Bio Unit 4 Objectives

Bio Unit 4 Objectives - 17.1 Evolution at Its Smallest...

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17.1 Evolution at Its Smallest Scale Large-scale evolutionary changes result from small-scale genetic transformations. Individual organisms do not undergo these transformations, nor generally do entire species , which are groups of organisms that naturally interbreed with one another but not with other such groups. Instead, evolution usually occurs as members of species are separated into geographically and reproductively isolated groups that then face the pressures of natural selection in their respective environments ( Figure 17.1a , Figure 17.1b ). It is these populations all the members of a species living in defined geographical regions at given times—that evolve as a result of genetic processes. An individual's genetic makeup, or genotype , determines its observable traits, or phenotype . Genes exist in variant forms called alleles . Although individuals generally have only two alleles for a given gene (one inherited from its mother and one inherited from its father), populations can possess multiple allelic variations of the same gene. The sum total of alleles in a population is its gene pool , which provides the materials upon which evolution operates. 17.2 Evolution as a Change in the Frequency of Alleles Evolution is fundamentally a change in the frequency of alleles in a population. That is, individuals with advantageous alleles survive more often and pass on those alleles to successive generations, thereby increasing the frequency of those alleles in the population. Microevolution is a change in allele frequencies in a population over a relatively short period of time. Evolution is any genetically based phenotypic change in a population of organisms over successive generations. Macroevolution is the evolution that results in the formation of new species or other biological groupings. 17.3 Five Agents of Microevolution Microevolution can occur as a result of five evolutionary forces: mutation, gene flow, genetic drift, sexual selection, and natural selection ( Table 17.1 ). Mutations are permanent changes in an organism's DNA ( Figure 17.3a , Figure 17.3b ). They are relatively rare and generally benign or harmful, and consequently do not appear with greater frequencies in successive generations. Nonetheless, rare beneficially adaptive mutations are vital to evolution in that they are the only means by which completely new genetic information can emerge in populations. Gene flow , or the movement of genes from one population to another, occurs through the migration of individuals from one population into the territory of another. Gene flow can occur for plants as well as animals, since wind and animals can carry plant seeds and pollen to new locations. Genetic drift
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This note was uploaded on 04/18/2008 for the course BIO 101 taught by Professor Kathleenwilsen during the Spring '08 term at N. Colorado.

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Bio Unit 4 Objectives - 17.1 Evolution at Its Smallest...

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