the population.

The concept of “Natural Selection” was first introduced by Charles Darwin in his 1859 book “The Origin of Species by Means of Natural Selection”, by saying “…individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind. On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection." Natural Selection provides one of the most important mechanisms by which evolution occurs.

Genetic Basis of Natural Selection
After genetics and population biology were incorporated into the studies of evolution, people get a better understanding of natural selection. The existence of genes was first suggested by Gregor Mendel, in the 1860s. In 1910s, T. H. Morgan introduced the chromosome theory of inheritance, R. A. Fisher showed how continuous variation could be the result of the action of many discrete loci. Morgan's student Theodosius Dobzhansky was the first to apply Morgan's chromosome theory and the mathematics of population genetics to natural populations of organism. Their works, as well as contributions from many other scientists shed light on the genetic basis of natural selection.

Traits of an organism are coded by their genes. But for each gene, there might be some variations, or so-called different alleles. The genetic variation arises from random mutation and recombination, and provides the sources for natural selection. Populations evolve by changes in the relative allele frequency brought about by random genetic drift, gene flow, and especially natural selection. Different alleles may give rise to different traits, or phenotypes. Individuals with the phenotypes favorable for surviving or reproduce have more chances to pass the alleles to the offspring, so that the allele frequency will increase in the population. The favorable genetic variants may have individually slight phenotypic effects and phenotypic changes are gradual. Given enough time, the gradual evolution could give rise to changes of great magnitude, such as reproductive isolation, and result in speciation or designation of higher taxonomic levels.

As an example of genetic natural selection and its relation to evolution, a group from University of California at San Diego demonstrated that even a few mutations in a given gene can create great phenotypic differences that would define a seperate taxonomic line. They investigated the cause of the major divergence that occurred between crustacean-like arthropods and insects 400 mission years ago, where crustaceans had multiple limbs while insects showed a hexapod body structure. They showed that by changing three amino acids at the end of the ultrabithorax and abdominal Hox proteins that are highly expressed in the abdomen of crustaceans, they could remove the inhibitory signal for a limb-repression gene. The removal of this limb-repression gene's inhibitory signal in the Artemia crustacean produced a mutant that represses embryonic limbs and showed a phenotype similar to the drosophila fruit fly. Such an experiment was the first evidence that showed that minor alternations in genes could bring rise to major morphological differences seen with evolution.

Natural Selection vs Artificial Selection
In the book "The Origin of Species", Darwin coined the term natural selection in analog to artificial selection, the process by which a farmer selects his breeding stock. But in fact, natural selection and artificial seletion are quite different processes. It would be beneficial here to make some comparisons between them.

Artificial selection often encourages the breeding of individuals possessing "desirable" characteristics over others, from a human perspective, either intentionally or unintentionally. The choice to encourage or decourage certain characteristics are usually clearly directed. The species formed under artificial selection do not necessarily have the fitness under natural conditions. Besides, people can only choose to change the frequency of alleles that have observable phenotypes.

On the other hand, as Francois Jacob pointed,“… natural selection does not work as an engineer works. It works like a tinkerer – a tinkerer who does not know exactly what he is going to produce but uses whatever he finds around him whether it be pieces of string, fragments of wood, or old cardboard… Evolution does not produce novelties from scratch.”.

It should be noticed that the underlying genetic basis for both artificial selection and natural selection are the same, and that the concept of artificial selection was first introduced as an illustration of the wider process of natural selection.

Selection Condition
Concerning the selection forces, natural selection can generally be divided into two classes, ecological selection and sexual seletion.

Ecological selection (or environmental selection) refers to the ecological processes that operate on inherited traits without reference to mating or secondary sex characteristics. Examples of ecological selection are climate and geographical changes, competitions for limiting natural resources, interactions among individuals of the same species (including relatives (e.g. kin selection) and conspecifics(e.g. competition, infanticide)), etc.

Sexual selection includes mechanisms such as mate choice and male-male competition. Sexual selection can happen both intersexually and intrasexually. Within a species, when one sex (typically females) acts as a limiting resource for the other, compititions (typically between males)will occurs over the limiting sex, and this results in sexual selection. In the case that females choose males, usually the most vigorous and best adapted males will have the greatest number of offspring, and therefore the alleles coding these favorable traits are more likely to be pass down to the progeny. Intrasexual selection is often associated with sexual dimorphism, including differences in body size between males and females of a species.

Natural selection happens at every life stage of an individual. Typical examples are shown in Figure 1. Selection at each of these stages can affect individuals' survivability and reproductive capcity.



Fig 1. Natural selection occurs at different life stages of an individual.

Antibiotic Resistance: Natural Selection in Hyperdrive
Antibiotic resistance is a biological phenomenon that vividly illustrates the concept of natural selection and is a medical issue that is important and timely. For years, antibiotics have been used for the treatment of infectious diseases by killing primarily bacterial pathogens. Each of these antibiotic drugs place survival hindrances for bacteria. Since bacteria multiply at a rapid rate, many generations of organisms are exposed to the antibiotic and each of these generations have slight mutations from the previous generation. Thus, this scenario naturally selects only the organisms that posses a mutation that thwarts the action of the drugs to survive. These traits are then passed to the next generation of bacterial organisms and will thus have resistance to the antibiotic.

Initially, penecillan was the drug of choice against bacteria that possesed peptidoglycan cell walls. The drug would bind to a cell wall protein, the penecillin binding protein, and prevent cell wall cross linking and thus prevent bacteria from making the cell wall. The organisms would thus die. After long-term exposure to penecillin, some generations of bacteria were able to generate enzymes that broke down penecillin - penecillinases. After several generations of antibiotics were used that resisted the penecillinase of particular bacteria, pharmaceutical drug design created a new drug for gram positive bacteria, vancomycin, which binds to the D-ala D-ala component of the cell wall and prevented cross linking rather than binding to the penecillin binding protein component of the cell wall. However, recently some gram positive bacteria which had mutations that changed the D-ala D-ala of the cell wall with a one oxygen atom substitution in the hydrocarbon ring within the cell wall. This one mutation resisted vancomycin's attachment to the cell wall and thus conferred another antibiotic resistance. Similar natural selection stories between bacterial pathogens and modern antibiotics have been seen with the latest antibiotics including linezolid (a very specific gram postive antiobiotic used against vancomycin resistant organisms). The endless saga of antibiotic resistance shows natural selection at work and has been the cause of much alarm in the field of infectious disease management in medicine.

Pre-Darwinian Theories
Until the early 19th century, through studying the fossil records, people began to recognize that organisms that lived in the distant past were often quite different from those that lived today. People tried to explain the dramatic changes and became aware that species might emerge by modification from ancestor species. Jean-Baptiste Lamarck is one of the important radical evolutionists at that time. He proposed the theory of "inheritance of acquired characters", that changes in physiology (adaptations) acquired by individuals during the life time might be inherited by their progeny, causing, in enough time, transmutation of species.

Darwin's Theory
The Darwin's Theory of natural selection is considered as a cornerstone in modern biology. Inspired by the observations during the trip on the Voyage of the Beagle, and by the economic theories of Thomas Malthus, Darwin conceived his theory of evolution by natural selection as an explanation for adaptation and speciation between 1842 and 1844, and put them down formally in his famous 1859 book "The Origin of Species".

It should be noticed that in 1858, Alfred Russel Wallace, a young naturalist, independently proposed the principle and described it in a letter to Darwin. Two short papers by the two were read at the Linnean Society announcing co-discovery of the principle. The following year, Darwin published the book "The Origin of Species".

Neo-Darwinism
Neo-Darwinism, or often refered as modern evolutionary synthesis, was initially established in the 1930s and 1940s, and led to a gene-centric view of evolution by the work of W. D. Hamilton, George C. Williams, John Maynard Smith and others in the 1960s. Neo-Darwinism integrates Charles Darwin's theory of natural selection, with genetics as the basis for biological inheritance, random genetic mutation as the source of variation, and mathematical population genetics.

Natural Selection of Behavior
It has been theorized that more than just genes are capable of being passed down from one generation to the next. Of course there is a direct link between genetics and some types of behavior. However, there are some behaviors that, independent of genes, are capable of undergoing natural selection. This process occurs in much the same way as its genetic counterpart. There must be a variation in behavior, with some form being more advantageouos than others, and there must be a struggle for survival. Here we may represent the struggle for survival in many ways. It may very well be linked to life and death. For example, if the the behavior in question was how one acquires food or fends off predators. The behavior may also be indirectly linked to survival as well (i.e. more socially apt individuals may be able to acquire more resources for their offspring which increases their rate of survival). This concept may also be expressed in another manner. Think of behavior that have become common to some specific societies, a social norm. Those who align themselves with these norms tend to "fit in", however, those who go against them stick out in the crowd and often are socially excluded. Thus there is a survival of the fittest where social skills are concerned.