Natural Selection and Evolution Help (page 2)

Natural Selection

Genetic diversity in a population of eukaryotic organisms is mainly generated by three mechanisms: mutation, independent assortment of alleles, and recombination. Relative to a given environment, some of these genetic variants will be better able to survive and reproduce than other variants. The genotype of an individual contributes to its various phenotypes, including the overall fitness of the individual. Fitness is the ability of an individual to survive and reproduce (i.e., pass its genetic information to the next generation). Fitness is generally measured relative to others in a population. Natural selection is a type of evolutionary force where the processes of nature (such as food type and availability, competition for resources, environmental changes, disease) act on the variation within gene combinations as presented in the phenotype. Selective forces may significantly affect the fitness of individuals within a population. Thus, certain gene combinations are favorable (adaptive) and tend to be perpetuated while less adaptive ones tend to be eliminated from the population. Evolution is the process whereby natural selection acts on the genetic diversity of a population in this manner.

Molecular Evolution

Evolution at the molecular level is driven primarily by mutation and selection. Mutations and recombination provide the genetic variation from which natural selection "selects" or favors transmission of the most adaptive gene combinations. Mutations can fall into three distinct classes based on their effects on the phenotype: (1) detrimental, (2) neutral, or (3) beneficial. Most detrimental mutations will tend to be eliminated in a population due to selection against individuals harboring this allele. Some deleterious mutations may survive in a population if they have a beneficial function in a heterozygous state. For example, in humans the sickle-cell hemoglobin allele produces a defective hemoglobin protein; it is codominant with the wild-type hemoglobin allele. When homozygous, this mutant allele causes sickle-cell disease, and, often, early death (decreased fitness). However, individuals heterozygous for this mutation are resistant to infection by the malarial parasite, Plasmodium. This advantage tends to perpetuate the detrimental allele in a gene pool where malaria is endemic. Neutral mutations, those that have neither advantageous nor nonadaptive phenotypic effects, are generally not affected by natural selection and either survive or do not survive in populations as a result of genetic drift. Neutral mutations are those that alter an amino acid codon in such away as to not change the amino acid coded for (e.g., GCU changed in the third position to GCC; both codons specify alanine) or in such a way that a functionally equivalent amino acid is substituted (e.g., one basic amino acid is substituted for another). Beneficial mutations or alleles generally survive due to the increased fitness afforded to individuals harboring these alleles. Ultimately, these alleles tend to become fixed in the population. During fixation, one allele replaces another (less fit) allele. Most mutations are deleterious and therefore tend o be lost due to selection. Therefore, most genetic diversity in a population is likely to be neutral or of little fitness consequence.

By comparing DNA or protein sequences from different individuals, evolutionary changes at the molecular level can be observed. In general, the more differences that two sequences contain, the more distantly related across evolutionary time they are. For example, one would expect the sequence of a gene in a bacterium to be very different from a homologous gene in a human. More specific terms describing homologous gene relationships are paralog and ortholog. A paralog (or paralogous gene) is a homologous gene within a species, while an ortholog (or orthologous gene) refers to a homologous gene sequence across species. Paralogs arise by gene duplication and differential mutations, and may have very different biochemical functions whereas orthologs arise from a common ancestor during speciation and generally have similar biochemical functions. From these types of sequence comparisons, phylogenetic trees (Fig. 9-1) can be built. These trees show the relatedness among different groups of organisms (commonly different species) based upon similarities in DNA or amino acid sequences of a gene.

Ribosomal genes are highly conserved and are thus commonly used as genes for observing evolution over long periods of time. Other genes, such as cytochrome genes or fibrinoprotein genes, are less conserved and can be used to observe evolutionary change over shorter periods of time. Noncoding regions, such as introns, are often used for short-scale ecological time periods.